(Custom-made) DMM (MM834?) Repair attempt - help and advice needed

[kblue]

Greetings!

I've got myself in trouble This seems to be a "custom made" unit without schematics and stuff (or nothing i am aware of). This unit has 4 modules (DC power supply, DMM, WoW&Flutter/Speed monitor, frequency gen (in steps 80 Hz to 14 kHz)).
The whole thing was tinkered with before i got it. The DC power supply seems to be working now. There was one bad filter cap and bad pot for voltage adjustment. But the DMM part is driving me nuts, i'm not an expert in such things and how they (should) work.

I know every pot is kinda messed with and i need some sort of "plan" what to do. I already checked and adjusted the voltages -5, +5, +12 and all seems to be okay now. The "ADC" board, consisting of a C520D (AD2020 copy) and BCD stuff works as expected. I was able to find some documents regarding the C520D with a nearly identical board as the one in this unit and how to "null" it and stuff. So all is fine and dandy here. A 900 mVDC input at pin 11 shows as +900 and -90 mVDC input shows -90 at front. As soon as i connect the ADC-Board back to the bigger board, everything is kinda messed up. I don't think its "messed up" because of broken parts. I know someone in the past was tinkering with every trimmer. If it's working again, this should be a gift for someone starting with electronics. Having a simple DMM, DC power and stuff would be a good start.

Main concern is eg. all the trimmers from VR1 to VR8. For example: VR1, 2, 3 and 4 are all 10k trimmer and connected to the opamps (TL080) pin 1 (N1/COMP) and pin 5 (OFFSET N2) with a pair of 47k resistors. I know all of them are tinkered with.

Area 3 for example, the single pot there changes the mA Output if i am in the Ohms-Range. Right now, pressing "ohms" and "1" (1k says the print at front while in ohms range) the output is 1 mA (1.000) between Hi and Lo.

Any plans how to proceed if the ADC part and voltages seems to be fine? Like "short the inputs from this IC, adjust trimmer XY until output is whatnot". Would be nice to have this custom build up and running again. I also started to work with KiCad to make schematics for this things but this could take some time. +10 V input DC shows as +8,XX and reversing the input still shows +8,XX Ohms is totally messed up and Amps too. I didnt even bothered with the AC-part.

In short:
- i know for a fact, every pot was tinkered with
- Area 5 ADC (disconnected from the big board) works just fine after some adjustments (was able to find documents with the same board)
- supply voltages are fine
- Area 1 to 4 every pot was tinkered with, so i guess the 4 TL080 (Area 1) and TAA861A IC (Area 2) are out of wack

Sorry if its a bit confusing. That's the problem having no schematics and not being an expert - but i try my best and help would be very nice. If it's not worth the time, that's okay too.

Best wishes and thanks in advance!

[Kleinstein]

A Ohms mode test current of 1 mA looks reasonable. The different ranges should differ in the test current and the voltage range used (e.g. 1 V or 100 mV).

The pot at the offest trim points would be for the offset of the OP-amps. Ideally these should not be used to fix other offsets.
Having extra resistors in series can be a way to reduce the adjustment range. It is not common because of the extra effort.
How to do the offset adjustments really depends on the circuit hard to tell without the schematics. Usually the adjustment should be first the offsets and only than the gain / scale factors.

The DC ranges 1 V / 100 mV should be the basis and these should be adjusted before the other ranges.

[kblue]

Hello there!

Thanks for the reply! Working on some sort of schematics but without the "switches" so its just the "0.1" path and connections. Thing is, i don't know how to make "custom switches" in KiCad.