Author Topic: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617  (Read 90457 times)

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Offline rastroTopic starter

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Several months ago I purchased a Keithley 617 electrometer (think I'm becoming a Keithley fanboy/collector).  It seemed like a fully functional unit with the exception of readings at the lowest 2 pico-amps setting.  The service procedure gives instructions for setting zero and offset on the electrometer board at the lowest pico-amps setting with no input.  The zero could be set with good stability but the there was too much variability in the 2 pico-amp range several hundred counts.  I did some research/probing but didn't find anything conclusive.  Prior posting: https://www.eevblog.com/forum/testgear/keithley-617-current-input-adjustment/msg450438/#msg450438

I held off cleaning the electrometer PCB for fear of adding or spreading contamination - thus adding new or exacerbating existing problem(s).  I finally decided to go for it.  I cleaned the critical half of the electrometer PCB with the best isopopal I could find.  I carefully scrubbed it and flushed it with IPA followed by drying with a heat gun.  I reinstalled the board and let it run for 3 days - It was better but still not there. 

A little frustrated I let it set in the closet for a few months while pursuing other projects.  I recently revisited the zero/offset procedure and surprisingly the unit was much quieter on the 2 pico-amps range.  After ~3-days of power-on, the unit was correctly zero/offset adjusted and is very stable.  This instrument is very sensitive - just moving across the floor makes it jump a couple hundred counts for a few seconds an then settle.

The FIX = TIME.  My best guess is the unit may have absorbed moisture at its previous owners location or while shipping.  It was not shipped in a sealed bag with descendant.  So altitude changes (e.g. air shipping) may have created condensation. 

While searching the web, I found some interesting information on design and cleaning/contamination for this type of instrumentation.
 
Reference Links:

http://www.edn.com/design/analog/4368681/Design-femtoampere-circuits-with-low-leakage-part-one
http://www.edn.com/design/analog/4375459/1/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

Cleaning PCB That Are Sensitive To Low Leakage Currents (Great before/after cleaning demonstration):
TI(National) Makes the LMP7721 (3 Femtoampere Input Bias Current Precision Amplifier) and a development board based on this chip. 
Op Amp EVM Cleaning for Femptoamp Bias Currents:
 

Offline jitter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #1 on: September 05, 2015, 09:12:56 am »
While searching for something else, I stumbled on this thread and found it worth bumping up.

Yeah, very familiar. In my job for an OEM, testing and calibrating measuring instruments that we manufacture, leakage always is an issue. We're talking about an instrument with a specified input impedance of > 1 TOhm.
Cleanliness of the boards is very important. All sensitive boards are washed in an industrial pcb washer using special detergents. This is to remove the spray on flux used in the wave soldering process. This washer also dries the boards, and surprisingly, not much time between washing and testing is needed at all.

We found the biggest contributing factors to leakage were actually what types of plastics were used in connectors in combination with the weather.
Certain types of plastic (like the green Phoenix connectors) can hold quite an amount of moisture and cause leakage in warm, muggy weather that we usually have here in  the summers. The seemingly trivial act of changing to a different type of connector solved a lot of issues we were having during these months.

So, with regard to your meter, just the weather conditions may have an effect on it that's not negligible.

Comment on the video: it looks like a crude way to clean the board, but it's tried and tested, been there done that. That curve on the monitor after cleaning looks very familiar...
Also note that there are no connectors on that board with plastic insulation, just individual pins with air around it. In these low current conditions, plastics turn out not to be that good an insulator at all since it can hold moisture (depending on the type of plastic, as written above).

Also, one of Dave's earlier vids:
sounds pretty familiar. But "my" instruments aren't that sensitive that "someone farting across the other side of the factory" :-DD has a measurable influence...
« Last Edit: September 05, 2015, 09:16:02 am by jitter »
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #2 on: September 05, 2015, 05:42:06 pm »
For most aplications a good cleansing with isopropanol will do. Do let it dry for a while, most people forget/neglect this and doing do can easely generate nanoamps of creepage currents. On your electrometer clean the connectors on both the cables and the device itself too.

Once you play with a (good) electrometer you will soon find out why people use expensive (triaxial) cables, and propper grounding is a must. I have made measurements where it was necisary to keep at least 2 meters distance, not to completely mess up the measurement. Just breathing, walking by or even turning your head or waving your hand can mess it up.
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Offline free_electron

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #3 on: September 05, 2015, 05:58:14 pm »
one of the things people do not realise is that, when the cleaner evaporates, it cools down ( evaporation creates a cooling effect ).
this allows moisture form the surrounding air to condense .... moisture that is pretty conductive.
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Offline sarepairman2

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #4 on: September 05, 2015, 07:42:17 pm »
you want to use a mixture of alcohol and diethyl ether for a final rinse (preferably dried under molecular sieves).

and probobly dry it with dry air or nitrogen

what free electron says holds too, waiting for a low humidity condition or using a dry hood/etc helps, you can also heat up the PCB prior to the final clean and then give it the ether rinse
« Last Edit: September 05, 2015, 07:43:48 pm by sarepairman2 »
 

Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #5 on: September 19, 2015, 01:51:34 pm »
I've joined this forum because of the Keithley 617. I've acquired one recently just for fun (I work with electrometers daily and with low current stuff in general for many years) and found several threads here about this device. My one came in a very good working state, with the input current meeting the specs (<4fA after a warm-up). However I've worked with the 617 before and it has one particular side which annoys me - the offset temperature drift. The input dual JFET used meet the leakage current specs but not very good in the offset voltage temperature stability. I thought about a cheap way to improve the stability and (possible) the input current level too, by replacing the dual JFET with a dual opamp - the venerable (and cheap) LMC662. The result was excellent in the DC offset stability, however I couldn't compensate the input current fully. After an investigation I've found that my 617 is an early unit, from about 1984, with a different pcb, a different input JFET and without the input protection transistors (compared to the service manual available on the web)! It also features a different input current compensation circuit, working in one polarity only (negative). For the LMC662 I had to put compensation to minimum (essentially none) and was left with about -2... -3fA cold and -7... -10fA after a warm-up. I will try to update the compensation circuit later today (making it a dual polarity as in later units) and see how stable the input current with the opamps in place of JFETs will be when properly compensated. I may also add some diode protection to the input however the opamp should survive in most overload conditions without it.

Cheers

Alex
« Last Edit: September 19, 2015, 01:57:36 pm by Alex Nikitin »
 
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Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #6 on: September 19, 2015, 04:37:24 pm »
I've just cleaned a 617 yesterday, after it failed calibration. 2pA was unusable, it had more than 450fA offset and more than 80fA noise (this should not exceed 15fA). Totally cleaned the input PCB, and replaced 2 bad caps and a bad potentiometer. It now works like new.

I've posted some pictures on TiNs FTP.
« Last Edit: September 19, 2015, 04:39:07 pm by Smith »
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Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #7 on: September 19, 2015, 08:15:38 pm »
Hello Smith;

Thanks - that's good stuff for future reference. Could you tell us which 2 capacitors and potentiometer?  What was was wrong with them (leak, ESR, capacitance)?  What led you to suspect these 3 parts?
 Also I have no idea how to access TINs FTP - wouldn't it be better to post a couple of pictures here?

 -rastro
 

Offline yodhe

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #8 on: September 19, 2015, 09:28:54 pm »
FTP details are on the xdevs site, but I don't know if you can access the uploads.

https://doc.xdevs.com/article/keithley-instruments/
 

Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #9 on: September 19, 2015, 11:10:00 pm »
Here is the input bias current of the Keithley 617 with LMC662A in place of the input JFET pair for over an hour after I've adjusted it. I'll leave the logger to work overnight to see how the bias behaves.

Cheers

Alex

« Last Edit: September 19, 2015, 11:12:23 pm by Alex Nikitin »
 

Offline TiN

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #10 on: September 20, 2015, 12:36:47 am »
There is no http mirror for uploads, but I usually check FTP often.  ;)

Here are Smith's photos, thanks!











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Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #11 on: September 20, 2015, 02:20:32 am »
Here is the input bias current of the Keithley 617 with LMC662A in place of the input JFET pair for over an hour after I've adjusted it. I'll leave the logger to work overnight to see how the bias behaves.

Cheers

Alex


Alex,
If I understand correctly, you've replaced the (unobtainable) Q308 low leakage dual JFET with an ultra low bias current op-amp??!!  The spec's on the LMC662 look very impressive but I did not think an op-amp would be an easy fit for the dual JFET.  I'd be interested in hearing more details on your modifications...
-rastro
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #12 on: September 20, 2015, 05:38:44 am »
Could you tell us which 2 capacitors and potentiometer?  What was was wrong with them (leak, ESR, capacitance)?  What led you to suspect these 3 parts?
 
 -rastro

I've replaced R348 which is used for setting the input current. It was 5K 3/4 turn pot instead of 10K used in the manual. I replaced it with a small Bourns 10K multi turn (don't know the type) and by offsetting the pins I could still set it via the original hole in the shielding (sorry, didn't make a picture afterwards). Its much easier to set a 10 or 13 turn pot than a 3/4 turn pot. Turning the old pot did nothing.

When I was at it I checked all old electrolytics because let's face it, its build around 1984. Most where fine, but two of them had quite a high ESR (capacity was fine). It were the two identical blue Ritchey (1000uF 16V if I remember correctly) caps next to the transformer on the (smaller) input PCB. I don't know the number as there was no silk and I could not find a silkscreen layout of the input board.

Unfortunately I had no time reading the EEPROM, it was version 4.2 as I remember correctly.

I am interested about the opamp conversion too. Maybe it's something for my Keithley 602 which I still have to repair. The opamp looks quite promising. Just looked at the temperature specs, and input bias should only be 2pA 2fA at 25C, and 4pA 4fA at 50C. I don't think this electrometer will hit 50C internally (although there are no ventilation holes).
« Last Edit: September 20, 2015, 09:50:44 am by Smith »
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Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #13 on: September 20, 2015, 09:14:10 am »
Alex,
If I understand correctly, you've replaced the (unobtainable) Q308 low leakage dual JFET with an ultra low bias current op-amp??!!  The spec's on the LMC662 look very impressive but I did not think an op-amp would be an easy fit for the dual JFET.  I'd be interested in hearing more details on your modifications...
-rastro

Fortunately, in the 617 the input FETs are just followers, so I've used opamp as two follower buffers. Works fine. 18K resistors are needed to imitate the FET's output impedance, these should be selected to match within 0.1% .The orientation of the opamp is important - pin 5 is better to use as the input pin as it is far away from the supply pins! Below is a part from the 617's circuit and a sketch of my replacement. as well as the overnight measurement of the input current (the temperature in the lab went down by about 6-7C during the night). The input current stayed almost in spec even in these conditions. I've also added 220nF film cap between the supply pins of the opamp (visible on the photo I've posted earlier), this cap could be 47nF-220nF in value, preferably film.

Cheers

Alex





« Last Edit: May 09, 2024, 01:29:50 pm by Alex Nikitin »
 

Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #14 on: September 20, 2015, 09:19:50 am »
I am interested about the opamp conversion too. Maybe it's something for my Keithley 602 which I still have to repair. The opamp looks quite promising. Just looked at the temperature specs, and input bias should only be 2pA at 25C, and 4pA at 50C. I don't think this electrometer will hit 50C internally (although there are no ventilation holes).

Femtoamps! Real input currents of the LMC662 are in a low fA region (if you keep it clean, obviously).

Cheers

Alex
« Last Edit: September 20, 2015, 09:34:29 am by Alex Nikitin »
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #15 on: September 20, 2015, 09:47:19 am »
Femtoamps! Real input currents of the LMC662 are in low fA region (if you keep it clean, obviously).

You are right, I was talking about femto amps. I might give this a try. I have some other older Keithley gear with blown inputs featuring unobtainium components.

I was wondering what other people here are using electrometers for. I use them for very low current measurement on light cells, high ohms resistor and leakage measurements (10M to >10Gohms and for measurements in circuit (high impedance voltage). I must admit I occasionally use their voltage source as a voltage supply or calibration voltage.  :-+

BTW Alex, I wondered how you determined the FET output impedance resistor as 18K. I would imagine this value to be quite a lot lower.
« Last Edit: September 20, 2015, 09:58:19 am by Smith »
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Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #16 on: September 20, 2015, 10:11:13 am »
BTW Alex, I wondered how you determined the FET output impedance resistor as 18K. I would imagine this value to be quite a lot lower.

These FETs have (roughly) the transconductance of about 2mA/V at 1mA (~0.5K output impedance in a follower). In the 617 circuit these FETS are run at a rather low current (~25uA). The transconductance in a JFET is (again, roughly) proportional to the drain current, so for 40 times lower current the output impedance would be about 40 times higher or ~20K. These resistors are needed as the second stage on the LT1012 is an integrator relying upon the previous stage output impedance.

Cheers

Alex
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #17 on: September 20, 2015, 10:28:14 am »
Thanks for the explanation  :-+
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Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #18 on: September 20, 2015, 10:30:18 am »
I see the LMC662A is available in a through-hole MDIP-8 package.  This might be a better choice over the SMD version.  I don't think the board is too cramped to accommodate the larger package.  It would also be easier to solder and clean up.  Also it seems like there would be more distance or physical isolation between the pins.

-rastro
 

Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #19 on: September 20, 2015, 10:46:10 am »
I see the LMC662A is available in a through-hole MDIP-8 package.  This might be a better choice over the SMD version.  I don't think the board is too cramped to accommodate the larger package.  It would also be easier to solder and clean up.  Also it seems like there would be more distance or physical isolation between the pins.

-rastro

Yes, a DIP-8 package will be fine too, I've just had the SOIC-8 version handy, that is all.

Cheers

Alex
 

Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #20 on: September 20, 2015, 12:36:50 pm »
Hello Alex,

The 617 I'm interested in LMC662A-retrofitting has the newer electrometer PCB because it has the input protection (dual) transistor(s).  I have 3 areas of concern with how well this modification would cross over from the older style electrometer PCB.

1. Different input current compensation circuitry:
-- This is likely a non-issue since the newer board probably just adds additional range.

2. Different input JFET (Q308):
-- How was it determined the Q308 is different between versions? (part number marking?)
-- Is there a meaningful difference between the new and old version of Q308?
-- Will the 18K resistors still provide the correct impedance matching for the newer Q308?

3. Addition of input protection circuits:
-- This should not be an issue since input off-set is adjusted out with R348.

Regards
-rastro
 

Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #21 on: September 20, 2015, 01:03:45 pm »
Hello Alex,

The 617 I'm interested in LMC662A-retrofitting has the newer electrometer PCB because it has the input protection (dual) transistor(s).  I have 3 areas of concern with how well this modification would cross over from the older style electrometer PCB.

1. Different input current compensation circuitry:
-- This is likely a non-issue since the newer board probably just adds additional range.

2. Different input JFET (Q308):
-- How was it determined the Q308 is different between versions? (part number marking?)
-- Is there a meaningful difference between the new and old version of Q308?
-- Will the 18K resistors still provide the correct impedance matching for the newer Q308?

3. Addition of input protection circuits:
-- This should not be an issue since input off-set is adjusted out with R348.

Regards
-rastro

Non-issues on all three items, actually. I had to update the current compensation circuit anyway, newer FETs are most likely similar in parameters , just from the photos it looks like the later ones have a smaller case, and the protection transistors should not be a problem. 

Cheers

Alex

P.S. - the last measurement graph I've done today, on a fully closed and warmed-up 617:

« Last Edit: September 20, 2015, 02:53:39 pm by Alex Nikitin »
 

Online Vgkid

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #22 on: September 20, 2015, 03:18:47 pm »
That is pretty stable, good job.
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Offline jitter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #23 on: September 20, 2015, 09:03:57 pm »
Those spikes in the measurements, could they be the result of popcorn noise?
 

Offline Alex Nikitin

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #24 on: September 20, 2015, 09:55:29 pm »
Those spikes in the measurements, could they be the result of popcorn noise?

You may call it popcorn noise if you wish  ;) . At these current level it is almost unavoidable to have some of these. Before I've closed the top cover on the 617 even some air movement may cause a spike. And even with a closed cover a small tap on the bench could cause a spike like one of those on the graph.

Cheers

Alex
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #25 on: September 21, 2015, 04:59:03 pm »
I tried to read the main EEPROM today, but unfortunately this type (Hitachi HM4827128G-25) was not supported by my programmer. 

Today I found a problem regarding calibration: The device won't calibrate the current ranges. When I save the fresh calibration  via program select => stor the calibration values disappear, and the previous calibration settings reappear. The manual states this action should copy the settings to the NVRAM  (Xicor X2443P NOVRAM) memory. Voltage calibration does work. Anyone has experience wit this kind of issue?
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Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #26 on: September 22, 2015, 12:08:20 am »
I tried to read the main EEPROM today, but unfortunately this type (Hitachi HM4827128G-25) was not supported by my programmer. 

Today I found a problem regarding calibration: The device won't calibrate the current ranges. When I save the fresh calibration  via program select => stor the calibration values disappear, and the previous calibration settings reappear. The manual states this action should copy the settings to the NVRAM  (Xicor X2443P NOVRAM) memory. Voltage calibration does work. Anyone has experience wit this kind of issue?

Smith,
See page 7-6 of the 617 user manual "Keep in mind that the calibration jumper must be in the correct position, as described in paragraph 7.4.4."
It's possible that the voltage calibration may already have the same value in NVRAM as the new value you are trying to save - so it just appears you have saved a new value.

-rastro
« Last Edit: September 22, 2015, 12:22:46 am by rastro »
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #27 on: September 22, 2015, 06:27:48 am »
I have set the jumper in the right setting. When saving calibration with the jumper on the wrong position the display says 'out' instead of 'stor'.

I am absolutely shure the voltage calibration worked as the device was far out of calibration (>1% on some settings). After adjusting the unit the voltage reading was within 1 or 2 digits of my Keithley 2000 & 6517a.

Unfortunately my programmer also doesn't support the NVRAM to make sure it writes anything. Also, the NVRAM has 256 bit for storing calibration. The device has 27 calibrated 'ranges'. So it has no more than 9,5bits for each setting, which will probably be 8 bits. Maybe I am trying to calibrate the device outside it's 'calibration working area'.

The device came in because it was out of cal on all voltage settings, some current and some resistor settings. As we don't use the coulomb measurement anywhere, none of our electrometer have calibrated coulomb settings. Maybe we should replace the damn thing, but it's only 31 years old  :D
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Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #28 on: September 23, 2015, 06:46:54 pm »
Complete fail! You can only calibrate 4 of the current ranges.  :palm:

Thankfully my colleague noticed. After some final adjustments the 617 now works like it should. Only thing left is the jumps every now and then. Sometimes the current jumps 30fA up and then settles down in 2pA 20pA and 2nA ranges. It took some other irradicate jumps as well, but 95% of the time there is no problem.  Calibrating the pA ranges took some time, it was difficult to find 100Gohm and 1Tohm resistors, but we did.
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Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #29 on: September 23, 2015, 07:25:44 pm »
Smith,

Glad to hear your cal problem is solved.  How did you instrument your 100G and 1T resistors?  Box and triax connectors for shielding?

