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That sounds alot like a bad capacitor or bad caps in the PSU
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It should have been the reason, but unfortunately voltages seem OK, at least their average values. I made a voltmeter check:
5,05
11,86
-12,04
I will check DC status by help of a scope.
I tried cleaning the ROM socket and pins but with no success.
To be more precise, the situation that happens the most is:
Black screen during 0,5s, then display gets a bit lighter and shows a constant gray color till I switch off the PSU.
What's a bit surprising is that I get same behaviour when I power up the system with no ROMS on the board. Normally I should get a red color because no ROMS is like having an error in the kickstart right?
But if the selftest is able to detect errors even on ROM, it means the test must be located not in ROM but somewhere as part of a custom chip.
Which component performs the selftest?