Re: Dead IIgs power supply gives power after turned off?

[mjmahon@aol.com (Michael J. Mahon)]

DT asked:

>When the power is turned on nothing happens.  But about 3 seconds
>*after* power is removed, there is a very brief "squeak" out of the
>power supply and the power LED glows weekly at the same time.
>From my limited knowledge I know that's probably from a capacitor
>draining, but it's still wierd.  I hope this sounds familiar to someone
>with supply repair experience.

Yep.

>I'm gonna try replacing the two 200V 100uF caps even though they
>look good.  The only thing that looks bad is a large brown spot on the
>supply's board to the left of the power transistor(?) between the two
>large caps and the large transformer.  The resistors and stuff in the
>center of that area all look OK.  I don't know what could of made
>that scorch(?) mark.

You are correct to doubt that it is a scorch mark--it is a mark made
by residue from a leaking electrolytic capacitor.  Replacing the two
nearby caps is exactly the right thing to do, and will very likely fix
the supply.

Usually the first capacitor to fail will be the one in the hottest
spot in the power supply--often adjacent to a power resistor
and/or a heat-sink.

>(I have read the power FAQ, hope I didn't miss something there.)
>
>I paid $8 for the IIgs with RGB monitor and 3.5" drive, would rather
>not pay $25 to $50 for a new supply... assuming they still can be had...

Used ones are available for much less--in the $5 to $10 range--but
are also not unlikely to need capacitors replaced.  At this time,
about 25% of all Apple II power supplies need new capacitors, and
the rest are working mostly because they have been, and still are,
lightly loaded.

The typical failure of an electrolytic is caused by loss of electrolyte,
usually due to extended operation at high temperature.  The result
is a gradual decrease in capacitance.  The capacitance required
for a supply to continue to operate in regulation is determined by
the load.  Eventually, even with a constant, moderate load, the
capacitance will fall sufficiently to cause the supply to fail, but
its operating margin has been steadily decreasing over a long
period.

Some capacitors last longer than others, both because of higher
temperature ratings and because of the luck of the draw.  But
after twenty years and thousands of hours of operation (heat),
most electrolytics are on their last legs.

-michael

 Email:  mjmahon@aol.com
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