I've only checked mine against 1G ohm.  Resistors  start getting real expensive in the higher ranges and low tempcos.  If someone was going to scrap and old electrometer it would be a good time to cannibalize a few high ohm resistors for future testing or baseline.

-rastro
 

Offline jitter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #30 on: September 23, 2015, 07:40:04 pm »
Those spikes in the measurements, could they be the result of popcorn noise?

You may call it popcorn noise if you wish  ;) . At these current level it is almost unavoidable to have some of these. Before I've closed the top cover on the 617 even some air movement may cause a spike. And even with a closed cover a small tap on the bench could cause a spike like one of those on the graph.

Cheers

Alex

The reason I asked is because with the instruments I work on, spikes like that may take it outside the specs set down by our customer. This happens with about 10% of the instruments. In almost all cases popcon noise in an AD797 is to blame.

It is a similar measurement taken on a 1k load with no voltage applied. The result shows offset and noise of which the latter must lie in a band of 4 nApp. Typical noise is around 2.5 nApp but peaks obviously take it outside the limits.
Tapping on the Faraday cage in which the electrodes, dummy load and differential amplifier are placed may indeed cause spikes.

I was wondering if the spikes in the graph of the Keithley could have a similar cause.
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #31 on: September 23, 2015, 08:40:38 pm »
Glad to hear your cal problem is solved.  How did you instrument your 100G and 1T resistors?  Box and triax connectors for shielding?
The are mounted in metal boxes with BNC connectors on the in and output. We use these boxes for current calibration. I have a separate shielded case (diy) for connecting loose high resistance resistors (and other components). The box itself has an impedance of >2Petaohm.

The resistors become quite expensive indeed, the ones we use are about 50 - 100 euro a piece. It comes down to absolute value, stability and voltage dependency.
« Last Edit: September 23, 2015, 08:42:25 pm by Smith »
Trying is the first step towards failure
 

Offline Alex Nikitin

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At last I've managed to build a proper triaxial cable for my 617. I may yet buy a proper low-noise one from Keithley, when I'll have £200 to spare though, as the Belden 9222 I've used is not a low-noise version and unless it is fixed in one position can produce quite a lot of charge. If it is not moving, I get the same 3-4fA input current readings (after the current settles down which takes a while) as without the cable, which is good. I've also measured the voltage source performance of the 617 - very good, with less than 1LSB (50mV) error over the complete range of +/-102.3V and the tempco about 30-40ppm/C .

Cheers

Alex
 

Offline Alex Nikitin

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A little update on the Keithley 617. A year ago I've considered <1LSB from the voltage source a pretty good performance. It is however, one area where the K617 can be improved - and there is a good reason to do it, as the voltage source error directly affects resistance measurements in the V/I mode. 25mV (1/2 LSB) error at 100V is only 0.025% but at 10V it is 0.25% and at 1V - 2.5% ! Unless you are prepared to tweak the pot at the back of the unit to set the voltage accurately at a particular point, this error makes this kind of measurement somewhat less precise. Fortunately, the performance of the voltage source can be upgraded by changing the DAC chip - the old and venerable AD7541AJN to an improved LT version - LTC7541AKN, plus changing LM308A in the x10 voltage amplifier stage to OP97. The result is a much improved accuracy (now the maximum deviation over the whole +/-102V range is less than 5mV and most of the time it is under 3mV) and also a considerably reduced LF noise at low output voltages.

Cheers

Alex
« Last Edit: January 16, 2017, 05:22:14 pm by Alex Nikitin »
 
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Online Kleinstein

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For the OP conversion I am wondering why you need the super high impedance LMC662 to replace both FETs. The feedback side is not very high impedance and could use a lower noise/drift type with no problem.

For the spike like noise, this could be cosmic or radioactive background radiation, hitting something like protecting diodes. Smaller diodes (less sensitive volume) and maybe radiation shielding could help.
 

Offline MadTux

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Anyone know where to find those black or red low leakage reed relays used on the electrometer input of the Keithley 617?

Just finished fixing a couple of broken K617s from ebay (collected over the years and never really had time fixing them ;-).
I now have 4 working ones and 1 with leaky relay, maybe someone knows where to find replacement.

It was the only one that refused to zero input offset current (R348 trimmer, always displayed something around -.1500pA). Initially started by cleaning electrometer board with IPA and deionized water (which actually fixed some of the other "broken" ones), which didn't help in this case. I then replaced the "magic" input JFET (Q308) with 2N5909 (~$5 on ebay), which actually works really great (one K617 had toasted input and got fixed that way), but unfortunately didn't solve the issue here. I then had the idea of disconnecting all unused relays (K307, K309, K310) from the input stage to isolate the leakage path. With all these relays disconnected, the K617 input offset current would zero without problems. I then simply reconnected relays until I the input offset current problem started again. Found 2 suspect relays this way, which were then removed from the board and measured for leakage using my trusty Radiometer IM6 Megaohm-Meter/leakage tester.

Good relay has a resistance >1000TOhm @ 900V, while the leaky one (K309) has around 5TOhms @ 900V, which apparently causes enough leakage so that the input offset current can't be zeroed any more.

BTW, the most important thing when zeroing the  input offset current is time, especially after the board was cleaned. I found that it sometimes takes a day or so until all moisture has disappeared. A hair dryer is also helpful. The +/- .0015pA  offset current mentioned in the manual are IMO also a bit optimistic. After a good warmup I easily could adjust them to +/- .0050pA, but adjusting them to within +/- .0015pA required constant fiddling on R348.
« Last Edit: August 04, 2017, 08:57:04 pm by MadTux »
 

Offline math_indy

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I could not find COTO 1240-0197 anywhere (aka Keithley RL-70) however I found COTO 7301-05-1000 relays (>1TOhm) in stock at Digi-key which may do if you cannot find anything else.  I "dead bugged" the new zeroing relay into the spot where the leaky relay was.

Here is a video showing the leaky relay that was replaced:


It would be nice to have some of the original relays AND a specification sheet for the original relays.  I can't find a specification sheet for 1240-0197 anywhere?

After replacing the relay it worked much better but getting full performance required cleaning BOTH circuit boards.  I got some laboratory grade methanol from eBay and used a small airbrush gun to blast the methanol onto those boards.  For safety reasons do that outside because of the flammable methanol and associated fumes that you don't want to inhale.

Does anyone know exactly how the zero-correct works?  Is it any different from suppress?  It appears that both zero-correct and suppress are executed by software.  I think this especially true because I cannot find any analog sample-and-hold so it seems just a software correction-no?
 
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Offline rastroTopic starter

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Tom,
Great video and follow-up.  Great troubleshooting techniques - I'll have to remember this work around.  Hope the subtitue relay works out.  Let us know.  Also if you are going to scrap the old relay it would be interesting to open it up to see how it's built.

-rastro
 

Offline MadTux

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I'm quite busy lately, so I somehow forgot to write on how I fixed it.

Anyway, these red relays are quite easy to fix. Just pull out the old reed switch tube with some pliers and put a new reed  inside the relay coil.

The real magic lies in selecting a good reed switch. Forget the cheap crap from china, only a few GOhm of isolation resistance (because they have no sulfur hexaflouride filling). They will also start to arc at relatively low voltages (300-500V, usually). The original reed switches inside the red relays aren't that great either. (in terms of arcover and isolation resistance). For replacement, I finally found some afordable Harmlin MARR5 reed switches that are rated to an isolation voltage of 1000V and an isolation resistance of at least 1TOhm and fit inside the old relay coils.

 Real isolation resistance is more like 50-100TOhm @ 1000V, just at the limit where I can measure reliably. Also couldn't measure breakdown voltage, because my isolation tester/megaohmmeter only goes up to 1000V.
http://www.ebay.com/itm/Hamlin-MARR-5-Reed-Switch-SPST-NO-1-3-A-1000-VDC-10-W-10pcs-1-Lot/122199268347?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649
http://m.littelfuse.com/~/media/electronics/datasheets/reed_switches/littelfuse_reed_switches_marr_5_datasheet.pdf.pdf

The only problem with these is that they won't reliably switch at 5V supplied by the 74HC04. So you need to overvolt the 74HC04 by applying something around 8V from the output of the first preregulator stage. (from emitter of Q309, placed some ceramic caps in there for good measure, too). I initially hacked an addon board so I can use 15V from C305 directly, but I got an noisy instrument afterwards, probably because of capacitivly coupled rectified AC noise. The it occurred to me that I might just overvolt the 74HC04 with some 8V from the 1th preregulator output, which worked.

Leaky black relays are a bit more difficult to fix, because the reed switch is epoxied inside. An option might be to drill it out. But I destroyed mine in the process, so I ended up replacing them all with Harmlin MARR5 reed switches and some reed relay coils I found on ebay. Doesn't look nice, but works just as good (maybe not as good as the red relay hack, because these coils have shielding inside) The coils are held in place by some copper wire and an eyelet soldered to the PCB, btw.
« Last Edit: October 03, 2017, 10:38:23 pm by MadTux »
 

Offline rastroTopic starter

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Hello MadTux;

Interesting information.  The 3 pictures you just posted look like different PCB's.  I looks like one has 3 of of the series transistors for the HV supply replaced with transistors that have different pin-out due to the crossed legs.

-rastro
 

Offline MadTux

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Yes, 2 different K617, both bought broken for little money on ebay over the course of a couple of years. The one with funny electrometer drive stage transistors was especially broken, because the previous owner (dubious dealer of all sorts of test gear on ebay  ;-) had no clue on how to fix it and replaced parts at random, apparently without success. Finally sold 2 rather broken K617 as a package for a good price, which I bought. Because I couln't find the original MMPS U60 for a good price, I replaced them by MJD350, which has different pin configuration, IIRC.
« Last Edit: October 03, 2017, 11:41:03 pm by MadTux »
 

Offline razberik

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math_indy: I see some problems in your approach. It is not very clear in your video, but it seems that you are touching these very sensitive areas with your fingers. Perhaps you know what are you doing and you also say that in video.
But anyway for everybody: Never touch picoamp parts with bare fingers. Of course I break this rule sometimes. ;D But I touch only leads, since there is no change in resistivity. Sensitive are insulation areas, like hi-ohm resistor bodies. When I manipulate with these parts, I use tweezers. When risk of touching, gloves are must.

I don't think methanol cleaning is the best idea when I am not absolutely sure what exactly the problem is.
Please read this topic, there are valuable information:
https://www.eevblog.com/forum/testgear/keithley-64856487-teardown/
There is also a schematic of input of this instruments. Since it is descent of K617, the principle is the same, ZeroCheck included.

Also that 1240-0197 COTO relay is a special shielded relay ! You definitely cannot replace it with 7301-05-1000. It can perhaps work in DC area since it has good insulation, but transient response would suffer. That guard shield helps condition material between contacts and outer world to zero voltage difference, so it wouldnt suffer stabilizing time.
Explained here: https://www.embedded.com/print/4375459

Modern replacement should be 1203-0147. Unfortunately no datasheet nor information on COTO website. I emailed czech and german representative yesterday about this relay since I am also interested in it! No answer yet.
1203-0147 is used in Keithley 6514 and 6517A.

I would use MadTux approach. Pull out old reed and replace it with new one. Dont screw guard foil. Perhaps Standex-Meder 1A66 reed should work too:
http://cz.mouser.com/ProductDetail/MEDER-electronic-Standex/KSK-1A66-1015/?qs=KFo7JewZbUHhbHIVxk%2FaJw%3D%3D
It seems that reed is covered with polyetylene (third best insulator after sapphire and teflon) and wrapped in shield foil. Therefore I would wrap new reed with foil and then try to insert it into coil package.

AD) Also, It is a luck this bad relay was discovered since it is really bad. But if relay has some small leakage problem, it wouldnt be discovered in nanoAmp range and unshielded arrangement.
« Last Edit: October 06, 2017, 01:17:59 pm by razberik »
 
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Offline rastroTopic starter

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This is a really interesting discussion on the reed relays.  It seems like faulty range-relays could be a hidden/frustrating problem on many of these electrometers as they age.

It would be great to find an comparable substitute for these reed switches.  One that could be inserted in the original solenoid.  Ideally a part that has the same AMPERE-TURNs (AT) turn-on level as the original parts.  This would eliminate the need for modification to the solenoid supply.  However it seems like reed switches with higher breakdown voltages also require higher activation current through the solenoid.  I've outline some of the considerations but these parameters still need to be evaluated.  I think determining the AT value of the original reed/solenoid would be good start.

* Reed Switch Requirements

** AMPERE-TURNs (AT):
   The product of the number of turns in an electromagnetic coil
   winding and the current in amperes passing through the
   winding. Used to quantify reed switch operate and release
   sensitivities.

** Breakdown Voltage:
   Higer voltage tolereance requires higher AT due to larger gaps
   required between reeds.  What is the design requirement of the
   current reed switches?

** Hermetically Sealed:
   Is this required for high impedence?

** Form:
   SPST n.o. (Normally Open)

** Carry Current:
   This sould be fairly low???

** Insulation Resistance:
   What leakage can we measure on known working switches?

 

Online Kleinstein

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If needed one could improve the sensitivity a little by adding a magnetic return path on top of the relay. It does not need to go all the way to the contacts and thus would not cause much extra leakage. Since the magnetic circuit is not closed, the gap at the contacts will not be that important for the sensitivity. 

Sensitive reed relay usually com in a ferromagnetic case. The case is not only a a shield for the outside, but also to improve sensitivity by a magnetic return path.

The separate reed contacts are usually just glass - these special relays use a plastics (e.g. polystyrene) cover directly around the glass. The direct contact might be important to prevent surface leakage. Usually the glass itself is a good enough isolator, but unless very dry, there can be a surface layer of water that can cause leakage.  Putting a new glass in the old plastic tube might not be enough and could be difficult. I have not seen the details, but my guess would be using a kind of heat shrinking (e.g stretching it first) on the polystyrene.
 

Offline razberik

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Does it happen to you too ? You want to buy your shielded relay, you open your favourit supplier, start browsing relay section and then ... you can't find shielded relays section !  :rant:

I have few old obsolete Meder HI12-1A79 which were replaced by 1A85.
I sacrificed one and tried to install shield foil. I wasn't able to install copper foil, it simply wasn't possible. I used aluminium foil and that wasn't easy to install though.
It was really difficult to arrange the foil not to touch reed switch. Gloves wearing is absolutely must.
See photos. I didn't have time to test it yet, but I am going to work on it.

The separate reed contacts are usually just glass - these special relays use a plastics (e.g. polystyrene) cover directly around the glass. The direct contact might be important to prevent surface leakage. Usually the glass itself is a good enough isolator, but unless very dry, there can be a surface layer of water that can cause leakage.  Putting a new glass in the old plastic tube might not be enough and could be difficult. I have not seen the details, but my guess would be using a kind of heat shrinking (e.g stretching it first) on the polystyrene.
This seems to be plausible. I firstly thought that the only reason copper foil is wrapped around plastic encapsulation is only manufacturing process. But I didn't think of you mentioned.
 

Offline rastroTopic starter

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razberik/Kleinstein;

I'm not sure what you mean by "sensitivity".  Does this mean lowering the amount of current required to activate/close the reed switch?
So are you saying that adding foil around the reed switch will make it activate easier?

Is the ferromagnetic casing focusing magnetic flux toward the reed switch?  Would this be similar to a torroid focusing the flux on a coil.??
 

Offline razberik

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Kleistein was perhaps talking about this (see picture).
 

Offline rastroTopic starter

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Kleistein was perhaps talking about this (see picture).
I'm not sure what the picture is supposed to convey.  At this point I'll just assume that 'sensitivity' is the current (VA?) required to activate the reed switch.
-rastro
 

Offline Alex Nikitin

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Reading this discussion about relays I've remembered that I have a quantity of encapsulated miniature reed relays - REMtech MRFL80005, 5V operation, internal screen for 50 Ohm line impedance connection, sub-ms operating time. Just out of curiosity I've measured the resistance between open contact pins (everything else disconnected, relay suspended in the air inside a metal box I use to measure high resistance with my Keithley 617). Using V/I resistance mode I've measured about 2.2 PetaOhm = 2200 Tohm at 100V, both polarities applied (about 45fA leakage), after ~30min settling time for each polarity. I did not even clean it, just soldered straight out of the bag. Not bad for a chance eBay purchase several years ago. I use these relay for audio with good results, but it looks like they are useful for high impedance stuff too (up to 125V as that is the contact rating) !

Cheers

Alex

P.S. - tried with the relay internal screen connected to ground - the leakage at 100V is around 10fA (measured resistance ~10 Peta Ohm, with the idle current of the setup, including the K617 around 0fA (+/-2fA fluctuations, with the voltage source at 0V).



« Last Edit: October 11, 2017, 09:43:27 pm by Alex Nikitin »
 

Offline rastroTopic starter

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I think you also want to check leakage from the reed contact pins to the solenoid connections also.
 

Offline Alex Nikitin

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I think you also want to check leakage from the reed contact pins to the solenoid connections also.

Obviously, it is in progress right now. These measurements take time... .

Update: With both contacts connected together , shield to ground and 100V to one of the coil connections, measured resistance is about 1000T (after an hour). Tomorrow I'll try to clean it and re-measure, as I did make four solder connections now to this small package.

Cheers

Alex

P.S. - in the morning the leakage dropped to ~20fA at 100V, about 5000T measured.
« Last Edit: October 12, 2017, 01:58:31 pm by Alex Nikitin »
 

Offline Alex Nikitin

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Did some more measurements on this REMtech relay, below are the results. Time scale is 10 sec per point, vertical scale 20fA per division. The leakage measured between the coil and both contacts together, the screen is connected to the screening box ground, voltages applied are 0V, +100V, 0V, -100V, 0V and +50V. The measured resistance after ~30 min is over 1000 Tohm for either 100V or 50V.

Cheers

Alex
« Last Edit: October 12, 2017, 02:30:50 pm by Alex Nikitin »
 
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Online Kleinstein

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Much of the shown "leakage" current seems to be due to dielectric absorption and not true leakage: Especially the curve at 0 V is pointing toward DA, as the transients look similar for the -100V to 0 step and the 0 to 100V step. While this is not true leakage, dielectric loss can be still a problem in an instrument.

It might be interesting to also have a step like 100 V to 50 V in the sequence, as here the initial DA current and leakage would be opposite sign.
 

Offline Alex Nikitin

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Much of the shown "leakage" current seems to be due to dielectric absorption and not true leakage: Especially the curve at 0 V is pointing toward DA, as the transients look similar for the -100V to 0 step and the 0 to 100V step. While this is not true leakage, dielectric loss can be still a problem in an instrument.

It might be interesting to also have a step like 100 V to 50 V in the sequence, as here the initial DA current and leakage would be opposite sign.

