IIsi power supply (Plessey) standby voltage fix

[Arisotura]

A while ago I got a Macintosh IIsi. Gave it a quick test when receiving it, and it was starting, although the HDD would just keep spinning down.

The motherboard had leaking caps, as typical, but the power supply (of Plessey brand) was absolutely clean, no traces of leakage whatsoever. I still decided to recap it for good measure.

So I recapped both motherboard and power supply. Then I eagerly went and tested it... and it didn't start at all.

Turns out the power supply was just not outputting any standby voltage.

I double-checked my recap work and everything was fine. I saw that the smaller daughterboard (on the high-voltage side) had sketchy solder joints, so I fixed a bunch. Still no dice.

I thought a bit more about how this power supply works, and it's kinda cute. There's a whole separate circuit for producing the standby voltage.



The switching for the standby circuit is controlled by Q3 (BUX85 power transistor, circled up there) and the small daughterboard. Upon checking around there, I eventually found out that Q3 was bad.

After receiving a replacement, I swapped it out, but still no standby voltage. I looked further and found that the resistor next to Q3 (also circled above) was bad too. It's supposed to be a 10ohm resistor, but it was measuring in the megaohm range...

So I replaced that part too, and suddenly my power supply was working again.

I figured I would post this in case other folks have the same sort of problem with this power supply...

 

[David Cook]

I also recently ran into one of those power supplies. Here are my recapping photos and notes...






My power supply had rivets.

I drilled them out with a 9/64 inch diameter drill bit.

I then tapped the main frame for screws.


And used nylon screws because I was concerned with making electrical contact inside the power supply. Rivets sit fairly flush -- and I didn't know the proper length for each screw position.

 

[bibilit]

Never dealt with that kind of IIsi PSU.

But, apart from bad capacitors, you should have a look at broken traces or broken components (i can see a lot of rust)

 

[Arisotura]

That's dust. The power supply is in good condition, I didn't see any leakage or corrosion.

David Cook, I'm amazed at how clean the smaller daughterboard is in your power supply. It's like, in pristine condition. Mine was basically toasted and had weak traces/joints (but oddly enough the components on it were fine).

 

[David Cook]

David Cook, I'm amazed at how clean the smaller daughterboard is in your power supply. It's like, in pristine condition. Mine was basically toasted and had weak traces/joints (but oddly enough the components on it were fine).

I wonder if yours just had a lot more years of power-on service? Or perhaps a big monitor was sitting on it, whereas mine perhaps had a monitor to the side?