Author Topic: Electrometer output stage , keithley 6517  (Read 24981 times)

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Offline TiN

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Re: Electrometer output stage , keithley 6517
« Reply #50 on: April 26, 2016, 04:34:19 am »
 :-+
Firmware B04? I have only C-versions on my collection. Perhaps upload for addition?
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Offline plesa

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Re: Electrometer output stage , keithley 6517
« Reply #51 on: April 26, 2016, 08:29:48 pm »
:-+
Firmware B04? I have only C-versions on my collection. Perhaps upload for addition?

It was typo, it has been upgraded to C04.
Today during calibration found that ranges 2nA, 200pA and 20pA are off by 0-75%, upper ranges are ok within calibration limits..
+2nA  1.5nA
-2nA  -1.4nA
+200pA  47pA
-200pA  -200pA
+20pA  10pA
-20pA  -15.7pA
Voltage measurement is spot on.
Needs to think about what is root cause, if some DG411, LMC6001 or something else.

Update - It was faulty cable :)
« Last Edit: April 26, 2016, 08:42:26 pm by plesa »
 

Offline Smith

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Re: Electrometer output stage , keithley 6517
« Reply #52 on: May 10, 2016, 06:14:03 pm »
I had recently replaced my display, and last week I cleaned the input on the connector and PCB after it had excesive noise and offset after being stored for a while. I cleaned in with IPA and let it dry in a dry cabinet. After that I had let it run for about 6 houres to stabilize the device and calibrate the offset.

After about 5 houres the display turned off. It was smelling like hot electronics. All voltages seem fine, except the VFD voltages, and the device reacted to the buttons. So it had to be the VFD module, or the PSU. Replaced the VFD with the old one, and the display worked. The two transistors driving the transformer where toast (BC868). It seemed like the 60V VFD drive voltage was overloading the PSU. I quickly reverse engineered the VFD module PSU, and found some VFD drivers, caps and resistors on the 60V rail. Then I unmounted parts until I found the problem. .Turned out C912, a 100V 2,2uF cap turned bad after less than 8 houres of use! It had a DC resistance of 450Ohm. I replaced it with the same cap of the old VFD board, and everything booted right up.
« Last Edit: May 10, 2016, 08:07:18 pm by Smith »
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Offline TiN

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Re: Electrometer output stage , keithley 6517
« Reply #53 on: May 12, 2016, 01:43:30 pm »
Interesting catch.
That trace from Q901 going to xfrmr looks unhappy too.
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Offline Smith

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Re: Electrometer output stage , keithley 6517
« Reply #54 on: May 12, 2016, 06:03:44 pm »
The trace is discoulored indeed, but there was no real damage. It's running for a few hours again to see if everything is fine, and to check the input. All seems fine, and the unit stays a lot cooler now. The input seems more stable too.
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Offline plesa

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Re: Electrometer output stage , keithley 6517
« Reply #55 on: May 13, 2016, 05:20:23 am »
Someone interested in repair not working 6517A from ebay?

http://www.ebay.com/itm/KEITHLEY-6517A-ELECTROMETER-HIGH-RESISTANCE-METER-/111994284036?

current bid is $350
 

Offline Smith

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Re: Electrometer output stage , keithley 6517
« Reply #56 on: May 13, 2016, 06:02:46 am »
It's getting more expensive. These are nice machines, but the service manual is quite limited. Looking at this unit is seems to boot fine, it just shows overflow on voltage setting. Current will probably do the same. My guess is a dirty connector or PCB. With such an input impedance the smallest leak will generate an overflow.
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Offline plesa

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Re: Electrometer output stage , keithley 6517
« Reply #57 on: May 13, 2016, 06:34:03 pm »
It's getting more expensive. These are nice machines, but the service manual is quite limited. Looking at this unit is seems to boot fine, it just shows overflow on voltage setting. Current will probably do the same. My guess is a dirty connector or PCB. With such an input impedance the smallest leak will generate an overflow.

Yes, the unit seems to be in good shape just upgrade firmware and repair input stage :) But actual price is too high.
I will check preamplifier output, This can help identify problem in input stage or in ADC.
With high probability it is related to input stage- check input protection (LS313 ) and opamp LMC6001.
If both are OK, I will check the 2N3904/2N3906 transistors in input stage.
 

Offline Smith

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Re: Electrometer output stage , keithley 6517
« Reply #58 on: May 22, 2016, 01:59:42 pm »
This may come in handy: I recently had troubles with a 6517B (rental unit this time). This seems to be the same problem I have heard a lot of complaints about.  I was measuring cable leakage, until at one time the AUTO setting gave me the default not auto-ranging right. If I remember the 20nA range and below. The current displayed seemed right, but it was not very stable. It was reading about 2nA on the 200nA range. But all ranges below gave me an overload. Shortly I was afraid I fried the input as I just had a short on the 1kV output.

I connected my calibration resistor (with good coax connections) and all ranges where working OK again. I took out the MM, and connected it to the rear pre-amp output. DC ranges where good, both with the cable, and the resistor. The cable measurements were not very stable though. I thought maybe it's AC related. On most ranges using the resistor it was less than 5VAC. With the cable it was just above that in some ranges. Then I switched to the 20nA and 2nA range, I had an overload warning on the Fluke 87V! Eventually it settled to about 180-200VAC. So AC was the culprit of the overload messages.
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Offline PREDATOR

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Re: Electrometer output stage , keithley 6517
« Reply #59 on: February 13, 2017, 11:07:11 am »
Hello!

