Notes on repairing the Apple Lisa power supply

From: John Lewczyk <jlewczyk_at_his.com>
Date: Fri May 26 16:35:55 2000

> > overlooked (or just ignored) that fact that the problem with this supply
is
> > that it never turns on and not that it turns on and is overloaded. The
> > NUMEROUS other Lisas that I know of that have "bad' power supplies have
the
> > same symptoms and I have to wonder if it really is a power supply
problem

The Lisa power supply has a signal line called "ON" which is supplied by the
IO board, and which apparently turns on the power supply's main outputs.
The PS has a "+5V standby" voltage which is always on (as long as the unit
is plugged in), and the integral safety interlock switch is pressed in. The
PS safety interlock switch is located in a rear corner of the power supply
unit and it is pressed in by the rear cover when installed). Also, the PS
"ON" signal is passed through a second safety interlock switch, which, when
the front panel is installed, closes, allowing the "ON" signal to pass from
the IO board to the PS. There are only 2 interlock switches on the Lisa. I
found them easily overridden to work on the unit with the covers off.

The Lisa's "soft power switch" shorts to ground a signal that goes to the
"IO board". The circuity on the "IO board" senses this, and then asserts
the "ON" line of the Power Supply (using a COPS 421 Microcontroller unit,
which also controls the keyboard and mouse). This setup gives the Lisa
control over its own power. I've heard that the Lisa could be programmed to
turn itself on at an appointed time! In addition, the Lisa won't shut down
until the O/S thinks its OK to do so (i.e. the hard drive has been properly
updated).

I've learned these things because I'm also in the process of repairing a
Lisa whose power supply won't turn on. I've yet to test the power supply out
of the the box with test loads to verify the operation of the supply. When
I've figure out the rest of the power supply scheme, I'll post what else
I've learned to the group.

John
Received on Fri May 26 2000 - 16:35:55 BST