Keithley 2000 with intermittent error on built-in self-test (400.2)

[Donny]

Hello there!

I'd like to ask the Keithley DMM specialists if you have a hint for me on my Keithley 2000 DMM.

The device is from 1997.
Caps have been replaced this year. Everything under them was dry and clean, no problems here.
An ESR-meter even showed acceptable values for C and ESR, after 23 years, not bad :-)
It has been calibrated by a service company like a year ago.

My only concern is, that _sometimes_ on built-in self-test, the error 400.2 appears.
Meaning: Testing 5 times in a row = 3 results with no errors, 2 results with 400.2 come up.
Results are independent from run-time. Happens cold, happens warmed-up (on for hours).

Precision: Almost all measures are bang-on, I compared them e.g. to my DMM6500.
The only thing that _may_ be, is, the AC volt and ACI, which are a little bit out of spec (as far as I can say), which in particular doesn't matter (to me) that much.

=> So, live with it, or chase down some rabbit-hole? :-)

Many thanks in advance for ideas and opinions!

Regards,
Sebastian

[picburner]

Attached is the response of the repair manual regarding the 400.2 error.
What firmware release does the instrument have?

[Donny]

Thanks for your answer!
-> Firmware-Version: Should be "A06" which is shown on the left side on boot-up.
(A02 on the right-side of the display).

[picburner]

I have a K2010 and sometimes it makes this error too since I upgraded from A10 to A15 firmware.
This error had never appeared before.
But this is not the your case as you haven't done any firmware upgrades.

When you replaced the capacitors, did you clean the pcb very, very well?
Several washing steps are required and then is better dry the pcb in a dryer.
Maybe you already know it but the pcb should only be touched with gloves.

And the most important thing: did you have this error before changing the capacitors too?

[Donny]

Thanks for your insight in that matter!
The error was there before exchanging the caps.

I think I was very careful and did the appropriate (normal) cleaning, but,
I did not know that it is advised to touch this pcb only with gloves.

For the moment this is what I'll probably do for now: As I have a pending order at Mouser for some project anyway,
I will 'blindly' order the only other 'wear part' in this device I can see, the 3 relays (they are cheap and available).
Then I'll do some _extensive_ cleaning and put it together again.

Will post an update in a couple of days.

[Kleinstein]

It is possible, though rather unlikely that a relays (K101) are cause of the error. The 400.2 self test is supposed to be for the AC part, so it could be related to the AC being possibly slightly out.
So one could to a few more checks on the AC part: e.g. check the signal during the 400.2 self test with a scope. Follow an AC signal (e.g. 1 kHz square) in normal AC mode.

The other part that seem to be involved is the higher ohms current source. So maybe to a check on high resistors (e.g. 10 M) too.

[Donny]

Great input. Makes sense (reading the repair manual excerpt). Will do that and post my results!