Amiga 3000T Power Supply

[Castellen]

Will an Amiga 3000T power supply "turn on" from the switch if the motherboard power cable is disconnected?

NM:  It does when it feels like it apparently.  I think this one is bad.  -9V on -12V rail.. when it feels like working.

Yes, without the main board connected the fan should run normally, and you should measure close to 5V on the 5V output.

Without a bit of load on the 5V side, don't pay too much attention to the regulation on the +12V and -12V outputs.

Was working on one of these PSUs the other day which was working normally aside from after an off/on cycle, the PG (power good) output would stay low (inactive), despite all the DC rails measuring as expected.  Meaning the computer would stay in reset state, because _FAIL is permanently low.  Since the supply contains a few proprietary parts with no information, it wasn't an easy fix.  So simply removed the PG output to the main board, and implemented a 5V watchdog the same as what's used in the A3000D.  Not too difficult and it worked fine.

[Castellen]

But then it just stops after a while and there's no power coming out.  I don't think that's normal

No, that's not normal.  When it does power up, does the A3000T run normally?

Have a check of the components on the PCB just in front of the fan.  As I recall, there's a couple of 5W 33k resistors with sleeving over them, and an optocoupler with a diode across the LED side.  At least in the 230V version.  I've seen it before where these components can get corroded and cause problems, especially the diode.  It's nothing special, probably just a 1N4148 small signal diode or similar.  If it appears rusted/corroded then you can safely replace it with a 1N4001 or whatever standard silicon diode you've got on hand.

And as much as I cringe when someone suggests electrolytic capacitor replacement as a sure fix for every problem imaginable, it might actually help in this case.