Notes on repairing the Apple Lisa power supply

From: Joe <rigdonj_at_intellistar.net>
Date: Thu May 25 22:44:52 2000

At 11:33 PM 5/25/00 +0100, Tony wrote:
>> (3) The CPU does not have to be installed for the PS to work but if it's
>> not installed the PS will keep resetting and will make a chirping sound.
>
>There are normally 2 things that will cause that (Note : I've not seen
>the Lisa schematics, and I've not seen inside a Lisa for many years, so
>these are general comments).
>
>The first is that the PSU needs a load -- without a load it overvoltages,
>fires the crowbar, shuts down, and repeats. The CPU board provides enough
>of a load for it to work (but I'd not want to risk an expensive/rare CPU
>board as 'just a load' :-))
>
>The second is that the PSU uses separate sense lines that actually
>measure the voltage at the CPU board and thus compensate for voltage
>drops in the wires/PCB tracks between the PSU and the rest of the system.
>These lines are connected to the +5V and ground lines on the CPU board
>and with the CPU board removed they float. Thus the PSU thinks it's not
>generating enough voltage, increases the output voltage, the crowbar
>fires, it shuts down, and repeats.
>
>Try connected a dummy load (a 6V car bulb is what I generally use)
>between the +5V line and ground on the backplane with the CPU removed. If
>it powers up properly now, then it was just lacking the load.

    FWIW I usually use an old disk drive. It's very handy since many
computers already have a power connector that will plug directly onto the
drive and it provides a load to both the +12 and +5 volt power supplies.

>
>If it still trips, then you need to identify the sense lines and connect
>them to the appropriate power lines. You need a schematic, or at least
>pinouts of the boards, for this.

    And some kind of extender cards for working on a Lisa. Otherwise
you're correct on all points. I should point out that the video section and
floppy drive were always connected on this one and provided a load. I wish
I could get a Lisa schematic! I'd love to know where all the interconnects
are that can prevent the power supply from turning on. RD completely
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
or an interconnect/interlock problem or a problem in the I/O card. I posted
those notes to alert others to that possiblity and to see if anyone could
shed any light on it.

   Joe
Received on Thu May 25 2000 - 22:44:52 BST