Hi gertsy,
Yes, it was working before. When the problem began, I did hook it up to another power supply (A600 one), but I used it before without problems, and its voltages seem to be OK.
The board is pretty clean for its age. No apparent corrosion.
I'm not the first owner, but as far as I know it was in original state when I got it.
When it began to fail I immediately suspected Denise or Fat Agnus because at the very beginning it started as a few garbled lines in another color on the screen. That rapidly worsened, and after maybe 2-3 boots I got the blue screen.
Because Denise and Fat Agnus check out fine I began suspecting HY1 (the vidiot). But would it fail in the manner described?? Other possibilities?
I read somewhere that the HY1 sometimes fails because the 3300 caps and its accompanying resistors begin to fail (especially on Rev 5 boards). That's why I replaced de caps, and measured out said resistors, which are fine.