Yes, it is mostly DA, as my overnight data also confirm. Unfortunately, DA is always present at these current levels, especially if voltages are reasonably high. I may try to do some more measurements tomorrow. I've tried 0 to 5V step and the leakage settled to <3fA step in less than 5min. 

Cheers

Alex
« Last Edit: October 12, 2017, 03:38:53 pm by Alex Nikitin »
 

Offline math_indy

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I got my unit working very nicely.  Here is a part 2 video that shows what I had to do to complete the repair and calibration.  Somebody mentioned that methanol maybe not a good idea but it seemed to work well and methanol is what the Keithley service manual calls for.  I sprayed it from a air-brush gun then gently dried the board with warm air.

https://youtu.be/-68-65swAag
« Last Edit: October 17, 2017, 05:12:37 pm by math_indy »
 
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Online Kleinstein

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I won't expect to much trouble from the vacuum leaking out  :-DD , even with air inside this would necessarily high leakage.
I am not even sure they have vacuum. Low pressure gas could be a problem at high voltages though. Fancy HV switches might use SF6 gas under pressure. I remember using an electron microscope that had some of the HV stuff under pressure with SF6.

The problem is more likely with the surface. This could be inside the reed contacts if they were exposed to too much voltage and did deposit some metal. It could also be on the outer surface from some kind of contamination or just the plastic cover getting old. Also some glass might take up humidity, not much but it can happen and make an effect. There is a slight chance that a simple bake out of the relays could make them isolate better again.
 

Offline rastroTopic starter

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I got my unit working very nicely.  Here is a part 2 video that shows what I had to do to complete the repair and calibration.  Somebody mentioned that methanol maybe not a good idea but it seemed to work well and methanol is what the Keithley service manual calls for.  I sprayed it from a air-brush gun then gently dried the board with warm air.

Thanks for taking the time to video/document this.  What do you think is the before/after performance on the LM662A modification?  Also could you post a few pictures of the electrometer PCB to see how the new parts fit mechanically.

-rastro
 

Offline cat87

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Funny thing... just the other day I was scouring the net for some info on what would be some figures for leakage of these kinds of Reed relays. From independent users, not from manufacturers. Of course, I didn't find anything.

And lo and behold, today I just stumble upon this thread and voila. Some useful data. Thank you  :-+

Offline math_indy

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PART 3 video showing the stability of the lowest current range:

https://youtu.be/NxTJ4Y7jZsY
 

Offline razberik

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I'm not sure what the picture is supposed to convey.  At this point I'll just assume that 'sensitivity' is the current (VA?) required to activate the reed switch.
-rastro
Sorry for not being explanatory. I attached picture of relay which has mu-metal shielding and it should avoid "cross-talk" of excitation current when there are more relays crowded closely together. And yes, it decreases excitation current. http://pickeringrelay.com/pdfs/reed_relaymate_web-1.pdf page 6.
But this shield doesn't help in transient characteristic.

Since I have built new version of Gyro's picoammeter with some test jigs I made similar measurement to Alex.

I have a small test chamber with feedthrough BNCs. It is basically inserted between picoammeter and test current source (1.5V battery + 1TOhm resistor). Thats the orange background in graphs.

1) I attached non-modified Meder HI12-1A79 with shorted contact to feedthrough line and turned the coil ON and OFF.
Experiment was repated with my modified relay with shielding foil connected to box enclosure potential.

2) Similar experiment like Alex did. Shorted coil and put on voltage. I tried only 30V, but results can be seen.
Non shielded and shielded.

Shielding helps a LOT. It doesn't really matter in DC region since modern relay has excellent insulation materials. But I don't believe that repaired K617 would meet it's nominal transient characteristics and settling times.
 
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Offline branadic

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #60 on: November 09, 2018, 07:31:05 pm »
Just curios, has someone tried Standex Electronics reed relay as a direct replacement: https://standexelectronics.com/products/hi-series-reed-relay/
They claim Insulation Resistance (IR) >10^14 Ω which looks quite good.
Two version are available HI05-1A66 and HI12-1A85 with a coil voltage of 5V or 12V.

-branadic-
Computers exist to solve problems that we wouldn't have without them. AI exists to answer questions, we wouldn't ask without it.
 
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Offline branadic

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #61 on: November 09, 2018, 09:56:41 pm »
Those spikes in the measurements, could they be the result of popcorn noise?

You may call it popcorn noise if you wish  ;) . At these current level it is almost unavoidable to have some of these. Before I've closed the top cover on the 617 even some air movement may cause a spike. And even with a closed cover a small tap on the bench could cause a spike like one of those on the graph.

Cheers

Alex

Looks more like what they call "Ionizing Radiation" in:
https://www.edn.com/design/analog/4375459/5/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

"...
Home> Analog Design Center > How To Article   
Design femtoampere circuits with low leakage - Part 2: Component selection
Paul Grohe, Texas Instruments Precision Systems Group -June 15, 2012

6 Comments
Power line noise
Powerline noise can manifest itself through many different ways. The obvious ways are the aforementioned ground loops, electrostatic and electromagnetic coupling, but there are some other avenues where line noise can infiltrate a high-impedance circuit.

For op-amps, instrumentation amps, CMOS switches and muxes, A/D's and other active devices with high impedance inputs, the input pins most likely have ESD protection or clamping structures on their input pins (as previously discussed). These structures have a direct connection to the power line, and also have inherent device capacitance across them. This small capacitance can couple noise directly into the high impedance input signal, bypassing any of the native Power Supply Rejection Ratio (PSRR) of the device. A few millivolts of "hum" or digital "hash" on the supply line can make its way through the protection diode capacitance and embed itself in the input signal.

Figure 12. Supply Noise coupled through ESD diode capacitance

Another entry point is power supply derived bias voltages, which are usually obtained through a resistive divider. The typical Vs/2 divider has only 6dB of power supply rejection without any filtering. Adding a bypass capacitor to the tap may boost this up to 20-30dB or more at high frequencies, but at low frequencies, the capacitance may not be enough to completely eliminate power line frequencies (and their harmonics). If this bias line is used to provide a bias voltages for the sensor (before the gain stage), then the noise can "modulate" the sensor signal and be amplified along with the sensor signal.

Obviously, the rule here is to keep the supplies and bias voltages as clean as possible. Very sensitive stages should be supplied from a separate supply. In most cases, a simple R-C-L filter on the supply lines will suffice.

A word of warning: When bench testing, digitally controlled power supplies can have a combination of hum, switching noise and digital "hash" on them. Also, many DMMs have a lot of digital sampling transients on their inputs. The DMMs, when connected to an analog supply to monitor the voltage, can inject digital noise into the supply lines. If you see noise on your signal that correlates with the DMM's display update rate, then turn the DMM off and see if the noise goes away.

Ionizing Radiation
One interesting phenomenon that occurs down at the femtoamp levels is the detection of naturally occurring ionizing radiation3. The most common effect is a sudden "pop" or step change in a measurement. As the particles fly by at the speed of light, they ionize the air in their wake. If the input conductors happen to be near by, they will accumulate some of this charge.

The radiation sources are both terrestrial, and extraterrestrial, and are all around us. The largest contributor is extraterrestrial. Energetic particles are generated from the sun, solar flares, exploding supernovae and other galactic sources. Those particles have come a long way to disturb your measurement! Terrestrial sources of radiation can include common materials such as ceramic, stone and granite. Radon can accumulate in enclosed underground areas.

Alpha and beta particles can be stopped with a few millimeters of aluminum, but the more energetic gamma and "X" rays (which can create the alpha and beta particles) are stopped by much more dense materials (centimeters of lead).

This phenomenon may occur once a week, or several times in one day, maybe once a year. The strikes are random in both time and amplitude, and frequency depends on circuit layout, sensitivity, altitude and construction materials.

With an integrator, this sudden accumulation of charge will look like a sharp step, but the slope of the line generally does not change. A transimpedance amplifier will show a sharp rising edge followed by an exponential decay.

The only way to minimize these events is to minimize the amount of air between the measurement nodes and the guard, as well as keeping the surface area of the input conductors to a minimum. Do not inadvertently create your own small-scale ion chamber!..."

-branadic-
Computers exist to solve problems that we wouldn't have without them. AI exists to answer questions, we wouldn't ask without it.
 
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Offline MiDi

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Keithley 617 incl. Fireworks
« Reply #62 on: June 13, 2019, 10:43:26 pm »
Got this interesting unit four weeks ago - including fireworks :popcorn:

Video from previous owner:



The eagle eye will catch instantly two obvious damages:



The first are two 35 year old spacers near primary tranformer that did not withstand careful handling by parcel service and dissolved to nearly dust, second one is a puff daddy.
Any suggestions for replacement of spacers?

Selection of pictures from attachement from unit as received







Strange black pcb cancer on electrometer board...

The fireworks section is located at the Diodes of HV supply for bootstrap amp with sharp edges meeting creepage distance of <1mm @>500V  :wtf:





Problem solved with piece of kapton tape and diodes as High Rider:





Next obvious site is the incontinent cap C316 (1000µF 105°C Nichicon, already replaced in ~91)



After that the unit was first fired up and first check revealed another problem.
When disabling zero check it went fast to overload.
Check of HV supply revealed unstable voltages, maybe only bad caps?

After a timeout all caps were replaced, another two of them decided to give high resistance - one was from the HV supply.
HV supply was now stable, but error still persisted - this seems to become a serious repair.

Check of supply rails showed ~0V for -5V bootstrap rail.
After desoldering LM337L (U307) it showed a bit of melted case on pins - this poor little sucker was toasted.
Guess what - the error still persisted after replacement.

Finally only Q308 remained to be the cause - this unobtainium matched dual input JFET.
Thanks to Alex the journey to femtoamps could continue by replacing it with LMC662.



Quick check: It's alive!  :-DMM

After several local cleanigs, the electrometer board was fully cleaned twice from both sides with special pcb cleaner "Kontakt LR" followed by blowing it with pressurized air from a can and repeat this with pure IPA.
After drying it at 50°C for about 2h it went back into its case, but drift and noise were horrible - even after several hours.

The next days noise did improve, now after 5 days it finally went down to ~1.2fA noise - this looks promising.
The offset could be adjusted to 0 only after shorting a resistor for balancing - in this unit this is done by changing the resistor values between Q308-1 and R335 (2 parallel resistors, one was cut from factory) and Q308-5 and R336 (pot + 2 parallel resistors - one was cut from factory)

The input bias current could not be cancelled out fully - Alex reported this.
Documentation of modification to dual rails for this early units appreciated.

Interesting that this unit seems to have some changes to that from Alex, maybe this has first revision of electrometer board.
Some parts have datecode of '91, so there was apparantly a repair/upgrade.

Big thanks to all contributors for repair documentation on 617, especially Alex for the JFET replacement!



--- to be continued ---
« Last Edit: June 14, 2019, 12:19:30 am by MiDi »
 
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Offline rastroTopic starter

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Midi thanks for posting your work/findings - keep adding to the collective 617 knowledge.

That was a bizarre and entertaining video.  At first I couldn't understand why it went dark  :-DD

rastro
 

Offline MiDi

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Midi thanks for posting your work/findings - keep adding to the collective 617 knowledge.

That was a bizarre and entertaining video.  At first I couldn't understand why it went dark  :-DD

rastro
I will try my best to keep adding to the knowledge, stay tuned  :popcorn:

I couldn't understand it either, after watching it several times I recognized it was a cut and not "bsssst - Strasse dunkel" (ziiip - street dark)   :-DD
Sorry, only for german speakers:


PS:
My unit has rev E printed on both boards, anyone can undercut this for Electrometer Board?

MiDi: E
r6502: F
Alex: G
Smith: G
MadTux: L, G
Dave: J
math_indy: L
baltersice (Marco Reps): L
« Last Edit: August 13, 2021, 05:22:30 pm by MiDi »
 
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Offline MiDi

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One sleepless night with the statement of Kleinstein in mind an idea came up...
There was an unused ADA4530-1 sitting on the shelf for some time - would it be possible to replace the whole input amplifier with it?
In a nutshell the input amplifier formed by Q308 and U309 is just a difference amplifier.

As Q308 was already replaced and the performance was not what could be expected (I will come back to this later), there was almost nothing to lose.
First attempt looked promising in the amps ranges, but suffered from crazy offset and random jumps in volt ranges - smells like instability.
Nice thing is the direct access to preamplifier on rear panel, so no deal to quickly hook up the oscilloscope - it's ringing like a bell, journey just ends here?

The ADA4530-1 is a remarkable device considering nearly all of its properties, for example the GBW and open loop gain are higher than that of LT1012 (U309).
The 617 manual mentions that the 10nF C319 in the FB of U309 is needed for stability - maybe just a bit of isolation at the output needed to calm it down?
I throwed a 220k trimpot in and the issues disappeared, no instability in any ranges anymore.
To get the edge value for stability the trimpot was adjusted and the value measured was ~700 Ohms.
For a bit of margin a 2k2 resistor was chosen and replaced the trimpot.

This ended in the following configuration:





The 33k should protect the negative input up to 300V (spec: 10mA max at the inputs), for the positive input there is already more than enough protection given through 100k R333 and 10Meg R334 - even for surges (R355 not present in this unit).
As there was no 10nF in 2.5mm grid at hand, for now I put a 47nF MKS in - needs further investigation what is best value.
Seems with this replacement input protection Q311 is no longer needed as the OP AMP has input protection already - my early unit does not have it anyway.






To get an impression what this ADA4530-1 delivers, the bias current compensation and the input jack was disconnected (offset compensation already leaved with the mod in this unit).
As I would not trust old 4.5 digit ADC in this unit, my 3458A was hooked up at the preamp output.
After settling, the offset was ~300µV and bias current ~48fA (calculated) - no good sign.

I continued with performance verification of voltage ranges with help of internal voltage source.
Everything in spec according to 3458A, but the voltage source seems to have like a weared out 0V, sitting at ~200mV with horrible DNL between +-50mV.
It took me a while to realize that the output of the voltage source - even if disabled - is set to 0V and enabled all the time.
As the AD7541A DAC is in feedback, I would suggest this is the source, so put a better replacement LTC7541A on the next oversea order.

The last days the big reed relays were disconnected one after each other and bias current and offset were measured, so finally only 2pA range (K312) remained.
Disconnecting K307 improved much on bias current and now it is according to expectation ~3fA.
Every disconnected relay improved on the bias current, seems all have to be replaced - this turns into a costly repair.
Offset did not significantly change - as expected.
I guess the offset is mainly due to other sources and not from ADA4530-1 - will try to measure it in circuit later.
There is some work to do and diving into details will remain to later post...

In the meantime the search for appropriate cables was done.
For now the plan is to replace the input jack with a 3 lug triax as the options for 2 lug cables/connectors/lugs are very small and an adapter from 2 to 3 lug seems not to be economic.
Has anyone done a replacement with Keithley 7078-TRX-TBC?
For the cable I came across the Keysight N1415A, this seems to have a much better price point in comparison to the Keithley cables, any experience with this?
« Last Edit: June 22, 2019, 01:04:15 am by MiDi »
 
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Offline TiN

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That's a rabbit hole, but you doing great. Enjoying the ride alone, thanks for all efforts.  :)
YouTube | Metrology IRC Chat room | Let's share T&M documentation? Upload! No upload limits for firmwares, photos, files.
 
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Offline rastroTopic starter

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...
For now the plan is to replace the input jack with a 3 lug triax as the options for 2 lug cables/connectors/lugs are very small and an adapter from 2 to 3 lug seems not to be economic.
Has anyone done a replacement with Keithley 7078-TRX-TBC?
...

zucca did this to a K220 which is the same generation of Keithley equipment.
https://www.eevblog.com/forum/testgear/keithley-220-output-triax-connector-mod/msg393616/#msg393616

I would probably try to save any heat-shrink/insulation from the cable between the bulkhead and PCB.  I'm not sure if it is special low leakage or not.  I'd try to stick it back on maybe using heat shrink to keep it in place if you had to split it along the center.

And yes, you need to add a K220 to your shopping list!   8)

rastro
 
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Offline MiDi

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Some updates on the unit:

EDIT: Relay replacement investigation & overview moved

To give unit offset correction back, a modern approach with DAC was tested (MCP4725 - 12bit EEPROM).
For units with offset correction as stated in Datasheet (> Rev E?), this mod would be different (Rev E has different circuit for that).



Messed up the polarity, so finally the DAC is connected to GND(B) (R341) and -5V(B) (R335/R336) and set to DAC-Code 2207.
As there was no 3R3 at hand, 2R2 and 22k was chosen.





Before any questions arise:
This is another ADA4530-1 salvaged from a DIY HIZ Buffer (not needed anymore - when 617 is ready).
The DAC-Mod was done twice, first time with previous 4530 and goofed up sanity check on open unit w/o top guard cover.
Forgot to install the screw with guard connection (near empty dip-8 socket) and it showed a lot of 50Hz hum, finally I thought the op died and replaced it - with same result, but then recognized where the error came from - damn, this circuit is so sensitive :-DD

Final test today revealed no problems and with removed reed relays K307-K312, disconnected bias current adjustment & TRIAX-Input the results are impressive:



Offset (w/o zero correct): -30E-18A / 3µV (Noise: 6E-18App / 0.6µVpp -> +-3 hidden digits from GPIB)
Bias Current (15-18h): 425E-18A (STD-DEV/AC-RMS: 284E-18A)

Output pre-amp on scope (bias current measurement):




Obligatory  :box:



« Last Edit: August 21, 2019, 10:31:10 am by MiDi »
 
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Offline rastroTopic starter

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I wonder if it would be possible to refurbish the old relays to bring back their original high impedance? 

Is most of the leakage failure between the reed's or also significant leakage between the reed and coil/housing?

Would methanol cleaning and then putting them in a rough vacuum a few days and maybe back flushing with nitrogen?  The setup would not be trivial but probably reasonably put together.  This would be an attempt to remove moisture and maybe some oil contamination.

Could it just be chemical reaction on the reeds and not possible to flush out?

Just some thoughts I thought I'd share.
 

Offline MiDi

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When my cables arrive and input jack is exchanged, I will measure all the relays and we will see.
If leakage is from inside glas there is no chance to do anything.
On the old potted brown/black there is not much you could do besides soaking/cleaning and dry at elevated temperatures...
 

Offline MadTux

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I wonder if it would be possible to refurbish the old relays to bring back their original high impedance? 

I once tried to drill out the leaky reed switch on one of these black relays, but failed horrible as the drill drifted away and mangled the poor relay coil. This were the days before I had good machining capabilities in my shed.

I would guess that on a good milling machine with rigid carbide tooling, it should be quite possible to remove the reed switch without destroying the relay coils. And then replace the reed with something good like MARR-5.

Cheap chinese reed won't do it, no SF6 filling and thereby horrible leakage and breakdown characteristics.
 

Offline MiDi

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Yesterday the Triax accessories arrived (and nearly quadrupled value of unit):



I do not want to bother you with Triax exchange pron, but pics are attached  8).



