Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

[vindoline]

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

[rastro]

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 

I've done that universal modification also...

[Vgkid]

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.

[MiDi]

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: 



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.



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 
Dremel-time: now EM-Board looks much more like a Keithley or - as we say - a swiss cheese.



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.

[mark03]

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 

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.

[HighVoltage]

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.

[mark03]

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?

[ch_scr]

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.

[JxR]

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.

[mark03]

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.

[JxR]

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.

[mark03]

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.

[HighVoltage]

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.

[balage]

I wish I would have such an easy job with my 237: https://www.eevblog.com/forum/metrology/calibrating-keithley-237-(236238)/msg2391528/#msg2391528

[MiDi]

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 
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

[HighVoltage]

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 

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?

[MiDi]

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?

[MiDi]

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.

[TurboTom]

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

[HighVoltage]

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.

[_Wim_]

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...

[_Wim_]

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!

[_Wim_]

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

[HighVoltage]

Congratulations, nice repair and explanation!

[_Wim_]

This is the result from logging the analog output (2V output) over night (+-12hour log), taking a sample every second using my picoscope.