Spent a very pleasant day working on this - then got sidetracked and re-capped the whole thing - Other than the one
Anyhoo Progress as follows
Cleaning went well, combination of IPA and Acetone
Removed the battery as suggested - it had just started to leak, hidden under the battery, thanks for the heads up - I have microscoped and multimetered this area carefully and there is no trace damage that I can find
Visual inspection didnt show any damage - will keep inspecting
Warmed it up with the hair drier for a bit to see if I could coax some life out of it - sadly no
Started checking continuity with multimeter which is where I left it for the day
At this point I realised I was going to have to narrow down my search a bit rather than just randomly beeping at stuff all over the board, thats going to take weeks.
I did a bit more reading on watchdog errors and the click of death - the best explanation that I found was here
http://newlifegames.net/nlg/index.php?topic=152.30;wap2
Bad CPU - unlikley
Bad Work SRAM - possible
Bad Backup SRAM - highly likley - given that both the stock bios AND the Universe bios complained about this 2 hours before it stopped working
Missing control signals on Work or Backup SRAM - somewhat likley
Shorted 74AS245 ICs buffering program ROMs on top board to CPU on bottom - top board is off so I cant see that being a problem
Short on address bus - possible
Short on data bus - possible
Missing address or data signals to chips - possible
So bad backup ram, or the traces/control signals associated with it are my prime suspects
This thread is absolutely halarious by the way - I'll roll the HC32 into the mix as well
https://www.arcade-projects.com/threads/mv1c-backup-ram-error.13969/
So I did some digging on bad SRAM and found this about diagnosing 4-slot SRAM locations
https://www.arcadecomponents.com/repairlogs.html#84069
SRAM is expressed as UPPER UPPER LOWER LOWER
So if backup RAM written = 55 55 and BACKUP RAM read = D5 55
55 is not equal to D5 so upper is bad
55 is equal to 55 so lower is ok
Backup RAM locations are given as the following on the MV4
Upper backup RAM = D9
Lower backup RAM = F9
Unfortunately the board layout on the MV4 is not the same as the MV4FT, but I'm guessing its the one at the bottom
@xodaraP Thanks for the pointers that you gave over on the "post new stuff" thread - this is starting to look a lot like a RAM problem, exactly as you predicted
So my next steps from here will be
0) bridge J2 to disable the watchdog to see what happens - this may be unwise.
1) Probe around those RAM traces some more to see if I can find any breaks
1a) either wait for my logic probe to arrive or learn how to use my osilliscope as a logic probe
2) reflow that LSPC2 (I'm not super hopeful about this, but it's good practise)
3) order parts to replace the Backup RAM and HC32
4) potentially order parts for the Palette ram, and Fast video ram just in case, depending on how much they cost