Apple power supply problems

From: Eric Smith <eric_at_brouhaha.com>
Date: Thu Oct 8 17:07:16 1998

I wrote:
> Don't *ever* short a fuse. The fuse is there for a reason, which is to
> protect the expensive components.

Marvin replied:
> *Ever* is a rather strong statement! I've been known to "short out" fuses
> in troubleshooting boards with a higher rating fuse than is called for for
> troubleshooting purposes, and yes, even short them out with a jumper wire
> (although granted very rarely!) One example was a shorted chip that for
> whatever reason, I didn't find with the VOM. Running it a short time with
> the fuse shorted out heated up the chip and I was able to feel where the
> problem was. Troubleshooting vs normal operation :).

Not so bad if it was something common and easily replaceable. But how
would you have felt if the problem had turned out to be a $0.05 shorted
passive compenent which caused then caused the failure of a rare or
exepnsive chip? Remember, the fuse isn't to protect the component that
failed and caused the problem, it's to protect the rest of the components
that are still OK (*).

IMNSHO, it is almost criminally negligent to suggest shorting a fuse to
anyone as a troubleshooting technique.

There are several proper ways to diagnose problems like the one that you
describe; typical is to use a current-limited lab power supply to operate
the board at a lower than normal DC voltage and determine where the current
is going. For AC equipment the equivlanent is to use a Variac (but
remember that a Variac does not provide isolation, so you may need an
isolation transformer as well).

If you know there is a short on a board in a certain signal, but not where
the short is physically located, a current tracer is quite an effective
troubleshooting tool. I had to fix a board with a solder bridge on the
component side of a board under a socket, and visual inspection didn't
reveal which socket. Rather than removing all eight candidates (or as many
as it took to visually spot the short), I was able to pinpoint it with a
current tracer.

I realize that not everyone owns lab supplies, Variacs, and current tracers.
However, I would point out that not everyone should be troubleshooting
faults in electronic equipment, either.

Eric

(*) The main purpose of fuses in consumer gear is actually simply to
prevent the equipment from overheating and bursting into flames, or causing
something else to burst into flames, or burning the user. But if that
serves to protect valuable components as well, why should you voluntarily
give up that protection?
Received on Thu Oct 08 1998 - 17:07:16 BST