Amiga 1000 Power Supply repair?

[Castellen]

Several sites suggest A1000 PSU schematics have been available but all downloads fail.

Oops, that would probably be my fault.  Helps when the HTML link matches the file it's linking to.  The A1000 PSU schematic download will work now:
http://amiga.serveftp.net/schematics.html
But that's for the 220V model, the American/110V one might be totally different, not sure.

Yes, of course it's a switch mode power supply, and a fairly old and simple one at that.  I've written a few notes on repairing A3000 power supply common faults here which might help also.

Power transistor or bridge rectifier failures are common in these supplies.  Though when they fail, it's usually in a short-circuit condition which blows the input fuse, but it sounds if this is OK.  Would be worth checking them anyway, easy enough to do with a multimeter on diode test range.  Also check low value series resistors (under about 47 Ohms) for being high resistance or open circuit.  During power on, the high inrush current through these resistors can damage them.  As stupid as it may sound, I've seen this many times in practice.

[Castellen]

So... do I trust it?  Probably not.  I'm still looking into my options, but being that I didn't really have the cash to do anything about it, it's nice that I got a bit more (admittedly, borrowed) time on it.

So it's obviously an intermittent problem then.  Agreed, it may stop working at any time.

Check all solder joints, particularly on larger components and components that dissipate significant heat.  Resolder anything that looks cracked or suspicious.

These older SMPS designs usually work around a self-oscillating principle as opposed to newer designs that have a dedicated controller IC.  There are always capacitors involved in the oscillation network and often these are aluminium electrolytic types.  As these components age, the internal liquid chemical electrolyte gradually dries out and the capacitence decreases as a result.  So what you may be seeing is that certain capacitors have degraded to the point where the oscillation is not self-starting reliably.  The solution is of course to replace all of the aging electrolytic capacitors.  Not expensive to do, but somewhat time consuming in ordering and replacing them.  But in doing so, the power supply is likely to be good for another 25+ years.

[Castellen]

I've never worked on such a thing, so the +80 and -80 volts coming out of it (with 110v AC going in to it) seemed somewhat reasonable.  I had expected it to be +-60 volts, but whatever.

I forgot to mention....
The incoming AC mains is full wave rectified to DC by the bridge rectifier, and on the DC output of the bridge rectifier there will be one large electrolytic capacitor, or two in series.

The voltage across this reservoir capacitor will be the mains AC waveform peak voltage, less the forward voltage drop of two silicon diodes in the bridge rectifier.

i.e.
Vcapacitor = Vpeak - bridge rectifier fwd drop
Vcapacitor = Vrms / SIN(45) - (2 x 0.7V)
      = 110V / SIN(45) - (2 x 0.7V)
      = 155.6V - (2 x 0.7)
      = 154V approx. across the reservior capacitor

If there are two reservior capacitors in series, there will be about 77V across each one.  Hope that makes sense.