Fluke 335A DC voltage standard repair

[alan ford]

Hello everyone

I'm trying to repair this calibrator so I was wandering if anyone encountered similar problem: the output on all ranges all settings is -1.234 V - it doesn't change at all.
This is when operating; in standby mode will be zero.

Work done so far : a bunch of electrolytic caps were changed - some of them by someone else, before I opened it , some of them by me - I changed everything that was below 80% and some more so about 90 % of electrolytic caps are changed. The supply voltages are OK ; the reference voltage is OK.
Chopper board and preregulator board are matching 335D schematics so it seems they were changed already using boards from D . I had to change CR1 diode on chopper board ( it was not the diode anymore - it was resistance in both directions ) Some other board look they were already changed by someone else - the differential amp is different vintage; same as null detector amplifier.

Any thoughts and advice would be greatly appreciated, Thank you

[coromonadalix]

https://www.eevblog.com/forum/repair/fluke-335a-dc-voltage-standard/

May help a little

ok  found the 335D service manual,  no schematics,  anyone ?

The section 2 and 3  can help

Fluke 332D service manual : https://doc.xdevs.com/docs/Fluke/332D/332d____imeng0100.pdf


EDIT
Maybe this thread too
https://www.eevblog.com/forum/testgear/fluke-332d-dc-voltage-standard-repair/

[alan ford]

Well thanks
It seems my problem is related to the chopper board - I do have signal coming in to it, but nothing getting out - the service manual shows test point at the output of op amp being in range of 0.4 Vpp and I have about 40 mVpp so next step will be to check components around op amp on the chopper board

[dacman]

Is the high voltage relay in the back being energized?  (It's the one with the exposed contacts.)

[alan ford]

At the moment I put this repair on back burner since there are some other things to do ( also I ordered IC from chopper board - MC 1709 - waiting for it  to arrive ) so I can't  try it but I recall a click from relay when switching from standby to operate so I'd say it's working

[coromonadalix]

 On my 343 thru the previous links, i may have found the bugs,  i think i have a stuck relay,   i recall if i force one of them i can go higher than 100 volts, but some resistors heat pretty fast.

And maybe the other on the output (the bleeder circuit) may give some problems too,  i have a surge on the 1kv selection, it goes into current limit.  Tested all the output parts and impedances, everything match the specs ??