GadgetUK
Ace Ghost Pilot
- Joined
- Sep 27, 2013
- Posts
- 1,323
I've got 3 boards here that I think have bad Palette RAM and the neo-buff associated, and it cause the same sort of behaviour you are seeing. Although I've not measured for shorts like you did there - so maybe you have a faulty MGA? With the Palette neo-buf it seems to totally kill the system as the neo-buf interferes with the 68K databus. The other thing I've found on the 8 boards I am working with (all 1A/1AX boards) is that often more than one neo-buf chip fails. I've seen the one you removed (Z80 interface) cause the 68K to watchdog with a diag bios or official bios. These boards are a pain to work on due to all the damn neo-buf chips. 2 or 3 of the boards have issues with nearly every neo-buf chip on them. You could remove the one from GA11 (used by the PROG interface to 68K) and fit it into the Palette spot to see if behaviour changes, but I've found that when that dies in that spot it can take the Palette RAM out (or vice versa). You can also btw remove GA11, GA7, GA8, and GA9 to rule them out as interfering with the 68K databus - the board should boot without them. But maybe the neo-bufs aren't your problem?!?
EDIT: That exact blue or green screen is what I've seen when Palette RAMs have died AND associated neo-buf.
Do you have the M1 test cart to go with diag ROM btw? If your system is booting, and the controller neo-buf is working, hold down D when booting and listen for sound, because that can provide a clue - if you hear the sound test being carried out despite a solid colour on the screen.
EDIT: That exact blue or green screen is what I've seen when Palette RAMs have died AND associated neo-buf.
Do you have the M1 test cart to go with diag ROM btw? If your system is booting, and the controller neo-buf is working, hold down D when booting and listen for sound, because that can provide a clue - if you hear the sound test being carried out despite a solid colour on the screen.
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