The Keithley 7078-TRX-TBC is direct replacement without any problems, only the heatshrink from original shell did not survive soldering and had to be replaced - maybe it will get teflon heatshrink later.



The Keysight N1415A fits perfectly, but needs quite some force to push in / pull out.

Finally the relays were measured quickly with crude setup and not warmed up unit (all values in T\$\Omega\$):

relay # |betw. contacts |C-ES long |C-ES short |
K30730515
K308>10040300
K309702070
K310<31010
K311>100100>100
K312>500>100>100
« Last Edit: July 04, 2019, 09:01:56 pm by MiDi »
 

Offline chickenHeadKnob

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To MiDi:

yes please bother us with triax pron, unlike a beautiful woman a triax connector is something I may actually hold in my hands in the near future!

I noticed you used black heat-shrink. Oh-OH, some of that stuff is no good for high impedance, carbon black or some other additive makes more conductive than is desirable for this type of application. You should measure a clean piece, maybe you got lucky. 
 

Offline MiDi

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unlike a beautiful woman a triax connector is something I may actually hold in my hands in the near future!

This depends how much it is worth to you  8)

I noticed you used black heat-shrink. Oh-OH, some of that stuff is no good for high impedance, carbon black or some other additive makes more conductive than is desirable for this type of application. You should measure a clean piece, maybe you got lucky.

This is a point where I am pretty unsure if it matters there.
Have to confess that my experience in fA/P \$\Omega\$ region is non existent  :-//

My thoughts:
There are three possible leakage paths for heat-shrink on input connector:
-contact with input conductor: (teflon) insulated & creepage >1cm to both ends to bare conductor
-contact with earthed backplate: clearance of some mm
-contact with guarded enclosure: clearance of <1 mm or contact (e.g. due to wiggling / bending enclosure)

Last seems critical in normal mode, in guarded mode both should have same potential.
The other seem not to be that critical, if cleaned properly.

Edit:
All seem not to be that critical, if leakage paths are cleaned and held properly clean .
For last one I thought the inner enclosure is always at input voltage level, but it is always at guard Level - corrected that.

The construction seems a bit odd to me, why did they not use coaxial e.g. RG178 where shield takes the place of the sleeve?
« Last Edit: July 31, 2019, 11:04:15 am by MiDi »
 

Offline rastroTopic starter

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To MiDi:
...
I noticed you used black heat-shrink. Oh-OH, some of that stuff is no good for high impedance, carbon black or some other additive makes more conductive than is desirable for this type of application. You should measure a clean piece, maybe you got lucky.

Perhaps you could first wrap the cable with plumbers tape which is Teflon/PTFE (probably clean it first to remove possible contaminates).  This would act as an insulation barrier to a heat shrink sleeve.

rasto
 

Offline chickenHeadKnob

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It would be simpler just to use clear heat-shrink which is cheap and available and should be  polyolefin without additives. I am always suspicious of any plastic from china as they could throw in all kinds of unhelpful additives. Some pvc is just loaded with lead, apparently it helps with the flow in molding.
 
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Offline 3roomlab

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relay # |betw. contacts |C-ES long |C-ES short |


Hi MiDi, what does CES long and short mean?
 

Offline MiDi

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C-ES: contact to electrostatic shield
long/short: distance of the pins, there is only one ES pin between coil pin and contact pin.

coto 1240-06-2104 datasheet
 
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Offline rastroTopic starter

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It would be simpler just to use clear heat-shrink which is cheap and available and should be  polyolefin without additives. I am always suspicious of any plastic from china as they could throw in all kinds of unhelpful additives. Some pvc is just loaded with lead, apparently it helps with the flow in molding.
Sounds good...
 

Offline JxR

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Fortunately, the performance of the voltage source can be upgraded by changing the DAC chip - the old and venerable AD7541AJN to an improved LT version - LTC7541AKN, plus changing LM308A in the x10 voltage amplifier stage to OP97.

I purchased a 617 a couple weeks ago, and so far this is the only modification I have done to it.  It was definitely an improvement on the voltage output accuracy.  It varies a bit, but it is always 8mV or less from whatever is set now.

I also discovered that apparently I purchased a 617-HIQ, although it wasn't advertised as such. It has a big ass 1uF film capacitor in the analog section that I haven't seen in anyone else's pics. It is an L-revision board, and has a B5 revision firmware (I copied and uploaded the firmware to xdevs, although probably not useful to anyone).

All of my accessories are just about here (including the 100G 1% resistor), so I hope to go through the calibration procedures with it this weekend (although it isn't too far off from my initial tests). I guess I will be skipping the Columbs cal, since I don't have the procedure for the HIQ version.  Overall, it is pretty clean on the inside and seems fairly stable in the 2pA range already.
« Last Edit: July 15, 2019, 07:08:47 pm by JxR »
 
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Offline rastroTopic starter

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I haven't seen any documentation on the 617-HIQ although I have seen a few references to it on some documents on the internet. 
 

Offline JxR

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I haven't seen any documentation on the 617-HIQ although I have seen a few references to it on some documents on the internet.
Same. I have a copy of the Rev G manual everyone else does I expect, which stops at revision K on the analog board. I'm keeping an eye on eBay just in-case I see a manual pop up that is later than revision G. I don't expect to find anything, but I will certainly scan and upload it if I do.

I even bought a 1000pF +/- 0.25pF capacitor to do the coulumb cal (although I honestly would probably never use that feature).  Just figured I would do it with the rest of the cal.  It is just a ceramic though, so maybe not the right cap for the job.
« Last Edit: July 12, 2019, 04:29:30 pm by JxR »
 

Offline rastroTopic starter

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There appears to be 3 versions of the 617-HIQ.

From an old K617 data sheet:
https://www.axitest.com/images/store/files/188931_K617.pdf

"For measurements of charge as high as 20µC, the 617-HIQ version offers three higher charge ranges. These higher ranges have proven useful on Faraday cup measurements and other static charge applications including photocopy research and development."

I wonder if the Capacitor affects the other measurement functions or just coulombs?
Is the difference among the three versions just the capacitor selection?

Another document:
https://www.nist.gov/sites/default/files/documents/2017/06/19/procedure03v430.pdf

"The charge measurements are acquired through the use of Keithley 617, 617-HiQ and 6512 electrometers. The internal capacitors of the electrometers are calibrated upon introduction into the system and any time the charge collection is suspect. Any necessary correction is applied as part of the air-kerma calculation. Five electrometers, three 6512 and two 617-HiQ, are maintained for use in the low- and high-energy ranges.  Four electrometers are maintained in the mammography range, two standard-capacitance-range electrometers (20 nC), a 617 HIQ (20 μC), and a midrange 617 (200 nC).  The HiQ is dedicated for use if a monitor chamber is required, and the midrange 617 is used for collecting charge on the Attix chamber.  The two 20 nC capacitance range electrometers are used for the customer chambers.  The procedure of calibrating a NIST reference-class chamber with each customer chamber is a quality-assurance check that rules out electrometer malfunctions."
 
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Offline rastroTopic starter

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It looks like the prior document also contains some calibration procedure; haven't read it closely.

"Test of high-quality electrometersNIST provides a test service (46030S) for high-quality feedback electrometers that are used in conjunction with current-type ionization chambers also being calibrated at NIST. The procedure involves electrically testing the electrometer using a feedback capacitor and computing a calibration factor, KQ.  A typical report form is found in Appendix 2.  As a check on this electrical test, the customer's current-type chamber is calibrated for one beam quality with both the NIST system and with the customer's system.  Agreement is usually within 0.2 %; if not, the calibration is reviewed and possibly repeated.Procedure for test of high-quality feedback electrometers1.  This procedure describes the calibration of a Keithley 617 electrometer, but may be applied to other electrometers if proper adjustments are made to setup and operational parameters."
 
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Offline JxR

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I wonder if the Capacitor affects the other measurement functions or just coulombs?
Is the difference among the three versions just the capacitor selection?

Thanks for the research into this and documents. I will look over them soon. Currently working on finishing up my test fixture, so I can complete the calibration procedures.  Also waiting for my third triax cable to come in, which should be tomorrow.  I will be calibrating it with the aid of a 2450.

For the first question, according to the datasheet it is only the charge measurement range that is different.  I've tested it down to about 100pA so far using the SMU.  It was off by about 10pA, but that was just using the alligator clips and some banana plug cables in open air (so not ideal).  The couple of resistance tests I did looked fine, and reading voltage was fine.  The normal ranges were all available that are listed in the datasheet.

I've never seen a schematic for the 6512, but it seems that until the 6514 was released, the 617-HIQ had the highest coulumbs measurement range.  Hard to say if there is any other difference except the firmware and the film capacitor atm. Other pictures I have seen of the Revision-L board looked about the same, but we don't even have a full schematic/part list for the L boards that I know of. 

I would guess that it is probably possible to turn a 617 into the HIQ version, my firmware should be available on the ftp site on xdevs in \root\firmware\617-HIQ\...
« Last Edit: July 12, 2019, 10:30:51 pm by JxR »
 

Offline JxR

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The HiQ is dedicated for use if a monitor chamber is required, and the midrange 617 is used for collecting charge on the Attix chamber....

I went to through the second document and it was interesting.  They only really highlight doing calibration on the 617 using a 1nF capacitor.  But they do list the values of their NIST calibrated capacitors, their highest being 100nF.  I'm assuming this is the capacitor they use to verify the 617-HIQ.

Based on other calibration procedures for the 617, Keithley usually don't use the lowest range for the calibration procedure, but one of the mid-ranges.  For the 617, you would calibrate on the 2nC range so the use of a 1nF capacitor makes sense.  For the 617-HIQ the mid-range is 2uC.  I would venture a guess that you would normally calibrate using a 1uF capacitor.  Although, in that document they don't mention having one, so unsure which value is correct.

Digikey has, 100nF 1% Mica capacitor for ~$20 (honestly more than I care to spend)
Mouser has some 1uF 1% Film capacitors around ~$3
Both have 1% tolerance Film/Ceramic 100nF that aren't too expensive.

I'm pretty much finished with my shielded test fixture, although I still need to work on the inner box for using guarded measurements.  Also need to make some more leads to connect everything.

So far the 2450 is telling me my 100G resistor is: 100.0937G
So with the calibration current of 190pA, I'm looking at a test voltage of: ~19.0178V

Hopefully I will get to go through all the steps on Sunday.  Sounding like the wife has some plans for me tomorrow...and still need to do some additional work on my test fixture.

The 617 didn't seem to appreciate me waving around a heat gun for hours melting PTFE heat shrink.  It still hasn't calmed back down to the normal ~1-5fA yet.  Hopefully it will stabilize in a few more hours.
 

Online Kleinstein

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I don't think a mica cap is a good idea. Mica caps have quite some dielectric absorption (slow type). They are nice at high frequency and rather stable, but not that good for slow things. So the capacitors of choice would be NP0 (ceramic), PS or PP.

With the 2450 available would could consider charge calibration from a programmed current pulse. Something like 100 nA for some 20 seconds should be more accurate than a 1% capacitor value.
 
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Offline JxR

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I don't think a mica cap is a good idea. Mica caps have quite some dielectric absorption (slow type). They are nice at high frequency and rather stable, but not that good for slow things. So the capacitors of choice would be NP0 (ceramic), PS or PP.

With the 2450 available would could consider charge calibration from a programmed current pulse. Something like 100 nA for some 20 seconds should be more accurate than a 1% capacitor value.

This is great information.  Thank you for taking the time to help me out.
 

Offline JxR

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Unfortunately, calibration may have to be put off for the weekend.  My last cable that was suppose to arrive today, but is now listed as "arriving late".

I did try the calibration voltage through the 100G resistor using a BNC w/banana adapter from the SMU to my test fixture, and jumpered it to the triax input of the 617.  The 617 read 190.10pA.  Technically, this is already in spec of the +/-1.6%(rdg) + 1 count for the 200pA range.

For now, will continue to work on my test fixture and hopefully the cable will arrive by Monday.
 

Offline JxR

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Finished up most of the test fixture to help with the calibration.  The stability of the readings for the 100G resistor have increased by nearly 400% when in the guarded box.  Hopefully I can revisit the 617 calibration early next week once the cable gets in.

 

Offline MadTux

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My Keithley Firmware dumps, as requested by MiDi, including the probably latest B5 version
(Is a 7z, but can't upload that here, so rename zip to 7z to unpack)
 
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Offline MiDi

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The comparison of JxRs HIQ and MadTux B5 FW revealed no differences, so the 617-HIQ uses same stock FW.
The only place left - if there is different scaling for Q-Ranges? - seems to be in NVRAM (256Bit EEPROM) U104.
I updated my unit now to B5 revision, old A3 revision and NVRAM attached and released into public domain.
« Last Edit: July 20, 2019, 04:02:29 pm by MiDi »
 

Offline JxR

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MadTux B5 FW[/url] revealed no differences, so the 617-HIQ uses same stock FW.

That's interesting that the FW is the same.  I figured there would have been a difference for the Q setting.

I finally finished up my calibration last night (well everything except the Q cal):

V/I Resistance measurement using internal voltage source:
Measured as 100.0356G (avg of 1000+ readings from 2450)
790293-0

20/200pA ranges:
790299-1
790305-2

200V range:
790311-3

It seems to be meeting the accuracy specs in all ranges now, and doesn't seem to differ by more than 5 counts off of what the 2450 says.  I did try and source 1pA directly from 2450 to 617, but gave up on getting a stable reading.  Although it was bouncing around values that matched the 1.5%rdg + 66 counts for that range.

Overall, I'm happy with the unit.

and...
The final form of my resitance box I used for calibration. Misread the cal values from the manual and bought a 10Gohm resistor by mistake unfortunately...
790323-4

For laughs, here is my monstrosity for calibrating: 10M, 1.9M, 190k, 19k (2x Caddock 1776-C621 plus a 100k/10k resistor).  One day I would like to get a nice 7 dial decade box with dials from 1 to 1Mohm, and ~10M total.
790329-5
« Last Edit: July 20, 2019, 05:33:57 pm by JxR »
 

Offline MiDi

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That's interesting that the FW is the same.  I figured there would have been a difference for the Q setting.
It is just a scaling factor of 1k, so instead of displaying nC/uC it could simply display uC/mC instead (or corresponding scientific notation).
 

Offline JxR

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With the 2450 available would could consider charge calibration from a programmed current pulse. Something like 100 nA for some 20 seconds should be more accurate than a 1% capacitor value.

I gave this a try, but it just resulted in the 617 overflowing the calibration range.

I also tried using a 100nF film cap I measured at 100.4nF (closest I had to the supposed cal capacitor).  I placed it in the 2uC range and supplied the 19V cal voltage in my shielded box.  I calculated that Q should have been: 1.9076uC and it read about 1.9248uC.  When I tried to correct to the calculated value, it displayed "Stor" like it took the value, but original value it measured didn't changed.  I also tried just forcing the cal value to 1.9uC, and it also wrote the value, but the displayed value still was ~1.9248uC.

So, I dunno.  A GR 1409T: 0.1uC 0.05% standard capacitors seem to go for about $125 on eBay.  Maybe I will pick one up one day, but it is far from a priority.  I would certainly put the money towards a nice resistor decade box before I bought a standard capacitor.

I went ahead a re-did the voltage calibration since I think my original setup of doing it with jumpers in the test fixture was flawed.  After just using the triax-to-alligator clips on some banana-to-binding-post adapter plugged directly into the 2450, it is spot on for every single range.

So, I think I am done with calibration for now.
 

Offline MiDi

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DIY Triaxial cable
« Reply #97 on: July 27, 2019, 11:26:28 am »
My DIY low (tribuelectric) noise Triaxial cable made of PL75-23 and Lemo 001 101.
The Lemo cable has nearly same dimensions as Keithley SC-22, but has all teflon insulation and is much cheaper if I got it right (~20$/m vs. ~50$/m).

795936-0

795942-1

795948-2
 

Offline JxR

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My DIY low (tribuelectric) noise Triaxial cable made...

How is your new cable working out?  Originally I planned to try and make my own cables too, but being in the US I was able to buy used 7087-TRX-10 cables for about the same price or cheaper than it would have costed me to make them.

I'm pretty much good on triax cables now, but if I ever have need for more I plan to cut some of my existing cables and use the excess I don't need to make one new cable and one shorter.

I'm seriously considering buying 7087-TRX-TBC to swap out the 2-lug bulkhead termination.  While I have a 6172 (2-lug to 3-lug) adapter, I find it a bit annoying and expect I could sale it easily for the price of the 3-lug bulkhead triax adapter.
 

Offline MiDi

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Thought it would be more complicated to build, the PL75 are very good to handle.
Tried to measure the resistance, but my crude setup was not able to give reasonable results above 100T \$\Omega\$.
Need to build proper Testbox and 2nd cable to get confidence.
This is ~100$ for 1.5m, lucky who lives in the us - low shipping costs and no import taxes...
 

Offline JxR

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....lucky who lives in the us - low shipping costs and no import taxes...

Yeah, one of the only perks living here as far as I'm concerned.  When I finish up my schooling, me and the family are looking to move back overseas.  We both miss living in Japan, but I might try and find work in Germany since we both like it there.  My wife is a dual French/US citizen and misses Europe. Both her parents live in Germany so I've been there a few times, and we have her family to make the transition easier.
 
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Offline balage

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Re: DIY Triaxial cable
« Reply #101 on: July 30, 2019, 12:43:17 pm »
My DIY low (tribuelectric) noise Triaxial cable made of PL75-23 and Lemo 001 101.
The Lemo cable has nearly same dimensions as Keithley SC-22, but has all teflon insulation and is much cheaper if I got it right (~20$/m vs. ~50$/m).

Hi MiDi,

Please share your experience on it, especially the resistance. I am interested if this cable as good as the Keithley SC-22.

Where have you bought that Lemo cable? Koax24.de?
« Last Edit: July 30, 2019, 12:47:16 pm by balage »
 
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Offline MiDi

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Re: DIY Triaxial cable
« Reply #102 on: July 30, 2019, 05:49:38 pm »
Please share your experience on it, especially the resistance. I am interested if this cable as good as the Keithley SC-22.

Where have you bought that Lemo cable? Koax24.de?

Yes, from koax24, min order qty. is 10m.

The Lemo should be better regarding resistance as it is PTFE against PE for SC-22.
But cable alone is useless, connectors and especially skills of builder add to it  ;)
 

Offline balage

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Re: DIY Triaxial cable
« Reply #103 on: July 31, 2019, 10:59:13 am »
But cable alone is useless, connectors and especially skills of builder add to it  ;)

I have made several of them, and the least ones tended to be better, as I had more practice. :) But it is sure that rubber gloves must be worn.

Have you found the resistance specs of 001 101?

I was wondering if you would like to swap 1 meter of that Lemo cable to a few meters of G_02330_HT. For the first row I would love to try that, before ordering for more than 120EUR.
 

Offline MiDi

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No resistance specs for 001 101 found either.

Regarding swap: PM sent.
 

