Hi folks
Am nearing the end of a MM renovation and there is one final annoying bug - the dreaded pot edit issue.
You will be turning a pot and seeing its corresponding value change in the display, only for it to suddenly stop increasing, as it is now displaying the value from another pot. Until you find the ‘rogue’ pot that (I assume) the MM thinks is being edited, you are stuck with a garbled value being displayed. Whilst things are stuck in this state, you can still control the synth’s parameters, but you get the annoying display thing happening.
The issue seems to be affecting certain pots, and some pots never become rogue. The synth will behave fine for 30 mins or so, and then it strikes. It has very rarely powered up with a garbled display.
Things I have tried:
FSB to separate the analog/digital grounds on the RSC
FSB that ensures the RSC 4051s are receiving a solid +15V connection
FSB that adds 1 nF cap on the DMUX board on U6 pin 3 to analog ground
All boards reworked to clean up flux, gunk, dirt etc; sockets replaced for all DIL 16 pin headers; all 0.1mm spaced headers replaced; all electrolytic caps replaced; all ribbon cables remade and tested
Swapping known good 4051s and associated buffers
Pots cleaned, resoldered
10V supply and ground ok on both LSC and RSC
Reseating RAM, PROMS etc
The tuning and autotune is solid however (thank goodness).
Just wondered if anyone has had this happen on their MM, and if so, is there anything that you could suggest to try and probe/replace etc to try and narrow down what is likely causing the issue?
Thanks very much for reading,
Tim
I am not owner of an MM, but a friend is currently cloning one and I helped him to analyze the circuits and logics. So just some thoughts:
The MM follows a standard Z80 microcomputer architecture (in the far far past I developed own Z80 boards). So within the MM the data from all peripheries (keyboard, triggers, pot values etc.) are communicated to the Z80 MCU via secondary data busses, isolated by bus transcievers, latches and logic ICs adressed by the address bus in combination of /RD and /IORQ.
According to the problem you describe I assume that there is a faulty 4051 or latch which connects different pots to the MCU the same time. So I would start checking / replacing all pot selecting logic ICs, especially the latches, bus transceivers and decoders (4051).
I recommend that as I have similar problems with faulty bus transceivers in my own current microcontroller project which (in my case) lets the keyboard play “ghost” notes.
And in my case it is a timing issue as well, but the other way round. My keyboard interface calms down after some time when the bus transceivers warm up, so warming up might cause problems.
Thanks very much analogmonster.
Yes I think it is now at the stage where I must place a mass order for 4*** and we 74LS*** chips to start a slow and ordered swap out procedure.
One interesting thing is that when you are turning a pot and have realised that the display is not incrementing correctly, it is always the next pot along that is the ‘rogue’ pot, which when you turn that, it resolves the issue. I don’t possess enough insight into the operation of the system as a whole to know what this might mean, however…? Also I am now just getting the issue on the RSC, whereas before it could also be on the LSC.
I noticed in the FSBs that Moog were particular about the fact that “flux leakage” can cause pot editing, so I think another check and clean of the RSC might also be worth doing.
Will report back if I get anywhere…
update - have found the culprit…U5 (LF351) on the DMUX board was causing the issue. Replaced and all pot editing now working correctly.
Am now facing another issue with slightly dimmed LEDs illuminating when the associated button is not pressed…will post that to a new thread once I’ve gathered a bit more info.
you found it!