I'm new here and I see a lot of skilled people, maybe someone could help me with a very strange problem that I have with Keithley 6517A.
First of all, I'm not an electronics engineer, so my knowledge in this area is limited.
I'm using this equipment to measure samples with very high electrical resistance (>100 GOhm). A few months ago I had to measure a sample with very dynamic resistance (ranging from hundreds of GOhm to several KOhm), so I was using autorange function.
I saw that when range changes from 200 pA to 2 nA, the displayed value is 10 times smaller (sometimes even negative) than it should be, so i decided to do a manual masurement. I sourced +10 V from the internal V-Source and measured the current through a 85 GOhm fixed resistor using Keithley low noise triax cable with Meter connect ON (it internally connects V-Source LOW to Ammeter LOW). The results were:

200 pA range -> +117 pA
2 nA range -> negative or 10 times lower current value (11.7 pA)
20 nA range -> +117pA
200 nA range -> +117 pA

Then, using the 85 GOhm resistor, I inserted one end of the resistor in V-Source High, and the other one (aprox. 10 cm long) in input High. Sourcing +10 V again, it measured:

200 pA range -> +117 pA
2 nA range -> +117 pA
20 nA range -> +117pA
200 nA range -> +117 pA

I extended the part that goes into the input High by 10 cm (20 cm total length) and measured again, using the same settings:

200 pA range -> +117 pA
2 nA range -> negative or 10 times lower current value (11.7 pA)
20 nA range -> +117pA
200 nA range -> +117 pA

It seems that if the input High connector is longer than 1x cm (no matter how good is it screened), the ammeter doesn't measure correctly only on 2 nA range, the other ranges work great. This problem doesn't always show up, sometimes the electrometer works fine, other times it doesn't.
I tried resetting the instrument using default bench settings, using digital filter, different integration periods (measurement speed), nothing seems to work. Any help would be much aprreciated.
I'm sorry for the bad english, I'm not a native english speaker, I hope I made myself understood.

Best regards,
Ovidiu
 

Online Kleinstein

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Re: Electrometer output stage , keithley 6517
« Reply #60 on: February 13, 2017, 09:40:18 pm »
Form the schematics shown so far, the positive input terminal works like the input of an trans-impedance amplifier, when measuring current. If there are no countermeasures, such an TIA might start oscillating, if there is to much capacitance on the input.

Usually there are internal caps parallel to the feedback resistors to make it less / not sensitive to input capacitance - so maybe there is a broken contact on the capacitor used in the 2 nA range.
 

Offline Smith

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Re: Electrometer output stage , keithley 6517
« Reply #61 on: February 14, 2017, 06:50:39 am »
Well, hearing you are not an electronics engineer: How about your measurement enviroment? Any heavy equipment / strong lights within several meters of your measurement? The 6517 is sensitive to noise, specially in the lower levels (read my previous posts).  Try to shut down some lights/computers/power supplies or try your measurement a few meters away.
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Offline PREDATOR

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Re: Electrometer output stage , keithley 6517
« Reply #62 on: February 17, 2017, 12:58:03 pm »
Hey guys! I would like to thank you for the answers.
This instrument is driving me crazy. It refused to work in the past few days in our lab on 2 nA range, so I decided to isolate it from the other equipments :1 DMM, 4 PCs, 7 lcd monitors, one  bench power supply, a fridge, 2 air conditioners, 2 lab ovens, 2 printers, one network switch, as Smith suggested.
I removed it from the lab and I plugged it in a power socket located many meters away from any electronic equipment and it worked like a charm. I returned to the lab, plugged it back in and worked again.  :-DD  :-//  :scared:  :wtf:  :bullshit:  :horse:
I guess it must be a bad solder joint...
« Last Edit: February 17, 2017, 01:01:52 pm by PREDATOR »
 

Offline PREDATOR

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Re: Electrometer output stage , keithley 6517
« Reply #63 on: December 12, 2017, 10:16:55 am »
UPDATE: I used the unit for current measurements, by-passing the 2 nA range and it worked like a charm until yesterday, when the V-Source refused to turn on.
If I set the voltage to any value and press the operate button, the operating light doesn't turn on and the output voltage is - x.xxx mV.  :rant:
I also tried to send the command through GPIB, then interrogate the instrument and it's saying that the V-Source is turned on.  :blah:
Well done Keithley, well done!  :clap: :-BROKE :horse: :-DD
« Last Edit: December 12, 2017, 10:20:54 am by PREDATOR »
 

Offline voltampere

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Re: Electrometer output stage , keithley 6517
« Reply #64 on: January 22, 2019, 05:01:37 pm »
Nope, I had contact with Keithley, and C03 is their final FW. C05 didn't work and was never meant to be released as far as they know. The device still has some problems, but it doesn't look like they will ever be solved

I am about to repair a Keithley 6517A that is running on firmware C05.
I am not absolutely sure that is was delivered with this version but i guess so.

The thread is here https://www.eevblog.com/forum/repair/repair-of-a-keithley-6517a-drifting-v-offset-and-i-bias-offset/

I would be happy if you 6517 ( and 617 ) experts could take a look at my problem.

Thanks

Joerg
 


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