Offline balage

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I have asked Lemo about the resistence, and they have forwarded me to the local distributor. Will see.

However this spec "operating voltage = 0,75kV" does not say much, either. Especially because while resistance can be measured, but the voltage specification cannot be measured.
 

Offline MiDi

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As earlier stated there was an offset of ~280µV at preamp out with zero check on - in either amps or volt mode.
When adjusting the offset DAC to give ~0µV in amps, it showed a doubled offset in volts mode.

Deaper investigation revealed yet another layout flaw (Rev E):



The trace of preamp out near C311 was drilled open and a bodge from R304 to ribbon cable connector was made, not shure what could have been the issue to do that.
The preamp out is the common for the +-24V rails on T301 as well.
The current source (for Ohms range) is fed from this 24V and so are the op amp U304 (LM741) and the voltage reference VR301 (1N4577).
This gives ~4-5mA ~14mA return current through the bodge and the traces from ribbon cable to K301 and this weird looking star point nearby.

Could this be the source?
The measurement between R305/R308 (real preamp out) and ribbon cable (goes down to the ADC) gave ~280µV - what a bummer!
Why did Keithley "improved" it that way? This problem should have been recognized at least when doing the cal - unbelievable!

So how to fix it? Best point seemed to be the star point where trace from transformer common joins.
A look at the later revisions: they did exactly that  :-DD
This was a quick'n'easy fix, the bodge wire could be reused and the offset is now in single digit region (+-10µV) in either amps or volt mode.
« Last Edit: November 15, 2019, 07:40:17 pm by MiDi »
 

Offline HighVoltage

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What a great thread of information ..

I am in the market of buying a 617 and would like to get a latest model.
- But how long did Keithley build these instruments and
- How can one detect from the outside, what is a latest model?
There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline JxR

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What a great thread of information ..

I am in the market of buying a 617 and would like to get a latest model.
- But how long did Keithley build these instruments and
- How can one detect from the outside, what is a latest model?

My 617-HIQ appears to have chips in it from 1996.  It wasn't until the 6517B I believe, until Keithley made another electrometer to have the same upper range for Coulomb measurements.  If going by the datasheet is any indication, the 6517A was already available when the 617-HIQ was still on the market.

The latest Rev-L boards have a date of 1984, and the latest manual I have seen (Rev-G), has a date of 1988.  Although I honestly have no idea when the instruments were actually discontinued.

There is the 6512 which has the same specs as the 617, but comes standard with a 3-Lug Triax.  Then the 6517A, and the 6517B that I believe is the latest model.

The serial number could probably be used as a rough estimate on getting a Rev-L model I would guess.

My serial number is: 0633900
 

Offline HighVoltage

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Thank you, very good info.

Your serial number (0633900) is the highest I have seen.

It seems very difficult to judge the functionality and quality of a 617 before buying.
And since most sellers are not capable to testing or do not even have a cable, its not easy.

There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline JxR

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Thank you, very good info.

Your serial number (0633900) is the highest I have seen.

It seems very difficult to judge the functionality and quality of a 617 before buying.
And since most sellers are not capable to testing or do not even have a cable, its not easy.

There are some Canadian sellers on ebay that ship worldwide and have a couple 6512s that are NIST Calibrated and come with a 90-day warranty.  Only downside is an increase in price, and I haven't seen a schematic for a 6512, although they provide a part list in the manual.  Color is a little weird for old Keithley gear...

They have a website as well: https://www.stratatek.com/

I've personally never bought from them, but they seem legit from first glance. 
 

Offline HighVoltage

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Interesting with the 6512, I was not really aware of that model.
I have a 614 and a 6517B, both in like new condition.

The 617 I want use for a client project were up to 100V for 10 GOhm measurements is just perfect and not too expensive.

Yesterday I bought a 617 in "unknown" condition.
Allegedly the seller does not have cables and/or capability to test.
It should arrive shortly.
There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline JxR

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Interesting with the 6512, I was not really aware of that model.
I have a 614 and a 6517B, both in like new condition.

The 617 I want use for a client project were up to 100V for 10 GOhm measurements is just perfect and not too expensive.

Yesterday I bought a 617 in "unknown" condition.
Allegedly the seller does not have cables and/or capability to test.
It should arrive shortly.

Gotcha.  I have to admit, I was a tad curious why you were first interested in a 617 since I knew you already had some really nice equipment.

The 617 can easily handle your intended needs once you perform the calibration procedures (which are fairly straight forward).  I think it can measure up to 10POhm in "V/I" mode.  I've gone as high as about 3.5POhm doing insulation testing while making the leads for my test fixture.
 

Offline MiDi

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After finishing my quick'n'dirty shielded triax box, it was time to do some measurements.
It is made of RF-case from shelf, BJ72-Triax (isolated outer shield), some SKS/Hirschmann AGF 1 clips, angled pinheader, safety banana jack, enameled copper wire (1.2/0.8mm) and some used solder wick.
Not a beauty and unsafe until properly enclosed, but seems to do the job for now:



As Reed relay K310 was worst of all, it became victim to do some tests if cleaning improves or even heals bad isolation resistance.
After soaking, cleaning and drying several hours at 50-60°C it was measured over several days - without significant changes in values - at ~10T\$\Omega\$ C-C and each C-ES at ~40T\$\Omega\$ - so no real improvement from cleaning  :horse:.

As this relay is dead, it was lead into its final designation - take it apart.
To remove case a heat-gun was used to make the resin gummy.
That worked good and it came apart in chunks quite quick:



Surprisingly it has a bit of magnetic shielding:



Trying to carefully separate glass-tube from coil, but finally the glass broke.
The contact from broken side does not seem to have any visible wearout:




There was no chance to slide it out as it is all glued together - so not the slightest chance to separate it without breaking something:







My conclusion is: if C-C resistance is lower than each C-ES, the problem has to be inside glass-tube of relay, because ES-Shield covers whole glass from outside with very thin glue in between.
Only other remaining leakage path could be direct on surface of outer glass, but that seems unlikely.
As the glue is really thin and seems not to provide high isolation resistance, this would affect the C-ES resistance as well.

To get a feeling what we are talking about, a MEDER HI05 Model 66 was ordered (spec'd with min isolation resistance of 1T\$\Omega\$) and measured at ~10P\$\Omega\$@~60%rH - impressive.
This was also a good verification, that the Box is suitable for this job.



Now waiting for the ordered RL-181 to arrive (spec'd min 100T\$\Omega\$) and we will see what they deliver ;)
They are still awailable from Tektronix/Keithley as spare parts (web-form) and seemed to be the best option.
« Last Edit: August 21, 2019, 10:31:56 am by MiDi »
 
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Online Kleinstein

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The magnetic shield is not only a shield, but also provides a shorter return path for the driving field.  The shield thus also reduces the required current. This can make quite some difference when it comes to thermal EMF.
 
 
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Offline HighVoltage

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Even the K200 reed relays in the Agilent DMMs have this kind of heavy metal shielding and then they are potted with some polyurethane.
Luckily these relays can be bought from Keysight.
May be all high end DMM have shielded relays?

But the Keithley 617 relays seem to be unobtainium.



 
There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline MiDi

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[Moved & Updated]
Investigation revealed that there are a couple of possible drop-in replacements for high insulation reed relays K307-K312 .
Even if there would be cheap salvaged parts awailable, better to be suspicious if they are untested and sold without any warranty.

-Original brown/black E-T R7526-1 (Keithley-Part# RL-70 - same as Coto 1240-0197, e.g. Keithley 617 < Rev L, 614): no source, no datasheet
Rcoil=600\$\Omega\$

-Coto 1240-0197 (Keithley-Part# RL-70, e.g. Keithley 617 >= Rev L, 199): Ebay (NOS) - 60$ (ask for stock & delivery outside us): Very high insulation resistance, 1FA, 350V, 0.35A/2.0A,10W Instrumentation Reed relay with an electrostatic shield and a 5V, Rcoil=625\$\Omega\$

No direct drop in:

-Coto 1203-0147 (Keithley-Part# RL-181 - same as ARI 300RCA - no datasheet found, Keithley 6514 & 6517 EM,  6521 & 6522 scanner cards): awailable from Keithley/Tektronix (spare-parts: web-form) - ~50$
spacing ES-Coil tighter

-ARI (American Relays?) SO051A 300RCA (Keithley-Part# RL-181 - no datasheet found, 6514 EM(at least from week 37 2008), Keithley 3761 scanner card): awailable from Keithley/Tektronix (spare-parts: web-form) - ~50$
Rcoil=300\$\Omega\$, spacing ES-Coil tighter

-Coto 1240-06-2104 (active part - confirmed by Coto, 1E14\$\Omega\$, 6V/5V compatible, RoHS): several sources, but ~100$
Rcoil=150\$\Omega\$


Other high insulation resistance reed relays:

-Okita TRY-105SV-15 (1E15/1E14\$\Omega\$, E/S-shield, 350V, 1/2.5A, 50W, 0.1pF, 50µV t-EMF) - source?, price?
Similar relays are used in Advantest R8340A Picoammeter (TRY-104S)

-Sanyu 95D-1A14N4 (1E14/1E13\$\Omega\$, E/S+M-shield, 350V, 0,5/2,8A, 50W) distributor, ~25$

-Standex-Meder HI-Series (mentioned by branadic, cheap: ~15$): no ES-shield (to be handcrafted - razberik), only 1E12\$\Omega\$ (Model 66) / 1E13\$\Omega\$ (Model 85) [Coto 1240-06-2104: 1E14\$\Omega\$], t-EMF? - preferred HI 05-1A85, but seems not to be awailable


« Last Edit: January 05, 2020, 04:39:07 pm by MiDi »
 
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Offline balage

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It is made of RF-case from shelf, BJ72-Triax (isolated outer shield), some SKS/Hirschmann AGF 1 clips, angled pinheader, safety banana jack, enameled copper wire (1.2/0.8mm) and some used solder wick.

Hi MiDi,

Why are you using isolated triax jack? I can see that a ground crocodile clip is tied to the case. But this clip is the same point as the outer shield of the triax, right?
 

Offline MiDi

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The case acts either as shield (~earth-potential) or guard (~input-potential).
With normal Triax-jack the outer shield of Triax-cable would always connect to case and so to earth-potential.
This would not work in guarded mode and not shure if it could cause problem in normal mode due to earth-loop formed by outer shield and inner shield.
So I decided to try it with an isolated triax which seems to be much simpler to get an universal box - have to figure out how it performs in guarded mode - so call it experimental ;)
 

Offline JxR

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So I decided to try it with an isolated triax which seems to be much simpler to get an universal box - have to figure out how it performs in guarded mode - so call it experimental ;)

Wow, I totally missed this.  Great find on the BJ72-Triax MiDi!  I made a single isolated Triax input on my test fixture using a PTFE washer covering the threads in PTFE heatshrink but this is much better!  I will have to get one of these in the future.
 

Offline balage

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Ah, that makes sense. And the isolated ones cost only a bit more than the BJ770.
 

Online Vgkid

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #121 on: September 05, 2019, 08:38:38 pm »
Looks like I won the broken 617 on ebay.  Gosh the connectors/cables alone cost more than my most expensive meter.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline JxR

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #122 on: September 05, 2019, 08:58:04 pm »
Looks like I won the broken 617 on ebay.  Gosh the connectors/cables alone cost more than my most expensive meter.

Making your own is always an option, but that isn't exactly cheap either. 

I somehow got extremely lucky and eBay was saturated with used Keithley triax cables and accessories around the time I was looking to purchase.  All of my cables are used and cost 1/5 to 1/4 of what they cost new. Buying used was even cheaper than I would have spent making my own at that time.  I just glanced at eBay and don't really see many good deals now unfortunately.

It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.
 

Online Vgkid

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #123 on: September 05, 2019, 09:11:34 pm »
Making your own is always an option, but that isn't exactly cheap either. 
/////////////////////
It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.
Thats the plan.
Huh, i always assumed that it was a 3 lug connector. Thanks for letting me know.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #124 on: September 05, 2019, 09:21:02 pm »
Congrats, welcome to the club  ;)
I really hope you have more luck with it than I have.

Last week there was a scary moment whilst preparing a measurement.
Wondered why the unit showed huge offset after couple of minutes after turning it on.
Then I smelled a bit of magic smoke.
Quick look under the hood and could not believe my eyes: glowing pcb near fixed fireworks-section  :wtf:
 

Offline vindoline

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #125 on: September 05, 2019, 09:28:48 pm »
Making your own is always an option, but that isn't exactly cheap either. 
/////////////////////
It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.
Thats the plan.
Huh, i always assumed that it was a 3 lug connector. Thanks for letting me know.

The cheapest is to take a sharp file and with a few strokes convert yours to a 1-lug connector. Now both 2 and 3-lug connectors fit  :box:
 

Offline rastroTopic starter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #126 on: September 05, 2019, 11:57:04 pm »
Making your own is always an option, but that isn't exactly cheap either. 
/////////////////////
It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.
Thats the plan.
Huh, i always assumed that it was a 3 lug connector. Thanks for letting me know.

The cheapest is to take a sharp file and with a few strokes convert yours to a 1-lug connector. Now both 2 and 3-lug connectors fit  :box:

I've done that universal modification also... :-+
 

Online Vgkid

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #127 on: September 14, 2019, 03:54:43 pm »
The brown box came(it was packed really well). All is not well, overrange on every range. The preamp out reads 23-24v, and using the shorting function does not change this. The a analog out reads 5.xx volts.
At least the V-source works correctly.
Power rails seem good, the digi 5v is good, and the analog rails seem good.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline MiDi

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RL-181 & swiss cheese
« Reply #128 on: November 15, 2019, 09:41:54 pm »
Couple of weeks ago the RL-181 for replacement of RL-70 were received.
After opening the package I scratched my head - obviously these were not the expected shining red Coto 1203-0147:  :o

872664-0

Received relays are marked with ARI (American relays?) 300RCA, did Keithley send me wrong parts?
After a quick search the picture of Keithley 3761 scanner card popped up and showed the same type - so they seem to be the newer versions of RL-181 (overview updated).

Next disappointment is that they are not direct drop in replacements - the spacing between Shield and Coil is narrower and Rcoil is 300\$\Omega\$ vs. 600\$\Omega\$ for RL-70.

872636-1

Investigation revealed that the Hex-Inverters U302/3 used for driving these relays are already operated near their current-limits - no resources left for the new relays.
My idea is to simply get 4 new 74HC04, throw out the current ones and stack two new ones on each other - any better suggestions?
The supply for 5V rail gets already damn hot - even without the old relays.
Seems to be a good idea to give U306 & Q309 some kind of heatsink - in particular with the more current-hungry new relays.


In the meantime the K617 demanded the next serious repair - never ending story  :scared:
Dremel-time: now EM-Board looks much more like a Keithley or - as we say - a swiss cheese.

872660-2

Yeah, this might be a bit of an overkill, but better to be on the safe side to have relaxed sleep when this unit is running unattended.
Hopefully this was the last surprise - steady I have less desire to work inside this K617 and more to work with it - but it was a good lesson whilst diving deeper into the world of attoamps & petaohms and how the circuits work to achieve that.

After repair the new relays were measured over several days, but this will remain for a follow up.
It is planned to measure the t-emf for self-heating - let's see if this will work out.
If there are more things that should be measured on these relays before they are soldered in, let me know.
« Last Edit: November 15, 2019, 10:27:48 pm by MiDi »
 
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Offline mark03

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #129 on: December 12, 2019, 12:36:16 am »
It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.
Thats the plan.
Huh, i always assumed that it was a 3 lug connector. Thanks for letting me know.

The cheapest is to take a sharp file and with a few strokes convert yours to a 1-lug connector. Now both 2 and 3-lug connectors fit  :box:

Hmmm... so I just picked up one of these "Keithly CA-240-3B" cables:
https://www.ebay.com/itm/Keithly-CA-240-3B-3-Slot-TRIAX-Cable-10-Foot/264492629709
I couldn't find any reference to it on the interwebs, but I'm assuming the "Keithly" name and the fact that it's triaxial implies it must be OK for electrometer use, even if it doesn't have the semi-conductive layer, etc.?

I'm planning to find a chassis-mount triax connector for a shield box.  If I can get two for cheap[-ish], I assume changing out the connector on the K617 is easy and straightforward?  If not, I'll file off a lug.
 

Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #130 on: December 12, 2019, 08:28:13 am »
You can also find 2-Lug to 3-Lug converter.
And once in a while there are cables on ebay that have one end 2-Lug and the other end 3-Lug connectors.

I have kept 2-Lug connectors on my older Keithley electrometer gear to keep them original.
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Offline mark03

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #131 on: December 12, 2019, 09:36:50 pm »
There are quite a few Trompeter BJ72 bulkhead connectors on ebay at the moment.  According to the catalog these are "insulated bulkhead jacks."  Anyone know if they can be magically converted from insulated into non-insulated by leaving out a washer or something?  (The diameter is also a bit larger than the regular bulkhead jacks, but for a shield box I wouldn't care.)

EDIT:  Hmm, looking more closely, I can't figure this out.  The main body of the connector (the outer shield) appears to be insulated from the mounting surfaces, but there are only two solder connections on the back---presumably the center conductor and the inner shield.  Does this mean there is no way to connect with the outer shield?
« Last Edit: December 12, 2019, 09:49:49 pm by mark03 »
 

Offline ch_scr

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #132 on: December 12, 2019, 09:56:06 pm »
Look closely, there are two gold coated solder connections in the center - a solder pot and a lug, as well as the gray looking outer shield solder lug to the side.
 

Offline JxR

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #133 on: December 12, 2019, 10:08:54 pm »
If your determined to switch out the triax connector, I personally would save yourself some hassle and just get the 7078-trx-tbc.  I think the Keithley version already matches the hole pattern in the back panel.  If you search back in the thread I know someone did a swap. 

There is also a thread on here you can search for about changing the bulk connector on a Keithley 220 current source.  I believe that person used a different brand and points out troubles they had and modifications they made.

I guess it all just depends on what you value your time at vs the cost of the connector.  It is around $50 for the Keithley one.  Others were certainly tell you to just file off one of the lugs and be done with it.  I have a 2-to-3 lug interconnect myself, but I got it used for an acceptable price.  If I had to do it over again, I probably would buy the 7078-trx-tbc.

You should consider picking up some ptfe heat shrink to finish it off.
« Last Edit: December 12, 2019, 10:16:19 pm by JxR »
 

Offline mark03

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #134 on: December 12, 2019, 11:12:10 pm »
Look closely, there are two gold coated solder connections in the center - a solder pot and a lug, as well as the gray looking outer shield solder lug to the side.

Ah!  I missed that.  So these might not be a bad choice for a shield box if I can get some cheap... we'll see.

Yes, for potentially replacing the two-lug connector on the back of a K617, it makes sense to buy the official replacement.
 

Offline JxR

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #135 on: December 12, 2019, 11:46:49 pm »
Look closely, there are two gold coated solder connections in the center - a solder pot and a lug, as well as the gray looking outer shield solder lug to the side.

