Advice/Suggestions for repairing Apple III power supply

dwight elvey dkelvey at
Tue Jul 24 08:41:27 CDT 2007

>From: "Ethan Dicks" <ethan.dicks at>

>Given how old an Apple III is, though, it wouldn't be a shock (no pun
>intended) to find out you had a defective cap or two.  Checking for a
>supply-line short to ground is a quick test that should help either
>narrow down or eliminate swaths of the board quickly.

I have a method that I've used over the years to locate shorts
on PCB's. It can even find a resistive short. A variation on the
method can fine multple shorts but that requires things that
most don't have.
First I'll discribe the principle then actually doing it.
Consider a open resistor that one can probe at any location.
Put a current through it. Now place one of the leads of
a sensitive voltmeter on some location along the resistor.
Pretend that you don't know where that probe is touching.
Move the other lead of the meter along the resistor.
notice that when both meter leads are at the same point,
the voltage on the meter reads zero volts.
This principle can be used to find a short on a PCB.
Lets say that we have a single trace that is shorted to ground.
We know it is some place along a 7 inch trace but we don't
know exactly where.
We take a current regulated supply ( or even a constant voltage
supply with a dummy load ) that can put out about 1 to 2 amperes.
We place the supply from end to end of the trace. The trace
is now like the resistor we had in the first example. The only
thing is that the point that is probed by the ground short
is unknown.
Place one lead on the ground.
Now probe along the trace that has the current flowing through
it. You'll find a small voltage along most of the trace but it will
go to zero at the location of the short.
The meter generally needs to be sensitive. A digital meter
that reads 200 mV is often enough. Most traces can handle
several amperes. The higher the current the higher the voltage
This is all fine but what if the short is between two plains on
the circuit board. The problem is that even when on find a location
that has zero volts, one finds that there are a lot of locations
that have zero volts along a line on the plain.
First, one needs to use much more current. Typically 10 or
more amperes to get enough voltage drop to measure.
The next probem is that just connecting the supplies at
two corners only gives a line of zero volts. The one thing
that is the same is that that the sort is somplace on that
One can move the power supply to the other corners.
This will form another line ( usually a curved line ). Where
these two lines cross will be the location of the short.
There are other methods of finding shorts but most include
driving current across the short. This can be destructive
to more than the failing part. It is better to know which part
is bad and replace it. This method only has current flowing
in parts that are normally designed to carry current.

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