Red screen on XEGS

mikro · June 7, 2020

I'm well aware of other threads on this topic, just wanted to ask something more specific. I've replaced PSU and RAM, installed the stereo + U1MB board from Lotharek a couple of years ago. Everything worked nicely until few days ago when I started seeing an alarming issue - when I was about to switch back to QMEG (SELECT+RESET) / U1MB BIOS (HELP+RESET), the computer would completely lost raster information, usually just switch to some colour. Pressing RESET or anything else didn't help. Sometimes it "crashed" into this state even when just being in QMEG. What did help however was to switch the unit off, wait random amount of seconds and switch it on again.

Ironically, the evening I had decided to look into this I got into the red screen state - I was running some demo, it ran for a good half an hour and again - I wanted to switch to QMEG, boom, red screen. And this time, switching it off didn't help even when switched on the next day.

I have suspected it must have something to do with ROM access as I find it very hard to believe that some IC would refuse to work just in the exact moment when I switched back to QMEG/BIOS. I have two theories:

for some odd reason, something went wrong with U1MB as it is emulating both the MMU and OS; however it is really strange that it would survive even multiple flashing operations and yet crashed when switching to its BIOS (randomly)
everything "new" is working (U1MB = ROM + MMU), RAM (don't get hot, replaced in 2016), PSU (new one) and CPU & ANTIC & GTIA is working as well (as I could run the last demo without any issue before jumping back to OS) so that leaves something else but what?

Can you think of a simple method how to verify/reject any of those theories?

Edited June 7, 2020 by mikro