Ah!  I missed that.  So these might not be a bad choice for a shield box if I can get some cheap... we'll see.

Yes, for potentially replacing the two-lug connector on the back of a K617, it makes sense to buy the official replacement.

It is probably a good idea to have at least one of those insulated triax connectors on a box.  If you are using more than one instrument connected to the box, they can keep you from creating a ground loop between the instruments. 

It all just really depends on what you are trying to do.  I wouldn't personally use only that type of connector though.  Often you do want the box to be grounded.
 

Offline mark03

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #136 on: December 12, 2019, 11:50:14 pm »
It all just really depends on what you are trying to do.  I wouldn't personally use only that type of connector though.  Often you do want the box to be grounded.
I would use a tin box so it is possible to "un-insulate" the insulated bulkhead connectors by soldering the lug to the box on the inside.
 

Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #137 on: January 05, 2020, 12:37:22 pm »
Last summer I bought a Keithley 617 and it arrived not working.
The instrument turned ON alright but showed no correct value and the display fluctuated a lot.

I dreaded the repair, expecting it to be complicated and difficult to find the problem.
So, I finally opened up the instrument and found, that when I bypassed the triax input connector and applied a signal directly to the input circuitry, I got perfect readings.

After inspecting the triax connector, I found that:
1. the ground was broken off
2. the inner conductor was broken in to two pieces
Hmmm, never seen anything like this.

So, where to get a new 2-lug triax connector?
Almost to good to be true, I found a brand new one on ebay Germany for Euro 10 and installed it today.
The instrument is now working perfectly.

One of the easiest repairs ever.

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Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #139 on: January 05, 2020, 05:25:20 pm »
Welcome to the Club HighVoltage ;)
You are lucky with your unit!


Difference between REV J and L is at least added fuse on COM, there is a screw on backplate between COM post and BNC connectors for fuse-holder - so watch out for this if you want at least REV L  8)
Edit: See attached pics.


Interesting: between my REV E and Dave's REV J (oldest known) there is maximum only 1 year in between according to highest datecodes on components (23/84 vs. 28/85)

Overview of known Revisions of EM-Board:
MiDi: E
r6502: F
Alex: G
Smith: G
MadTux: L, G
Dave: J
TurboTom: J
HighVoltage: J, L
_Wim_: K
math_indy: L
baltersice (Marco Reps): L

« Last Edit: August 13, 2021, 05:41:56 pm by MiDi »
 
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Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #140 on: January 05, 2020, 09:14:02 pm »

Difference between REV J and L is at least added fuse on COM, there is a screw on backplate between COM post and BNC connectors for fuse-holder - so watch out for this if you want at least REV L  8)

Very interesting... Thanks.
Are there any known improvements between J and L REV?
Is there a known date code from where to where the different revisions have been built?
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Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #141 on: January 05, 2020, 10:53:09 pm »
Are there any known improvements between J and L REV?
Is there a known date code from where to where the different revisions have been built?

Feel free to pull that out of the Revision notes p.158-166
For convenience Rev Notes from EM & MB Component Layout attached - happy decipher ;)

Edit: R355 & Q311 were added after REV J, Q311 is for better protection of Q308 input JFET, not sure what R355 does - maybe isolation to C308 to protect against input transients?
« Last Edit: January 06, 2020, 12:01:14 am by MiDi »
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #142 on: January 05, 2020, 11:24:41 pm »
My MB is REV E with max Datecode of 48/84 (besides repairs with DC from 91-94), Dave's is REV E too with max DC 48/85.
Interesting that mine does lack the CM-Choke after mains input, there are only jumpers installed instead...
I do not know if there is other than REV E of MB in units.
 

Offline TurboTom

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #143 on: January 06, 2020, 09:57:50 pm »
My 617 input board is Rev. J. Funny thing is, the date code of the components on the input board point to some time in Q4/1984 while the main board components have been manufactured in Q1/1986. I didn't find any traces of previous repair / tampering with the instrument, it was "clean as a whistle" inside. No indication of a board swap whatsoever. It may well be possible that Keithley just assembled batches of pre-produced boards that they stocked, into instruments whenever they needed them and in no particular order.

Edit: I have to correct the date codes since I found components with later stamps on the analog board -- the "youngest"  ;) part is a reed relay from 23/1986. So the whole thing has probably been delivered Q3 or Q4 of 1986. S/N is 338269 if that's the number on the silver sticker on the rear of the casing.

What I don't quite understand: Even though the electronic side of Keithley's "brownie" instruments is highly sophisticated and elaborate, from a mechanical / rigidity point of view, the instruments are more than amateurishly built. Big electrolytics installed above the PCB, "flapping around in the breeze", TO220 voltage regulators installed vertically with push-on type "wannabe" heat sinks without any further reinforcement, quite heavy mains transformers installed with just two flimsy screws mounted to the PCB, far off any casing mounting points... I could continue like that. It's not uncommon to find considerable mechanical damage inside these "brownies" after shipping. Had these instruments been built to a budget or why did Kiethley pay so little attention to mechanical detail? When looking at HP or TEK gear from that era, one can learn how things are done properly.

Anyone has some further information on this?

Thanks,
Thomas
« Last Edit: January 07, 2020, 04:14:37 pm by TurboTom »
 

Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #144 on: January 07, 2020, 04:33:23 pm »

What I don't quite understand: Even though the electronic side of Keithley's "brownie" instruments is highly sophisticated and elaborate, from a mechanical / rigidity point of view, the instruments are more than amateurishly built. Big electrolytics installed above the PCB, "flapping around in the breeze", TO220 voltage regulators installed vertically with push-on type "wannabe" heat sinks without any further reinforcement, quite heavy mains transformers installed with just two flimsy screws mounted to the PCB, far off any casing mounting points... I could continue like that. It's not uncommon to find considerable mechanical damage inside these "brownies" after shipping. Had these instruments been built to a budget or why did Kiethley pay so little attention to mechanical detail? When looking at HP or TEK gear from that era, one can learn how things are done properly.

Anyone has some further information on this?

Thanks,
Thomas

To me it seems that Keithley must have had some amazing engineers, when it came to the "specialty" of their brown instrument with current sensitivity. But the rest was done by a team that may have been not so amazing and they just got away with it.

I have several brown case instruments now and I am truly positively amazed by some circuitry and then surprised by the cruel mounting of the transformer or switches and so on.

And may be in those days, the market was limited in sales quantity and one could not ask for too much money for an instrument of this class.
In comparison, a new 6517B (more or less, the successor of the 617) cost now Euro 10.733,80
Isn't that some crazy money for an electrometer? But it seems, people are willing to pay so much for it.

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Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #145 on: February 07, 2020, 09:39:37 pm »
I recently bought a Keithley 617 on Ebay for “parts or repair”. In the description it was stated that the Keithley showed overload on all ranges and all functions, but voltage source was working correct. The pictures also showed unbroken warranty void stickers, and the unit looked well cared for. I was very glad I won the auction.

The unit arrived double boxed with additional protection for the front and rear panel. The seller really did a great effort in packaging this one, a rare thing nowadays.
When I opened the unit it was immediately some repairs were made in the past: quite a few IC’s were socketed, most caps (apart from the high voltage ones) were replaced by some unknown brands like ChongX. But overall impression was clean, an no parts became loose during transport.

There were no immediate visual defects, so I started with checking the power rails. The -9.1 volts rail was only  -0.7V, and series resistor R311 was running very hot, but Zener VR303 was cold => not the Zener that has failed, but some big power consumer. I powered the unit down and measured for a short, but got a high impedance, so this means looking for some kind of active short. Luckily this –V rail only goes to a few chips, and these were already socketed during a previous repair. Removing U145 did not make a change, but when I removed U136 I got -9.6V. U135 was a CD4015, which I had in stock. Replaced it with a new one, immediately same scenario (rail to -0.7V). Started to look more in detail at the schematic, I realise that U136 should not have been a CD4015, but a CD4053. Not a good sign, as it appears the unit never could have worked since the last repair attempt. Warranty void stickers cannot always be trusted apparently…

Replacing U135 by a CD4053 did fix the –V rail, but did not make the unit work. Checking  the other power rails also showed a 5V rail was only 4.2V, this was caused by a faulty 7805 (U308).

To be continued...

Edit: the "as found" picture was already during debugging, the bent pins on U128 were to test if that was the reason the oscillator did not work...
« Last Edit: February 07, 2020, 09:46:06 pm by _Wim_ »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #146 on: February 07, 2020, 09:40:50 pm »
As power rails were now ok, I first tested the preamp section by measuring the preamp & 2V output with a multimeter, and connecting a power supply to the triax input of the 617. This seemed to work fine in voltage mode, but display kept showing overload.  I did not test other modes. This meant the electrometer board at leastworks correctly in voltage mode, which was a good sign.

Tracing down the signal to the ADC showed the ranging amplifier (U130) and buffer (U138) also worked correctly, but AD conversion did not match the signals in table 7-13 of the service manual. None of the clock pulses were seen. 

I figured U128 would be broken, as this is used to make the oscillator. But instead it was a problem with the 1.2288Mhz crystal itself (U128 was also broken, but port C, not port D). I seem  to remember having read that this happened to somebody else also, but cannot find it back.  This crystal is not readily available any more, but a complete oscillator was: (https://www.mouser.be/ProductDetail/ecs/ecs-2100a-12288mhz/?qs=fIFOtpKAkpD%2FYFsyA4ulVQ==&countrycode=DE&currencycode=EUR)

To install the oscillator I removed Y102, and replaced C121 and C122 by a wire link. GND of the oscillator was connected to were Y102 was, the output of the oscillator to R131 (side where it is connected with R133), and the 5 volt was connected to C123. Pin 13 of U128 was removed.  Now we had the clock signals listed in table 7-13, but the integrator ramp was still incorrect. The display now show zero on all ranges instead of overload, unless an actual overload was present. We have some progress!
« Last Edit: February 07, 2020, 09:48:17 pm by _Wim_ »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #147 on: February 07, 2020, 09:42:05 pm »
Switching between the test mode and the normal mode I also found that U129 was not working correctly  (port B).  Unfortunately this still did not fix the unit. As I ordered all of the digital and analog chips in the ADC section, at this point I decided to replace them all at once, as probably many of them suffered damage anyway.  This made the unit work.  :-+

Next was clean-up time. The sockets used were of the low cost type, and as this is a sensitive unit, I opted to remove all sockets and install the adc chips again directly as it was originally. I also replaced all caps by Nichicon and Panasonic caps, because I was unsure of the life expectancy of the previously installed Chinese ones. While doing that  I found that some of the 10µF 25V caps (still Keithley original ones) had seen better days. The original high voltage caps still measured ok, but were replaced anyway.  Remark: when ordering caps, check also circuit diagram, because part list contains some mistakes!  :--
I also already changed U141 (AD7541 => LTC7541AKN) and U143 (LM308A =>OP97) as suggested by Alex Nikitin earlier in this thread. I did not yet replace Q308, but have the parts ready.

After lots of cleaning with flux remover and IPA, the unit is now again assembled.  It has been running now for a couple of hours, and is slowly getting more stable: upon powering up it had an input bias of 800fA 800count (80fA), and has now dropped to 80fA (fluctuating +-30fA) 80counts  (fluctuating +-30counts) (. I will leave it running for a couple of days, before I do any adjustments. 


Edit: corrected fA to counts
« Last Edit: February 08, 2020, 08:27:48 am by _Wim_ »
 
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Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #148 on: February 07, 2020, 10:33:19 pm »
Congratulations, nice repair and explanation!
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Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #149 on: February 08, 2020, 08:31:24 am »
This is the result from logging the analog output (2V output) over night (+-12hour log), taking a sample every second using my picoscope.
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #150 on: February 08, 2020, 02:16:23 pm »
Looks good, was confused short moment  :o, because graph did not correspond to last post - but now recognized your edit.
You could measure effective Frontend offset voltage for V&A from Preamp output when in Zero Check.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #151 on: February 08, 2020, 08:31:53 pm »
Yes, when making the plot I realized I made a stupid mistake before. Just finished exchanging Q308 by an LMC662AIN. Was quite a fiddly job, should have ordered a DIP part instead of SMD. Running with the case open at the moment to let traces of remaining IPA vaporize. Noise spikes at a first glance seem lower, but unit is still drifting due to the cleaning (I think).

Will test offset later on, still need to adjust the unit also. Looked into calibration, but those 100Gohms resistors cost quite a lot, even for "only" 1% accuracy and not the stated 0.08%  :o from the manual.   

 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #152 on: February 08, 2020, 09:05:55 pm »
BTW, my unit is boardrev K, latest datecode I can find is 86 week 29.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #153 on: February 09, 2020, 08:02:15 am »
These are the logs after changing to a LM662AIN (with case still open). Results look very similar to the Q308, peaks a little lower. I measured the offset voltage when in zero check, and was able to adjust it varying between +16µV and -18µV on the 2V output.
 

Online Kleinstein

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #154 on: February 09, 2020, 09:56:12 am »
Some of the spikes could be from radioactive decay (radon gas) or cosmic rays. These can produce ions in the air and that cause such spikes. Air is a good isolator, but the air gap should not be to large and DC electric fields can matter.
 

Offline HighVoltage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #155 on: February 27, 2020, 07:38:54 pm »
Just bought one more Keithley 617, this was the cheapest one so far.
This time I watched out for the "screw in the back" and got a revison L in like new condition and never opened.
Revision number on input board: 617-162-02L

Date code are around 9323

Oddly, one electrolytic cap looks bulged, will check that one first.

« Last Edit: February 27, 2020, 07:44:11 pm by HighVoltage »
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Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #156 on: February 28, 2020, 07:24:57 pm »
I wonder if others see similar behaviour:

when switching from resistance mode to current mode (after being in resistance mode for a longer period), it takes almost 1 hour for the current to settle to around 0fA (see attached plot, 1mV=1fA). When switching ranges in current mode (or switching to coulomb mode and back) I do not see this behaviour, it only happens after measuring resistances. Also briefly switching to resistance mode and back to current mode cause some offset, but this settles to zero within a couple of minutes.  Seems like some sort of dielectric absorption happening, as it always nicely comes back to zero. No cable connected in current mode, for resistance a cable was connected.

 

Online Kleinstein

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #157 on: February 28, 2020, 08:16:38 pm »
The description for the settling after coming from resistance measurements sound like dielectric absorption. This can also be a problem for a cable if there is no driven shield (triax) used.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #158 on: February 28, 2020, 08:35:29 pm »
The description for the settling after coming from resistance measurements sound like dielectric absorption.

That is what I thought also, but I wonder if it is normal behaviour for this unit, or I still need to fix something. To repeat what I did:
1) measure some high value resistors using a triax cable with crocodile clips (or any other cable-test fixture)
2) disconnect the cable and put a shield cap on the input of the 617
3) switch to the lowest range of current mode and wait until it settles around zero
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #159 on: February 29, 2020, 09:36:10 am »
This may indicate bad reed relays.
I did some measurents for settling of old and new ones and the new ones settle a lot faster towards >100T \$\Omega\$.
« Last Edit: February 29, 2020, 09:50:57 am by MiDi »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #160 on: February 29, 2020, 10:02:42 am »
This may indicate bad reed relays.
I did some measurents for settling of old and new ones and the new ones settle a lot faster towards >100T \$\Omega\$.

That is also what I am afraid of. Did you replace also the smaller COTO/Wabash-relays (like K303)?

These would be my initial suspect, as I do not see the behaviour when switching between ranges within current and coulomb, but do see it when switching to resistance/voltage.
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #161 on: February 29, 2020, 06:51:02 pm »
Does this also apply when there is short circuit in resistance/voltage mode?
Open circuit in these modes lets the input settle to one of the HV-rails (~+-250V) and if this is applied longer, the relays could suffer from DA.

Just to be sure :
Quote
but do see it when switching to resistance/voltage
means when switching from?

Currently my repair is on hold, I want to check several things before putting the expensive and fragile reed relays at last in.
Have thought of replacing the small ones for resistance range switching, but their aging should not be that relevant.

PS: Would appreciate high res pictures of bottom EM-Board Rev K/L.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #162 on: February 29, 2020, 06:57:26 pm »
Does this also apply when there is short circuit in resistance/voltage mode?
Open circuit in these modes lets the input settle to one of the HV-rails (~+-250V) and if this is applied longer, the relays could suffer from DA.

Good point, did not test that yet. Will try tomorrow

Just to be sure :
Quote
but do see it when switching to resistance/voltage
means when switching from?

Yes, when returning from resistance/voltage to current mode.

PS: Would appreciate high res pictures of bottom EM-Board Rev K/L.

I will try to take some tomorrow (rev K board in mine).
 
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Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #163 on: February 29, 2020, 09:01:04 pm »
I think the only relay that could be affected by DA this way is K307 for zero check.
It seems to be the most sensitive relay as it is always connected in parallel to measurement path.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #164 on: February 29, 2020, 09:15:10 pm »
When I enable zero check in current mode the display immediately goes to zero+-1, no slow effect is seen.
The slow settling continuous in the background while the zero function is on, and it makes no difference switching to zero and back or not, the slow settling takes about an hour in the lowest current range. 

From this I would think K307 cannot be the culprit, but I could be wrong. As is it not easy to find suitable replacements for the COTO relays, I have no easy way to test this.


 

Online Kleinstein

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #165 on: February 29, 2020, 09:57:35 pm »
The slow response / settling looks like some DA. A tell tail is that a short time in resistance mode also results in faster recovery. The DA can be in a capacitor, but also one of the relays or possibly the board itself. The parasitic capacitance is relatively small, but the material in the capacitor is usually much better.

The effect of the resistance measurement mode with a high resistance is that the input voltage goes up and thus DA is activated.
Switching to a voltage reading may be another case that applies a voltage to some of the caps / relays.

While testing the relays is tricky, as a replacement is hard to find,  one could check  the capacitors. PS caps are still available, though not a regular type anymore.
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #166 on: February 29, 2020, 09:58:55 pm »
DA cannot be reset by anything (except by time), it is modeled by parallel path with resistor and capacitor in series (or multiples).
So I do not think that this excludes K307 as source, but it may not be the only possible source.
Only simple way to check that comes to my mind is to desolder/cut lead of K307 from input side.
« Last Edit: February 29, 2020, 10:22:22 pm by MiDi »
 

Offline _Wim_

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While testing the relays is tricky, as a replacement is hard to find,  one could check  the capacitors. PS caps are still available, though not a regular type anymore.

PS caps (electrolytic) have already been replaced during the repair (as some low cost caps were installed by the previous owner). I did not touch any of the ceramic and foil caps so far as this seems risky to make things much worse.
 

Offline _Wim_

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DA cannot be reset by anything (except by time), it is modeled by parallel path with resistor and capacitor in series (or multiples).
So I do not think that this excludes K307 as source, but it may not be the only possible source.
Only simple way to check that comes to my mind is to desolder/cut lead of K307 from input side.

