Hi Folks
First time on here, and first time faced with a Memorymoog to try and rejuvenate. (!) Tuning issues.. So far I have gone through the voice cards replacing pin headers, changing the 16 pin DIL sockets that the ribbon cables connect to, general cleaning off corrosion/flux, cleaning the socket bases, removing all chips checking for bent pins etc, replaced all trimmers, replaced the ribbon cables and DIL plugs.
My first hurdle is at the above stage in the subject - I cannot get the zero trim to 0.000v as mentioned on page 45 of the service manual. Minimum I can get is around + 140 mV.
I’ve swapped a few parts around that area of the DAC - the DAC itself from a broken SDE3000 delay that was lying around, which made no difference, and the 351 op-amp..the trimmer itself is working and providing a +/- 15 V range at the top pf R9.
Can anyone advise me on what might be causing this, and what to focus in on in trying to find the source? Is it something that can be negated by setting the fullscale adjustment to 10v plus this offset? Any advice warmly received. Other than tuning issues the MM seems to be operating ok otherwise.
Thank you all for your time.
I am not owner of a Memorymoog, so just some thoughts while reading the schematics:
A zero adjust normally operates around the middle position of the corresponding trim wiper, so it is very strange that you have a MINIMUM of 140mV.
I would check the voltage at pin 6 of the current2voltage opamp U5 first. This shoud be 0V.
If not: A 0V output of the DAC should be the result of the bit combination of 0b000000000000 at D0 to D11 of the 6012. So I would check the correct bit pattern (meaning all 0V) at those pins first.
Next dependencies are the +REF and -REF voltages. +REF should be exact +10V, -REF is a fraction of it determined by the FULL SCALE voltage divider trim. So I would check those voltages as well, perhaps by changing the FULL SCALE trim.
Last but not least the environment can inject error voltages at the current2voltage opamp U5 (351) output (pin 6). To ensure that this is not the case you can remove R12 (47R) which feeds the 4051 switches and you can remove the U6 opamp (tagged as 393?). Now pin 6 of U5 should be 0V.
If you see 0V there, but not at R67, the 4051 might be broken.
Hope that helps…
Good luck
Carsten
Thank you very much Carsten for looking into the schematics and sharing your insights.
I managed to resolve this issue yesterday: after suspecting the 4051s, I swapped a replacement to each of the 8 positions, I noticed a change in the offset, but it could still not be trimmed to 0V.
So I then went through all the 353 S/H op-amps (which had been changed to TL072 previously). Someone had put in a TL071 in one of the positions…needless to say there was a DC voltage being injected (as you postulated) from this chip - which was also running hot. I also found another TL072 which had way more than the odd mV between its input pins. After replacing these 2 TL072s, all appears well, and it trims to zero.
I made up a test jig/cable to speed through the S/H op-amps and measure the input pin voltages - it’s hard to probe safely with the tight space of having those other boards hinged up - hope this helps someone else in the future if they have similar issues. Just connect to 2 DVMs and quickly move the 16 pin DIL socket along, ‘mounting’ each chip. Once mounted it was not too liable to slipping.
I will get on with the calibration… and I should think will be back here before not too long…!
Cool you solved the problem! 