X68030 Power supply blues

[mikkohoo]

I'm having an on and off relationship with my X68030 (non-Compact). I bought it several years ago from a friend who had repaired it himself and converted the power supply to ATX. It worked for a while, then died on me without warning.

Turns out it had been recapped with tantalum capacitors and one of them exploded on the motherboard. I replaced it with an electrolytic version, but couldn't get it to run again: the power light stayed red and would only turn orange, not green, and the system would not boot. I let it be for several years.

I heard that this could be caused by the LS244 at IC202. I tried replacing it, and also decided to do a full recap with electrolytics, as I'm firmly of the opinion that capacitors should not explode. I also replaced the old messy wiring with one of Edu Arana's X68000 ATX PSU adaptors, and tried cleaning the old leaked electrolyte residue off the board with isopropyl alcohol. There was a lot of it.

This changed the behaviour: the system would now turn on and show the disk screen, but would not turn off, no matter what I did. The power light would blink but the machine stayed on.

I read in the wiki that this might have to do with bad contacts on the daughterboard's 74 logic chips, so I reflowed their solder joints, using a microscope so I wouldn't create any new shorts, and checked continuity.

As a result of this, or something else that happened at the same time, the system's operation changed again: if the soft power switch is off, the power light goes off completely. If I turn it on, it will light up green, but the system will not boot or give me video sync. A couple of times, it greeted me with the "error has occurred" screen, but mostly it will just stay dead.

I'm having a hard time following the service manual as it is fully in Japanese. However, I'm quite sure that the problem is on the daughterboard, and it might be in the components near the soft power switch. As replacing parts at random is a less than favourable approach, I would be most grateful for any pointers regarding which components I should be looking at.