My thinking was that because the voltage over K307 changes between output voltage and ground when enabling/disabling the zero check, I should see a difference in slope.

But if we are indeed talking about DA charged to one of the high voltage rails, this little difference in voltage might not produce a noticeable effect in the slope.

I will do some further testing this afternoon hopefully.
 

Offline _Wim_

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PS: Would appreciate high res pictures of bottom EM-Board Rev K/L.

Here you go
 
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Offline _Wim_

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Does this also apply when there is short circuit in resistance/voltage mode?
Open circuit in these modes lets the input settle to one of the HV-rails (~+-250V) and if this is applied longer, the relays could suffer from DA.

When measuring a short, the effect is similar but smaller. Measured current is also positive instead of negative. The settling time is directly related to the time the meter was is resistance mode (so it is for sure a DA effect):

20 sec in resistance mode with a short: 1min40 sec to settle to <+100fA (waiting to zero takes way too long...)
20 sec in resistance mode open circuit: 4min10sec to settle to >-100fA
60 sec in resistance mode open circuit: 9min15sec to settle to >-100fA

Would be great to see results from another unit to see what is "normal".
 

Offline Noopy

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Hi all!


MiDi sent me a leaky Q308 and I took some pictures of the die.
A very small thing...  :phew:





There is some dirt caused by the opening of the can but on the right gate there is a spot that´s not "normal dirt":



With different illumination that thing at the side of the metal structure looks always different than "normal dirt".
Perhaps that´s a production problem leading to a leaky JFET?


Some more pictures here: https://richis-lab.de/K617.htm

 :popcorn:
 
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Offline HighVoltage

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@Noopy,
Thanks for these nice insight pictures.
Do we actually know, who the manufacturer of Q308 is and is there a datasheet around ?
There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline Noopy

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@Noopy,
Thanks for these nice insight pictures.
Do we actually know, who the manufacturer of Q308 is and is there a datasheet around ?

I was happy to get such an special IC!  :-+

Unfortunately I also wasn´t able to identify the manufacturer. Couldn´t assign the "M" logo...  :-//

Online Gyro

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With different illumination that thing at the side of the metal structure looks always different than "normal dirt".
Perhaps that´s a production problem leading to a leaky JFET?

Degraded performance caused by an ESD strike? :-\   It presumably tested fine at manufacture.

It would be interesting to compile a 'rogue's gallery' of such damage.


P.S.
Quote
Couldn´t assign the "M" logo...   :-//

Motorola maybe?

« Last Edit: March 02, 2020, 01:59:52 pm by Gyro »
Best Regards, Chris
 

Offline Noopy

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With different illumination that thing at the side of the metal structure looks always different than "normal dirt".
Perhaps that´s a production problem leading to a leaky JFET?

Degraded performance caused by an ESD strike? :-\   It presumably tested fine at manufacture.

I´m no expert regarding semiconductor damages but I´m pretty sure ESD causes little holes. Here it looks like there is too much metal.
I could imagine that the part was good but some manufaturing residue has degraded the performance over the years...


P.S.
Quote
Couldn´t assign the "M" logo...   :-//

Motorola maybe?



Possible but I haven´t found a Motorola-Logo looking like the M on the Q308...

Offline Noopy

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@Noopy,
Thanks for these nice insight pictures.
Do we actually know, who the manufacturer of Q308 is and is there a datasheet around ?

I was happy to get such an special IC!  :-+

Unfortunately I also wasn´t able to identify the manufacturer. Couldn´t assign the "M" logo...  :-//


NEWS:
The "M" stands for Micro Power Systems!  :)
Unfortunatelly you don´t find very much about the company and nothing about this FET.  :(
 
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Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #177 on: September 14, 2020, 11:50:18 am »
Hi fellow 617 users,

I will join the club soon! I have bought a non-working as-is 617 on Ebay that does not turn on. The device seems cosmetically good in and outside, so I am eagerly waiting to get the item and start the repair. I am sure I will need your help and experience.

I hope all the reeds are good.

Cheers!
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #178 on: September 14, 2020, 06:47:52 pm »
I will join the club soon!

Welcome! I wish you lots of fun, both with repairing the 617 and playing with it afterwards (or even doing some actual work with it)
 
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Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #179 on: November 08, 2020, 07:05:46 pm »
Here I am! I was sure it is not a "fuse replace complexity" of problem but it seems I have to go deep into the digital circuitry (first...).

I can see that the input stage was repaired; the input protection transistor pair and the input JFETs are changed. Some LDOs on the analog board are also changed. But the main problem is that the device does not turn on.

What it does is turning on some LEDs and digits on the front for 2-3 seconds. Power rails are okay, but the motorola microcontroller does not run the crystal. Now I am waiting for a new crystal.

But anyway, has anybody seen such symptoms?

Thanks,
 

Offline Johnny10

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #180 on: November 08, 2020, 07:36:23 pm »
Very Cool
Tektronix TDS7104, DMM4050, HP 3561A, HP 35665, Tek 2465A, HP8903B, DSA602A, Tek 7854, 7834, HP3457A, Tek 575, 576, 577 Curve Tracers, Datron 4000, Datron 4000A, DOS4EVER uTracer, HP5335A, EIP534B 20GHz Frequency Counter, TrueTime Rubidium, Sencore LC102, Tek TG506, TG501, SG503, HP 8568B
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #181 on: November 08, 2020, 08:15:04 pm »
What it does is turning on some LEDs and digits on the front for 2-3 seconds. Power rails are okay, but the motorola microcontroller does not run the crystal. Now I am waiting for a new crystal.

But anyway, has anybody seen such symptoms?

Thanks,

I also had a broken crystal, but the one used by the ADC, not the one used by the micro. I find it strange the leds & digits go again out after 2-3 seconds. That does seem like the micro is doing something, or a power rail that shuts down (overload?).
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #182 on: November 08, 2020, 08:28:14 pm »
I also had a broken crystal, but the one used by the ADC, not the one used by the micro. I find it strange the leds & digits go again out after 2-3 seconds. That does seem like the micro is doing something, or a power rail that shuts down (overload?).

It seems the power rails (at least around the micro) are stable and noiseless.

But, I've discovered that the micro's reset pin is tied to the ground while it should go up to ~4 volts after a second. I cannot find where the reset signal is coming from. On the schematic there is a label for the reset signal so maybe it is controller from somewhere else.

Fast access to the schematic, page 163: https://www.univie.ac.at/photovoltaik/umwelt/ws2015/K617_original.pdf

UPDATE 1: The reset signal goes to the GPIB controller IC U113.

UPDATE 2: There is a typo in the manual. It says the reset pin is pin 3 but in reality it is pin 1. See the section below. And the real reset pin goes to high as it should.
« Last Edit: November 08, 2020, 09:28:17 pm by balage »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #183 on: November 09, 2020, 06:04:56 pm »
Do you see any other pulse trains on for example pin 4 and pin 6? Does the unit has any reaction to button presses?
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #184 on: November 10, 2020, 08:58:55 am »
I have changed the xtal but nothing has changed. Maybe the MC146805 uC is dead?! But it is only avaliabe on ebay and that's what I have when you have to wait for a component for 1 month.  |O

Anyway. There is totally no response from the microcontroller, buttons have no effect. What I also see is that the display board does not produce the button release pulses. Its strange because the display board has the power supplies and there is nothing there that could likely go wrong.
 

Offline TurboTom

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #185 on: November 10, 2020, 09:11:46 am »
If supply voltage is present at the 6805 and the oscillator isn't running, I'ld check the 10M resistor parallel to the crystal and the two load capacitors. If all that checks okay and you still won't get oscillation, it's likely the processor is at fault. As a final test, you may try supplying an external clock signal to pin 39 of the microcontroller. If it then shows some signs of life, only the oscillator section may have shifted out of tolerance (gain / offset).
« Last Edit: November 10, 2020, 09:17:13 am by TurboTom »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #186 on: November 10, 2020, 04:59:32 pm »
What I also see is that the display board does not produce the button release pulses. Its strange because the display board has the power supplies and there is nothing there that could likely go wrong.

The buttons are scanned by the micro controller via pins 9,10,11. If the micro is not running, no pulses will be seen.
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #187 on: December 03, 2020, 08:34:58 pm »
Hey,

Sorry for not reacting. I was busy (and lazy) and had only time to order the microcontroller. Has just arrived! It works now! Scans the buttons and runs the oscillator. :-+

However it seems this is the only thing that is working good. The voltage source was bad but it seems only one of the optos is wrong.

The instrument is reading the followings:
Input state                    Opened                 Shorted
Volts:                increasing to overload         90mV
Ohms:                      inc. to overload          900Ohm
Current:                         0,9mA                   overload

For me it seems the input circuitry gets 90mV or something from somewhere. That is why it measures 9xx in different configs.  :palm:

Do you have any idea where should I start? The input protection BJT pair (near the end of the red minigrabber) is replaced a rude way.
[images later]
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #188 on: December 03, 2020, 09:23:37 pm »
What do you measure on the preamp output at the back? The preamp output has the signal before is goes to the ADC, so this way you can already see if the problem is on the analog board or the digital board.

When you are in manual ranging, does it behave the same on all ranges? Or are only some ranges faulty?
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #189 on: December 03, 2020, 09:49:27 pm »
The preamp outputs 0,09 always in all ranges. At least it is stable  ;D  For current is outputs -0,09. So changes the polarity.

Both the preamp and the display show the same value in all ranges, e.g. I mean 90mV in all ranges.

NO! Sorry, I was wrong. For volts it shows 90mV in all ranges. But in resistance it shows this in the ranges, with rounded values:
0,09G -> 0,090G -> 0,0900G
9,00M -> 0,900M -> 0,0900M
9,00k -> 0,900k -> 0,9000k

0,90mA -> 0,9000mA
0,90uA -> 0,900uA -> 0,9000uA
0,90nA -> 0,900nA -> 0,9000nA
For picoamps the sequence is the same. Actually for current it is straightforward, it is always 0,9xxx.

It seems an offset voltage is injected somewhere in the measurement loop.
« Last Edit: December 03, 2020, 10:04:34 pm by balage »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #190 on: December 04, 2020, 06:41:23 pm »
Ok, so the problem is definitely on the analog board. 

What happens is you actually measure a resistor, voltage or current? Does it go up with the correct amount, or do you see no change?
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #191 on: December 04, 2020, 07:57:00 pm »
The voltage source is now working. U124 needed to be changed.

So the displayed value is not changing in any of the functions. The LT1012 (U309) has a moving bootstrapped power supply as far as I can understand it. It is good. The reeds are also clicking when I change function.

I have measured that 89mV is going back to the motherboard on J1014 between pin 1 and 10 and 4. At pin 10 there is signal if K301 is operated. Should I brave enough to move W303 jumper to + or -5V? What is this for?
Also should I put a voltage here to the middle of W303?

UPDATE:
I measured voltage between U309's pin 2 and 3. It was 10mVolts. When I have applied a voltage to the input, the voltage was the same. But while doing this the displayed value changed.
Also I have measured the output of this opamp and the output was constantly -77mV while I changed the input voltage.
« Last Edit: December 04, 2020, 08:17:31 pm by balage »
 

Online Kleinstein

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #192 on: December 04, 2020, 09:36:33 pm »
The W303 jumper is for coarse trim / compensation of offset of the FET pair.  Remember where it started and watch out for ESD as the FET can be ESD sensitive. 

10 mV across the input of U309 is normally too much - especially for an LT1012. So the U309 should be close to one side of it's supply (bootstrapped +-5 V) With a fixed output, there should not be much movement for the bootstrapped supply - more like close to +-5 V. The 90 mV shift is not much.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #193 on: December 04, 2020, 09:46:12 pm »
I measured voltage between U309's pin 2 and 3. It was 10mVolts. When I have applied a voltage to the input, the voltage was the same. But while doing this the displayed value changed.
Also I have measured the output of this opamp and the output was constantly -77mV while I changed the input voltage.

Did I understood you correctly that, when applying a voltage between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminals of the 617 no change is seen on the display. This would indicate something is probably wrong with Q308...
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #194 on: December 04, 2020, 09:57:48 pm »
What happens when you do "zero check". Does it change the reading on the display? I presume it makes also no difference?

If you measure on pin3 of Q308 with a voltage applied to the input terminals, do you see any change with the input voltage on or off?
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #195 on: December 04, 2020, 10:23:44 pm »
I measured voltage between U309's pin 2 and 3. It was 10mVolts. When I have applied a voltage to the input, the voltage was the same. But while doing this the displayed value changed.
Also I have measured the output of this opamp and the output was constantly -77mV while I changed the input voltage.

Did I understood you correctly that, when applying a voltage between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminals of the 617 no change is seen on the display. This would indicate something is probably wrong with Q308...

Yes, I was not so accurate. So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So any voltage on the triax input has not effect. But zeroing works fine.
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #196 on: December 04, 2020, 10:37:46 pm »
The W303 jumper is for coarse trim / compensation of offset of the FET pair.  Remember where it started and watch out for ESD as the FET can be ESD sensitive. 

10 mV across the input of U309 is normally too much - especially for an LT1012. So the U309 should be close to one side of it's supply (bootstrapped +-5 V) With a fixed output, there should not be much movement for the bootstrapped supply - more like close to +-5 V. The 90 mV shift is not much.

Then the 90mV is coming from the center rail of the bootstrapped supply. I mean from between R306& R307. Confusing.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #197 on: December 04, 2020, 10:38:24 pm »
Yes, I was not so accurate. So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So any voltage on the triax input has not effect. But zeroing works fine.

"Zero check" works fine? That would mean Q308 is at least doing something, and I would also expect you see the effect of a voltage at the input terminal (triax). "Zero check" grounds the input using K307 just before Q308.

Could it be you confused "zero check" with "zero correct" (which is only a micro-controller delta function)?
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #198 on: December 04, 2020, 10:43:15 pm »
So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So when you probe between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminal (triax) of the 617 no change is seen on the display. This still would indicate something is probably wrong with Q308... But than the "zero check" should not have any effect.
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #199 on: December 04, 2020, 10:55:08 pm »
 :D Sorry for this. So zero check has no effect but I hear the reed-relay clicking.

Q308 was replaced so something was around here before I get the 617. 9121TN is written on the case. Google shows nothing for that. The paired BJT is replaced a rude way...

So if at the inputs of LT1012 nothing changes when a voltage is applied to the triax then Q308 is dead, right? +/-5V from the bootstrapped supply is good.
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #200 on: December 04, 2020, 10:59:50 pm »
So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So when you probe between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminal (triax) of the 617 no change is seen on the display. This still would indicate something is probably wrong with Q308... But than the "zero check" should not have any effect.

Okay, then I should start looking this site: http://www.linearsystems.com/product.html  :-//
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #201 on: December 05, 2020, 07:50:08 am »
So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So when you probe between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminal (triax) of the 617 no change is seen on the display. This still would indicate something is probably wrong with Q308... But than the "zero check" should not have any effect.

Okay, then I should start looking this site: http://www.linearsystems.com/product.html  :-//

 :o, that was indeed done in a crude way!

No,I would not look for a replacement fet, but I would do the excellent modification described by user Alex Nikitin: https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg759354/#msg759354

I had a good original Q308 and also did the modification, and the LMC662A is readily available.
« Last Edit: December 05, 2020, 08:48:11 am by _Wim_ »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #202 on: December 05, 2020, 08:01:59 am »
Had another look at your picture, it is Q311 that is replaced, Q308 is the smaller FET that still seems original. Q311 is used as a protective diode. I would start by removing this one, maybe it is shorting to ground. The 617 works perfectly without, it is just less protected at its input.

Q311 is not needed any more when doing the mod described by Alex Nikitin.
« Last Edit: December 05, 2020, 08:49:35 am by _Wim_ »
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #203 on: December 05, 2020, 08:08:37 am »
Please also read the beginning of this thread thoroughly about the cleanliness required of this pcb. Wear gloves at all this because fingerprints can ruin already the performance of this unit. It does seem however the previous owner did not really care about this, to some extensive cleaning might be required, but would still very much avoid additional contamination!

 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #204 on: December 05, 2020, 09:12:39 pm »
Had another look at your picture, it is Q311 that is replaced, Q308 is the smaller FET that still seems original. Q311 is used as a protective diode. I would start by removing this one, maybe it is shorting to ground. The 617 works perfectly without, it is just less protected at its input.

Q311 is not needed any more when doing the mod described by Alex Nikitin.

At the bottom of the PCB there are soldering residuals at Q308, that is why I think it is also replaced. On the top it seems clear around.

I have removed Q311, but it has no effect, everything is the same. Actually the reading become a few milivolts more. So the source of this 90mV is one question, and the other is that why the input nis not reacting for input signals. Where should I proceed? Maybe I buy a new LT1012.
 

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #205 on: December 05, 2020, 10:19:15 pm »
I would start with the replacing Q308 by an LMC662A as described by Alex Nikitin. If you have to order parts, I would order an LT1012 also as it possibly also took a hit when Q308 died.
 
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Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #206 on: December 06, 2020, 05:39:25 pm »
I would start with the replacing Q308 by an LMC662A as described by Alex Nikitin. If you have to order parts, I would order an LT1012 also as it possibly also took a hit when Q308 died.

First thing to blame is Q308 if voltage NOT slowly rising to overload (leaky Q308).
If you are brave you can replace whole frontend with ADA4530-1.
Than you can get rid of TC and bias offset compensation (cut R332), transient protection (cut Q311) and voltage offset compensation (cut R351 & R352).
« Last Edit: December 06, 2020, 07:25:25 pm by MiDi »
 
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Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #207 on: December 08, 2020, 08:44:47 am »
I would start with the replacing Q308 by an LMC662A as described by Alex Nikitin. If you have to order parts, I would order an LT1012 also as it possibly also took a hit when Q308 died.

First thing to blame is Q308 if voltage NOT slowly rising to overload (leaky Q308).
If you are brave you can replace whole frontend with ADA4530-1.
Than you can get rid of TC and bias offset compensation (cut R332), transient protection (cut Q311) and voltage offset compensation (cut R351 & R352).

Thanks for the tip. After I have repaired the device and still have spirit than i'll try. Your 0,000pA reading looked nice. :)

But anyway, components for Alex Nikitin style modification are on their way.
 

Offline balage

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #208 on: December 11, 2020, 09:50:41 pm »
Components arrived, old JFET removed, LMC662 is put in, LT1012 replaced. But now the 617 displays overload. :(

So I have measured the voltage going to the AD from the EM board, and now between pin 1 and 10 there is -225V. For sure, -225V!.  :-BROKE

How the f..ing hell can this can happen? Previously it was this:

...
I have measured that 89mV is going back to the motherboard on J1014 between pin 1 and 10 and 4.
...

