Apple Lisa Power Supply Repair

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Aug 8 15:47:49 2004

>
> Hello,
>
> Does anyone out there have any words of wisdom for someone looking into =
> repairing an Apple Lisa power supply? I have 3 power supplies, 2x 1.2A =

I have never sorked on a Lisa (if someone wants to donate one....), but I
would start out by regardign them as normal SMPSUs and faultfinding
accordingly.

> and 1x 1.8A, and each has its own set of issues. One is just plain =
> dead, the safety switch engages OK but nothing powers up. Another one, =

Check the internal fuse. If blown, look for shorts in the rectifier
diodes or chopper. Check low value resistors in this area too, which may
well act as fuses!.

See if you can find a startup resistor (a few hundred k from the +ve end
of the mains smoothing cap to some point in the chopper circuit). It may
well be open.

> the 1.8A unit, pops the GFI on the circuit as soon as it is plugged in. =

Something before the power control 'switch' (is this a relay or a triac,
or something?). Maybe a leaking capacitor in the mains filter circuit or
something like that?


> The third one makes a chirping sound when the power switch is pressed. =
> The chirping continues as long as the switch is pressed. The PS that =

A short circuit on the output side? Maybe a shorted rectifier or a
leaking output-side capacitor. Or maybe the current sense circuit is
malfuncitoning and detecting an overload when there isn't one. But don't
be tempted to disable it!. Or maybe the crowbar is trippinf and shutting
the supply down due to a problem in the voltage regulation loop.

-tony
Received on Sun Aug 08 2004 - 15:47:49 BST