When you find yourself in a hole, stop digging - that was the good advice that I did not take...
I needed parts for the MV4 repair so I bought this MVS-MV2FS for $20 - seller was very clear. "Does not work, parts only". In hindsight this was kind of a stupid thing for me to do, But it turned out OK.
Got it home and powered it up just for fun before I started stripping it for parts, and it worked
It didnt work very consistently; sometimes it would boot and sometimes it would not, sometimes the sound would work and sometimes it wouldnt
So today turned into MVS-MV2FS repair day instead!
It also had these strange graphical glitches. At first I thought they were "cart slot needs a clean" glitches. But they were not.
And then I opened it up and found this horror show
As
@stj mentioned earlier, it's possible for this kind of damage to occur even when the battery is apparently ok. The battery was sparkling and imaculate - everything around it was an acid-ravaged wasteland.
At this point I was starting to doubt the wisdom of continuing with the repair so I didnt spend much time cleaning, but I did beep out all the traces to see where the problems were. The back of the board has some track rot that I decided to deal with later when I knew if it was repairable or not. The front had two broken traces coming out of ZM02 which does clock stuff. The graphical glitches looked very "bad clock" so I ran some bodge wires back to where they belonged on the 257
Then I fired it up, smug and certain that I had fixed the problem
Aaaaaand nothing - would not even boot. Then I left it alone for half an hour and it did boot to jailbars, then I moved the carts around and it did a slow watchdog, then I moved them around again and it worked perfectly but with bad sound.
Intermittent weirdness normally = bad physical joint somewhere
The Universe Bios was consistent in not working - but the Diagnostic Bios kept giving bad backup memory errors, probably more likley to be bad addressing for the backup memory (thanks
@xodaraP!) - so I started poking around HC32 which does lots of address stuff - thanks to
@Raph_friend and
@Kujako for the tips in this thread
https://www.arcade-projects.com/threads/mv1c-backup-ram-error.13969/
Eventually I traced the fault to the little 22k resistor that sits between the HC32 and +12v - on the outside it looked perfect, but the legs on the resistor had corroded away - so the resistor was only resisting intermittently, HC32 was only addressing intermittently, and that was my intermittent weirdness for backup ram, sound, and boot
So I salvaged a 22k resistor off the MV4 board (ironically) and now the MV2 works perfectly. Graphics Glitches cleared, Sound works, boots every time, all diagnostic tests passed
Now I just have to go back, wash with vinegar to arrest the corrosion, clean up all the track rot, remove the bodge wires and re-build the traces and generally tidy up. I've been practising that on old waste boards, still got a way to go...
Success! Thank you to everyone for sharing your knowledge and subject matter expertise. I was really thankful to everyone that you had taught me what I needed to be able to complete the MV2 repair myself without having to ask a single queston.
Still no progress on the MV4... I'll get to work on that again when I get another source of spare parts