Supply of LT1012 is still stable +/-5V. Maybe this bootstrapped bridge puts the negative supply to the central line that is going to K301. This must have damaged the AD. Great.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #209 on: December 11, 2020, 10:09:37 pm »
Components arrived, old JFET removed, LMC662 is put in, LT1012 replaced. But now the 617 displays overload. :(

So I have measured the voltage going to the AD from the EM board, and now between pin 1 and 10 there is -225V. For sure, -225V!.  :-BROKE

How the f..ing hell can this can happen? Previously it was this:

...
I have measured that 89mV is going back to the motherboard on J1014 between pin 1 and 10 and 4.
...

Supply of LT1012 is still stable +/-5V. Maybe this bootstrapped bridge puts the negative supply to the central line that is going to K301. This must have damaged the AD. Great.

225 volt can go to pin 10 (same voltage as preamp out), as the preamp out still has the full voltage swing.
 

Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #210 on: December 11, 2020, 10:27:13 pm »
I think you best should try and read at least chapter 6 of the service manual (theory of operation), and also chapter 2 (operation). This should give you a better idea what to expect, otherwise debugging will be very difficult. The service manual is very well written, but there are some complex parts. After those 2, start from chapter 7.6 (trouble shooting) and follow it step by step. Report back which step failed, this way it might be easier to follow along.
 

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #211 on: December 11, 2020, 10:34:54 pm »
I think you best should try and read at least chapter 6 of the service manual (theory of operation), and also chapter 2 (operation). This should give you a better idea what to expect, otherwise debugging will be very difficult. The service manual is very well written, but there are some complex parts. After those 2, start from chapter 7.6 (trouble shooting) and follow it step by step. Report back which step failed, this way it might be easier to follow along.

Yes, you are right! It is now not a simple transistor replace-type of repair. Sorry for exaggerated questions.  :D
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #212 on: December 12, 2020, 01:30:47 am »
Do you get overload on zero check?
Is the overload present on highest amp range?
If bias current compensation still present, it needs adjustment.
 

Offline analogNewbie

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #213 on: December 12, 2020, 05:24:36 am »
As far as I understand, the IV converter circuit should work like this
1129562-0" alt="" class="bbc_img" />

However, K617 connects the Rf to the In+ of LT1012. Why?
1129566-1" alt="" class="bbc_img" />
« Last Edit: December 12, 2020, 05:26:54 am by analogNewbie »
 

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #214 on: December 12, 2020, 06:54:41 am »
Yes, you are right! It is now not a simple transistor replace-type of repair. Sorry for exaggerated questions.  :D

You can ask many questions, that is no problem. But when you fully understand the background, you will have much more fun doing the repair, and also learn quite a bit from it. And especially when a good service manual is available, it is the ideal opportunity to really learn how the unit is working. For many of the test gears I have, the most fun I had with them, was during the repair, not once it was repaired, and that includes my 617...
 

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #215 on: December 12, 2020, 07:12:06 am »
As far as I understand, the IV converter circuit should work like this
(Attachment Link) " alt="" class="bbc_img" />

However, K617 connects the Rf to the In+ of LT1012. Why?
(Attachment Link) " alt="" class="bbc_img" />

That is because LT1012 is only functioning as a part of the amplifier (gain stage), and is not the complete amplifier. If you include Q308 (a differential input pair, which causes inversion), the figure is correct.

See also here https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-differential-pair/ for a nice description.
 

Offline analogNewbie

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #216 on: December 12, 2020, 09:48:23 am »
Thanks.

But Q308 is connected as follower not common source amplifier, right? the phase is not inverted in this stage
« Last Edit: December 12, 2020, 10:05:20 am by analogNewbie »
 

Offline TurboTom

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #217 on: December 12, 2020, 11:11:46 am »
That's the result of the ingenious concept of the 617's output stage that keithley enabled to reach such an extremely wide output voltage range while still using "relatively" common input / gain stage components.

It's obvious, from a common point of view, the tranconductance amplifier stage appears to be connected the wrong way round (inputs reversed). And it's also correct, the input buffer (Q308) doesn't cause the polarity reversal. Instead, it's very important to have a close look at the power supply and especially the "grounding" scheme. The PSU for the output stage (+- 210V) isn't referenced to signal ground. Instead, the output terminal of the output stage that usually drives the load, is grounded, thus pulling the whole output voltage PSU "around", providing an inverted output signal at the output stage PSU's common terminal. And that's how Keithley's transconductance amplifier works, and also, why they use such funny (sophisticated...), staged and screened supply voltage transformers. This concept allows Keithley to drive the output stage with the low output voltage range available from the LT1012 by basically converting a voltage follower output into a true (common emitter) voltage gain output. Yet, this concept still keeps the voltage follower advantage from a biasing / driving point of view, eliminating all the voltage-shifting otherwise required.

The schematic is somewhat confusing since the multitude of supplies and ground symbols aren't always in direct view. It may be recommendable to print the page with the PSU schematic and have it always as a reference when working with the other schematics.

Edit: For a better understanding of the explanation, please see pages 6-5 and 6-6 of the manual. Observe where "Gain Stage Common" is connected to.
Edit2: Only now I noticed that Keithley made a mistake in the schema of the current output configuration: Both the "lower" supply and the transistor are of the wrong polarity (is NPN, should be PNP).
« Last Edit: December 12, 2020, 11:28:25 am by TurboTom »
 
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Offline _Wim_

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #218 on: December 12, 2020, 12:25:32 pm »
That's the result of the ingenious concept of the 617's output stage that keithley enabled to reach such an extremely wide output voltage range while still using "relatively" common input / gain stage components.

It's obvious, from a common point of view, the tranconductance amplifier stage appears to be connected the wrong way round (inputs reversed). And it's also correct, the input buffer (Q308) doesn't cause the polarity reversal. Instead, it's very important to have a close look at the power supply and especially the "grounding" scheme. The PSU for the output stage (+- 210V) isn't referenced to signal ground. Instead, the output terminal of the output stage that usually drives the load, is grounded, thus pulling the whole output voltage PSU "around", providing an inverted output signal at the output stage PSU's common terminal. And that's how Keithley's transconductance amplifier works, and also, why they use such funny (sophisticated...), staged and screened supply voltage transformers. This concept allows Keithley to drive the output stage with the low output voltage range available from the LT1012 by basically converting a voltage follower output into a true (common emitter) voltage gain output. Yet, this concept still keeps the voltage follower advantage from a biasing / driving point of view, eliminating all the voltage-shifting otherwise required.

The schematic is somewhat confusing since the multitude of supplies and ground symbols aren't always in direct view. It may be recommendable to print the page with the PSU schematic and have it always as a reference when working with the other schematics.

Edit: For a better understanding of the explanation, please see pages 6-5 and 6-6 of the manual. Observe where "Gain Stage Common" is connected to.
Edit2: Only now I noticed that Keithley made a mistake in the schema of the current output configuration: Both the "lower" supply and the transistor are of the wrong polarity (is NPN, should be PNP).

Thanks for the detailed explanation, I missed that completely before.  :-+
 

Offline MiDi

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #219 on: December 12, 2020, 12:34:01 pm »
There are a lot of mistakes all over the place - not only in the documentation, but also in design - at least the first revisions.
 

Offline analogNewbie

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #220 on: December 12, 2020, 02:32:52 pm »
That's the result of the ingenious concept of the 617's output stage that keithley enabled to reach such an extremely wide output voltage range while still using "relatively" common input / gain stage components.

It's obvious, from a common point of view, the tranconductance amplifier stage appears to be connected the wrong way round (inputs reversed). And it's also correct, the input buffer (Q308) doesn't cause the polarity reversal. Instead, it's very important to have a close look at the power supply and especially the "grounding" scheme. The PSU for the output stage (+- 210V) isn't referenced to signal ground. Instead, the output terminal of the output stage that usually drives the load, is grounded, thus pulling the whole output voltage PSU "around", providing an inverted output signal at the output stage PSU's common terminal. And that's how Keithley's transconductance amplifier works, and also, why they use such funny (sophisticated...), staged and screened supply voltage transformers. This concept allows Keithley to drive the output stage with the low output voltage range available from the LT1012 by basically converting a voltage follower output into a true (common emitter) voltage gain output. Yet, this concept still keeps the voltage follower advantage from a biasing / driving point of view, eliminating all the voltage-shifting otherwise required.

The schematic is somewhat confusing since the multitude of supplies and ground symbols aren't always in direct view. It may be recommendable to print the page with the PSU schematic and have it always as a reference when working with the other schematics.

Edit: For a better understanding of the explanation, please see pages 6-5 and 6-6 of the manual. Observe where "Gain Stage Common" is connected to.
Edit2: Only now I noticed that Keithley made a mistake in the schema of the current output configuration: Both the "lower" supply and the transistor are of the wrong polarity (is NPN, should be PNP).

Thanks for the details.  I have tried in the LTSpice.
 
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Offline snik

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I bought recently a defective K617 and the Input Dual-Fet was destroyed in this unit also.
Because of restoring it for original look inside, i don't want to make the OpAmp mod.
Now i managed to get 5 Transistors for replacement (I also found an old original Micro Power Systems inc. Datasheet).

Since they arrived, i asked me, for which values Keithley have selected the FETs for the Electrometer and if i can do a selection too ...

The 2N5906 is the one in this series who has the best values, maybe a selection is not necessary and i should dice for one.  :-//

Do anybody know how and for what they selected the Input Dual-FETs ?
 
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Offline r6502

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Hello all,

I also got now my 617 from EBAY.

When it came, it showed in all ranges Overflow - ok, for Ohm's this would be fine if connections are open - ;-) ...

A check of all pats of the device showed, that U307, Q303, Q305 and Q307 on the electrometer board where blown.

U307 is easy to fix, it is an LM337. But the transistors Q303, Q305 and Q307 are MPS-U60 types from good old Motorola.

I have looked at different mfg sites of discrete bipolar transistors and found out, that MJD340 (NPN),
MJD350 (PNP) will fit mostly to the parameters of the original MPS-U10 / MPS-U60 types, but the parameter "Emitter−Base Voltage" is only 3V with the Motrola  type. The Transistors specified with 6V here.

Does anybody have an Idea, if MJD340 and MJD350 will be fine replacement for MPS-U10 / MPS-U60 - yes, thy are SMD, , but I think, could be adapted ...


Kind regards

Guido
« Last Edit: August 04, 2021, 09:35:49 pm by r6502 »
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Offline MiDi

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Since they arrived, i asked me, for which values Keithley have selected the FETs for the Electrometer and if i can do a selection too ...

The 2N5906 is the one in this series who has the best values, maybe a selection is not necessary and i should dice for one.  :-//

Do anybody know how and for what they selected the Input Dual-FETs ?

As this instrument has input current in range of couple of fA, at least the input FET has to be in that order.
Input current has to be canceled anyhow (+-20fA adjustment range), but the lower the input FET bias current is, the better.
One way is to put them into the unit, disconnect cancelation circuit (R332) & all reed relays at the input node and get the reading as mentioned in performance verification for input bias current after ~1day settling.
K307 has to be bridged manually for zero check & zero correct while operating the unit (! dangerous, high voltage present).
Very good cleaning is mandatory when dealing with fA, especially the teflon standoffs and between legs of FET.

Alternative is to meaure FETs with another EM ;)

Chances are that some of the critical reed relays show significant leakage/DA, which would degrade input current - see previous posts for details.

I have looked at different mfg sites of discrete bipolar transistors and found out, that MJD340 (NPN),
MJD350 (PNP) will fit mostly to the parameters of the original MPS-U10 / MPS-U60 types, but the parameter "Emitter−Base Voltage" is only 3V with the Motrola  type. The Transistors specified with 6V here.

Does anybody have an Idea, if MJD340 and MJD350 will be fine replacement for MPS-U10 / MPS-U60 - yes, thy are SMD, , but I think, could be adapted ...

MJE340/350 are the THT versions of MJD340/350, but have different pinout than MPS-U10/U60, either should work as a replacement.
Veb is not relevant for this (and most) circuit, in general the emitter base breakdown voltage has wide spread and DS numbers are given for safe operation w/o breakdown in reverse bias only.
« Last Edit: August 05, 2021, 09:48:35 am by MiDi »
 

Offline r6502

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@MiDi:
Thanks for the reply. Theese I can get really easy I think.

@snik:
As MiDi wrote, one parameter would be the input current of the JFETs and how equal both JFets fit together. Another thing ist the thermal drift. Also the chacteristic from input voltage to drain current will be cheked I think. The 2 JFETs are already good matched, but some will be better than other, and these are beeing selected to use for the instruments.

Question would be, how to measure all these parameters  - for the input current you will need an electrometer ...

I'm going to order some transistors, and I will also replace all electrolytic caps - some look not so well.  Then I will continue to check the rest of the instrument. Calibration will be easy for me, especual the current range - own a 263 current source ...

Does it make sense, to replace the internal reference of the device or is it just fine for the application?

Kind regards

Guido

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Online Kleinstein

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One can check the input current by using the amplifier / meter itself to watch a capacitor accumulate charge.
So the input bias translates to voltage drift.

With the very low currents / very high impedance part the scale factor and thus the reference is not that important. A well aged reference is often better than a new one. So unless known broken, there is no need / use to change the ref.
 

Offline snik

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Meanwhile it turned out that my five 2N5906, which i ordered specially in America and not in China, seems to be Fake. Though the Electrometer works with them, but the Input-Current was always very high 300 - 600 fA and i couln't regulate that very much. After changing the current poti and three Transistors i cracked one open and the Die doesn't look like the other FETs (See my picture). So after that i have changed the Input-FET to an 2N5909 which i ordered here in germany and now the input-current is, how it should be. So the question for selecting one of the 2N5906 was obsolete but interesting to know.

As summary : After changing the Input Fet (to often ;) ), the LT1012, two MPS-U60, the LM337L (which was 8V instead of 5V), a poti, a broken 100 R Resistor (which was 6.5kOhm) and all Capacitors (one was bursted), my Electrometer works now as expected.  :-+ Last step is now calibrating the unit.

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

@r6502
When my MPS-U60 transistors was on order, i've used for further testing MPSA92 Transistor as a replacement. Maybe they could work also. I orderd my MPS-U60 Transistors via eBay in Italy.
 
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Offline r6502

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As summary : After changing the Input Fet (to often ;) ), the LT1012, two MPS-U60, the LM337L (which was 8V instead of 5V), a poti, a broken 100 R Resistor (which was 6.5kOhm) and all Capacitors (one was bursted), my Electrometer works now as expected.  :-+ Last step is now calibrating the unit.

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

@r6502
When my MPS-U60 transistors was on order, i've used for further testing MPSA92 Transistor as a replacement. Maybe they could work also. I orderd my MPS-U60 Transistors via eBay in Italy.

Very interesting, so the defect end stage of the input amplifier seems to be a common problem with this instrument?

Question: Where in Germany did you by the double JFET sitting in the input?

I'm not really sure, how to proceed - improving the instrument by replacing the defect part wit newer better components or putting it in the original setup and replace the defect parts with the originals.

Guido
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Offline r6502

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For the OP conversion I am wondering why you need the super high impedance LMC662 to replace both FETs. The feedback side is not very high impedance and could use a lower noise/drift type with no problem.

For the spike like noise, this could be cosmic or radioactive background radiation, hitting something like protecting diodes. Smaller diodes (less sensitive volume) and maybe radiation shielding could help.

What OP do you have in mind? The LMC662 has an ultra low Ib, that's what you want in the input stage to get the high input impedance here. It combines also 2 amplifiers in the same chip, that's from the thermal point of view also very nice. Also power supply is needed only one time ...

If the ADA4530 would be available as a double OP, I would use this one instead of LMC662 I think.

Guido
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Online Kleinstein

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Yes the LMC662 and some other very low bias OPs come as dual.  Some, like the LMP7721, LMC6001 also come as singles.
I would todays consider something like an OPA202 for a simple low noise follower for the less critical path.
The second OP of the input stage could also be just skiped alltogether. The input side has a source impedance of about the required value - so no real need for an extra buffer. A slightly differenten resistance would change the BW a little, but not much and a different cap could correct this.

The 2 nd not really needed OP would add to the drift an noise. With the JFETs it makes sense to have 2 to get drift compensation.
However with most OPs the offset drift of the duals does not correlate that much. So the 2nd OP is not really compensating the drift and definitely adding to the noise.
 

Offline David Hess

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Linear Systems has some dual JFETs specified to have lower than 1 picoamp leakage.  It seems like an old style 3N series MOSFET would be better for leakage, but they only test them down to 10 picoamps.

Yes the LMC662 and some other very low bias OPs come as dual.  Some, like the LMP7721, LMC6001 also come as singles.

Before the LMC6001 became available, I used the LMC6081 which later is what National graded to get the LMC6001; they are identical parts.  If you are willing to grade the parts yourself, then you can buy like 8 LMC6081s for every LMC6001 and I never saw an LMC6081 with more than the typical input bias current specification.

Bob Pease mentioned what they went through to test LMC6001s for input bias current.
 

Offline MiDi

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Very interesting, so the defect end stage of the input amplifier seems to be a common problem with this instrument?

I'm not really sure, how to proceed - improving the instrument by replacing the defect part wit newer better components or putting it in the original setup and replace the defect parts with the originals.

As common as leaking/blown input JFET and leaking reed relays  ;)
Imo recapping all electrolytics is mandatory, even if they seem to be fine.

One could improve a lot in this instrument, it is an old design with old parts and rather bad layout...

My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

Richi has already done die shots of my leaky JFET, but it is first gen, would be interesting to see if there are differences to newer gen  :-+
« Last Edit: August 05, 2021, 07:21:45 pm by MiDi »
 

Offline snik

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My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.

Richi has already done die shots of my leaky JFET, but it is first gen, would be interesting to see if there are differences to newer gen  :-+

That's exactly the reasons why i've contact him.  :)
 
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Offline r6502

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Hello All,

one question, my 617 seems to be an earlier device - it does not have the input protection Q311 and the Jumper W303 for (better?) offset trimming of the preamp is missing as well.

In this older instrument the offset trimming then is done only with the trim pot R314 correct?

Does somebody have a schematic for the older version of the device?

Kind regards

Guido
Science can amuse and fascinate us all, but it is engineering that changes the world - - Isaac Asimov
 

Offline MiDi

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What revision has your EM board?
 

Offline r6502

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The board has the marking 617-162-02F

Guido
Science can amuse and fascinate us all, but it is engineering that changes the world - - Isaac Asimov
 

Offline MiDi

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one question, my 617 seems to be an earlier device - it does not have the input protection Q311 and the Jumper W303 for (better?) offset trimming of the preamp is missing as well.

In this older instrument the offset trimming then is done only with the trim pot R314 correct?

Does somebody have a schematic for the older version of the device?

Yes, the voltage offset trimming is implemented differently in the earlier revisions - seems rev. F is like rev. E in this regard (see my previous pictures), IIRC it trims the right resistor R336 of JFET Q308 instead.
Added Q311 (R355, R353) & voltage offset trimming are the major differences in the schematic, the layout had some other changes in later revisions, too.
I could not find any schematics for other revisions, but there is no need for.
« Last Edit: August 13, 2021, 07:54:34 pm by MiDi »