Keithley 2010 Repair

[Kleinstein]

The change from +-0.03 to overload is huge for 305.2 step. This does not looks like noise, but more like a relay not closing or similar. Without more information on what the test actually does, the information it gives is pretty limited. My guess would be something like a bad relay contact or maybe a cold solder joint / loose connection or bad front rear switch.

As far as I understand it, it is testing the 4 wire ohms low side. So the question is if the normal 4 wire ohms mode give correct / stable readings.
This would be something like testing a 4 wire short and switch between the 4 wire and 2 wire ohms mode several time and check how stable the zero readings are. Similar switch between front and rear to look for changes.

The 400.x tests should be for the AC section. It is a bit surprosing that the readings are chaging quite a bit. Does one only get a single value, or is there a contineous reading that changes over time ?
Where did your get the partial info on the tests from ?

[KungFuJosh]

2W/4W seems pretty stable, especially 4W.

Front:
-00.010181 2W
-00.000611 4W
-00.009837 2W
-00.000527 4W

Rear:
-00.009351 2W
-00.000359 4W

Front again:
-00.012645 2W
-00.000248 4W

I tested a lot more than that, but only wrote down the more significant changes.

Thanks,
Josh

[KungFuJosh]

The 400.x tests should be for the AC section. It is a bit surprosing that the readings are chaging quite a bit. Does one only get a single value, or is there a contineous reading that changes over time ?

The values written are what is displayed on the screen when you press the shift key after the test is performed. It's always a single value shown.

Where did your get the partial info on the tests from ?

Which partial info? The basic descriptions of the error are shown when the fail summary lists each error. For example, if it says 400.2 on the screen, press shift key, and it will say NON INV PATH on the screen.

That matches the list from the 2000 service manual. Unfortunately, the 2000 manual doesn't have 305.x. As @Cyclotron noted, 305.x wasn't added until a later fw version on the 2010.

Thanks,
Josh

[Kleinstein]

To me the 4 wire ohms part does not look that stable: With a 10 mA test-current in the 10 ohm range 0.1 mohm change still corresponds to 1 µV or 10 ppm of the range.  The specs call for 9 ppm as 24 hour accuracy. So there may be some issue, possibly from not fully warmed up as the drift is in one direction.
The front / back part looks OK for the 2 wire mode. At least in those tests no visible contact problem.
The K2000 plan has an extra buffer for the low side sense part - this could be a source of drift. The low side sense path would be a part to check.


It is interesting to see that the ohms part still looks very much the same as for the K2000. Even the part numbering seems to still largely match. Only the FET for the protection seems to have changes from Q120 to Q153 (TO220 case) - may well be needed for the higher current. There is however also an extra 10 mA current source added somehow.
So the description for the 400.2 test may indeed still work with the 2010.
The question of the inverting / non inverting path should be for the AC input amplifier, not the RMS converter or test signal.
It is a bit unclear what could cause so much variations. Maybe some intial charge in a capacitor or a relay at the AC input.

[KungFuJosh]

Yeah, I guess it really isn't that stable. It was slowly drifting down overall, but maybe a little weird after switching between 4W/2W. It goes back up slightly after switching, and then resumes its descension.

4W: 0.000099 to 0.000112
2W: 0.000896 to 0.000904
10 minutes later:
2W: 0.000833 to 0.000845
Switch to 4W then wait 10 minutes:
4W: 0.000112 to 0.000128
A few more minutes...
4W: 0.000100 to 0.000118
90 minutes later:
4W: 0.000093 to 0.000108

[KungFuJosh]

Short-term stability is about 15uΩ. I'm guessing it will continue to drift down, shifting the 15uΩ window. I'll run TC overnight and see where it's at in in the morning.

Thanks,
Josh

[KungFuJosh]

It drifted down over night as expected, but the short-term stability was worse after running all night.

[Kleinstein]

The curves look like they use something like 10 PLC. AFAIK the specs are for 100 PLC and thus with some additional filtering (like 10x average) and this may bring the noise just to the spec limit. The occasional larger jumps and drift (likely with temperaute) are still not great. Still there is no large error that should trigger a self test to fail.

[KungFuJosh]

Correct, it is 10 PLC. 15 PLC is the max.

So if this isn't the source of the issue, should I follow the suggested path in the repair manual for 400.2, or is there something you suggest I check first?

Thanks,
Josh

[Kleinstein]

I don't think the errors are related as 305.2 is ohms (low side) and 400.2 is about the AC mode.
The ohms mode still seems to work at least reasonable.

The rather different readings for the 400.2 mode looks a bit like a poor contact at K101 or K102. One could check the contact resistance.
Another point would be to test if the low AC ranges work. E.g. use a stable 1 V AC signal for the test and switch between AC and DC  and the 1 V and 100 V range. To operate the relays. A bad contace should also show up in thsese test, at least from time to time.

[KungFuJosh]

I pulled the short a while ago. In DCV, the meter is at -11.906V with the last 3 digits after that unstable with nothing inserted.

[KungFuJosh]

Looks like that's a decent clue. I think your theory about a bad switch or relay is looking more and more likely...now, which one? 🤔

I power cycled the meter to remove the case. The voltage was immediately closer to 0. It didn't change much for a few minutes, so maybe not heat related.

I switched back and forth between 2W & 4W, and then back to DCV. -11.9V is back now.

Power cycled again, started immediately climbing back to -11.9V.

Switched to ACV, audible relay click, now the meter won't go back to -11.9V in DCV.

U139 is still running hot at around 92C. I don't think it's the cause of the issue, but it might be a symptom.

[KungFuJosh]

20 minutes later and it's back to -11.9VDC. I wasn't paying attention, but I heard what sounding like a couple relay clicks on its own.

I clicked through all the mode buttons and back to DCV, and it immediately went back to -11.9V (starts near 0, then -1.x and climbs back up to -11.9V).



Thanks,
Josh