Yes, I'm stuck here in
Antarctica for the next 9 months or so; I expect to resume Amiga repair work when I return to NZ in November. It's kind of sad being stuck here with no Amigas, just annoying windows boxes with no useful software and a 32kbps satellite data link. Hence WinUAE gets a lot of use, no idea what I'd do without PageStream, CubicIDE, etc.
A test ROM could simple send it's output on the serial port.
Serial - OK
CPU - OK
ChipRAM - OK
Paula - OK
Denise - OK
etc..
Agreed, that would be incredibly helpful in the right situation. Unfortunately the UART (the bit that makes the serial port work) lives in Paula. And for Paula to work, it first needs valid clocks that are divided and generated by both Alice and Lisa, so the bulk of the video sub-system needs to be working. And Paula lives on the other half of the Amiga sub-system on the chip memory bus side. And to talk to that, the data bridge needs to be completely working in both directions (Bridgette in the A4000). And to talk to that, the address decoder (Gary) needs to be working. And to talk to that, the entire CPU subsystem needs to be working. And... hey that's most of the computer already.
The best ChipRAM diagnostic is to remove the SIMM. As we all know, quite early on in the standard ROM software, it does a basic ChipRAM write/read test and turns the screen green if it's unhappy. So if that's not working, it's probably a simple but fundamental problem causing everything not to work. Unfortunately because the operation of any one sub-system relies on most other sub-systems to be working, you can chase yourself in circles looking for the fault. Sometimes the best approach is to compare various signals with a known working system. This is extremely time consuming, but I've repaired many difficult non-booting faults in this way.
The other way is to replace various bits with known good bits - also known as the 'shotgun' approach. The problems there are that you need a source of known good bits, and you need to make sure all of your soldering work is 100% else you'll introduce new problems which simply compond with the original one. Also, this doesn't help at all in finding PCB related faults, which are fortunately rare except in case of battery and capacitor corrosion - unfortunately which are common in the A4000.
Sorry for possibly sounding negative, but that's the harsh reality of the situation.
One possibly useful piece of basic diagnostic ROM software you could develop if you had time, would be to toggle a pin in the Centronics port. This only requires a working CPU sub-system, address decoder, CIA U350 and most system clocks. i.e. the computer would only need to be about two-thirds working properly for the diagnostic software to run.
But seriously, if there's been any form of battery corrosion, start with removing both U177 and U178 - the computer doesn't need them to boot. Replace both U975 and U976 and verify the integrity of all connections to these shift registers (primarily the clock and data lines) are all correct. It's a bit of a blind stab in the dark, but this area is a very common cause of non-booting faults in the event of battery corrosion. Even if everything visually "looks" OK.
Also check the analogue bias voltage is around +2.4V on U400/Paula pins 33 and 34. Capacitor corrosion does bad things to the op-amp U402 which can ruin the bias in the entire analogue stage which can sometimes cause Paula to do some very unusual things.
