Hi all, me again. I am really appreciative of everyones help with things so far. Unfortunately one of my problems got worse!
The machine has never liked sitting since trying to resurrect it, if I leave it overnight unplugged, the next day it will often fail to start without fidding.
I could never figure out what exactly caused it to happen, but removing cards seemed to get it to boot, and after lots of switching it off, unplugging drives from the disk controller, plugging different ones in, swapping things around, etc, it would boot with the disk controller card inserted again. And with the video card, I could unplug it, it would boot, then I could turn it back off, plug it in, and turn it back on again. The disk controller was more commonly the "issue" though.
However it is now doing that with no expansion cards installed, I can't get it to boot at all. I even took the language card out and put a RAM chip from it in the socket where it had previously plugged into the board.
I removed and re-inserted every chip I could get access to, all the RAM chips, the ROMs, the CPU, and all the others, aside from the ones under the keyboard which I can't access without taking the machine fully apart.
If I remove the CPU and power the machine on it behaves exactly the same...
I feel like this might have been my original issue where the cards were causing it not to boot, so it was the machine itself not the cards. (the Videx issue remains though, of course)
Any ideas? Pulling out my hair lol. The RAM including the language card, and all the ROM chips tested OK a few days ago.