Keithley 175A not working

[MikeK]

But if I lift one leg of the 5.1V Zener and the op-amp output is zero it tells me that the op-amp is indeed faulty.

[MikeK]

I lifted the 5.1V Zener and the op-amp output goes to -3.4V, so it must have been providing some current.  I measured the current through the 5.1V Zener at 20mA.

[Dr. Frank]

I lifted the 5.1V Zener and the op-amp output goes to -3.4V, so it must have been providing some current.  I measured the current through the 5.1V Zener at 20mA.

See? That does not give a clear conclusion.
Either one of these 3 other ICs draws too much current, or the TLC271 is defect on its output stage.
It should deliver -30mA at least.
I would remove the TLC271 and measure the supply current for the other ICs, first.
Frank

[MikeK]

I was assuming that if the 3 other ICs where drawing too much that the 5.1V Zener current would be much higher.  I'll pull the op-amp and test it.

[MikeK]

I haven't tested the op-amp, but I may not need to.  With it removed I provided 50mA at the op-amp output point (pin 6) and it still took it.  That is, the other IC's are pulling at least 50mA, which is way too much.  More searching...

[MikeK]

I ran it for 10 minutes supplying 40mA and the three supplied IC's (U101, U102, U104) only got a couple degrees above room temperature.  Using an inexpensive infrared thermometer.  U101 was just a couple degrees above the other two.  Not sure if that was enough current to see the culprit heat up.  Temps higher still were seen in the area of Q109-Q113, but those don't appear involved in the -7V supply.

[MikeK]

Found something promising.  When I raised the -7V rail current to 70mA, op-amp U105 (another TLC271) heats up to 110F (43C).  The other 3 IC's powered by the -7V rail stay at room temperature.  The schematic says that U105's negative rail is provided by the -7V line!  And a resistance check agrees with less than 0.1ohms between the two points.  U105 is in the bottom-right corner of the image.

[Dr. Frank]

Good find, I missed that "-7" label.
So then you got the stinker, probably.

Check the B-C diode of the transistor behind it, Q10X (unreadable). A fail is quite unlikely , but who knows.
Frank

[MikeK]

I pulled U105 and the current draw dropped to 5mA.  Cool.  So what could have caused it to fail?  The connected transistor (Q10x, 2N3904) tests fine, but I'm tempted to replace it as well.  I tested the op-amps inputs for resistance to each of the other pins and they're all in the megohms range.

[Kleinstein]

Failure of U105 is likely not from the output side, but more from the input, e.g. from ESD at the input.
Another possiblity would be some kind of latch-up, possibly from high energy radiation ( a muon hit or even stronger)  - it is rare but can happen. Odd power supply sequencing (e.g. fast on/off cycles) may also contribute.

[Dr. Frank]

I pulled U105 and the current draw dropped to 5mA.  Cool.  So what could have caused it to fail?  The connected transistor (Q10x, 2N3904) tests fine, but I'm tempted to replace it as well.  I tested the op-amps inputs for resistance to each of the other pins and they're all in the megohms range.

Therefore, the OpAmp broke down, of course not from its inputs, but from internal stages, because the negative supply draws too much current.
This simply could happen from a timely deterioration.

ESD, EOS and energy particles are extremely unlikely, compared to that.
 
Please, simply replace this OpAmp and test the completed circuit, first.

Frank   

[MikeK]

Yes, I ordered the op-amp from Digi-Key and should have it by Wednesday.  I plan to install it and run the meter for a couple of hours before reinstalling U103 and the 5.1V Zener.  I can set my power supply to trip and turn off at 30mA.  That way I can see if something else was causing U105 to fail.

Very excited to see this work.  Thanks very much for the help, guys.  Without the help I would have replaced the 6.4V reference Zener, saw that it didn't fix anything, and then given up.  Definitely learned some things with this.

[MikeK]

I noticed that Keithley changed the part spec for U105 between the 1983 and 1990 manuals.  I wonder if this had something to do with it failing.

[Kleinstein]

The LF411 should be OK as well. The TLC271 would be a BOM simplification and lower power.  Unless they use way more than 200 ms integration at a time the noise should not be a real problem, as the resolution of the meter is not that great.
I don't think either of the OPs has an expecially high failure rate - both should be OK. Sometimes it is just bad luck.

[MikeK]

My parts won't arrive today so I did some resistance measurements over breakfast, comparing the dead TLC271 (U105) to the good [pulled] TLC271 (U103).  I think the damage happend via GND?

[MikeK]

I replaced the parts today and it's working.  I tested several ranges in Volts and Amps and it appears to be functioning as it should.  Great!  Thanks again, guys.

[MikeK]

One thing I didn't mention before was this suspicious capacitor.  C119.  It has a black sticker on top for some reason.  It's also much different than the schematic/BOM call for.  BOM says 4.7uF 25V, but the installed part is 1000uF (!) and only 16V.  The former owner's quick fix?  Or is the larger cap needed because of the switch?

[Kleinstein]

16 V voltage rating should be enough, as there is a 12 V zener to limit the voltage.  For my feeling 4.7 µF may be a bit on the low side, but 1000 µF is quite large. It could add some stress to the switch from a current pulse.  A somewhat larger cap may be needed if a weak battery is used. More capacitance could also help of the other 1500 µF capacitor in the other side of the switch is weal.