Re: Slightly O/T - Apple /// question - PSU failure?

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

Matthew Russotto wrote:

>In article <jake.gittes-B9A486.17335026052004@nnrp-t71-03.news.uk.clara.net>,
>Jake Gittes  <jake.gittes@chinatown.com> wrote:
>>Sorry, but I can't find an Apple /// newsgroup. My question is:
>>
>>I've got a couple of Apple ///s and I've had the same problem with both. 
>>I switched them on after not using for some years, and after a few 
>>minutes heard a 'pop, pop' sound and saw white smoke coming out from the 
>>machine around the on/off switch. All accompanied by smell of burning.
>
>Capacitors went boom.  Find them (easy, they'll be the ones half off
>the board with lots of white stuff coming out of them), replace with
>ones with similar ratings, probably things will be good again.
>
>>Naturally I disconnected the power. I opened one machine up and it 
>>looked like it was the PSU which had gone, but it seems to be encased in 
>>a mass of metal and inaccessible.
>
>I'm pretty sure it's openable, you probably have to get the PSU out
>before you can open it.
>
>>The bizarre thing was that I reconnected power and rebooted one of the 
>>machines and it seems to work! Before I continue to experiment - anyone 
>>got an idea what's going on? How terminal is it?
>
>The capacitors in question are probably filter capacitors, and
>probably shorted open.  So it may work but the output isn't as clean
>as it should be.

I concur.  But I would also recommend replacing all the "similar"
electrolytic capacitors at the end of the power supply nearest the
AC power connector.  They are all about the same age and have
experienced the same environment, so it is reasonable to assume
that they are all in about the same state of "readiness to fail".  ;-)

The fact that one of your machines is now working indicates that
at least two of the capacitors are in parallel, and only one of the
pair has failed (open).  Since this is almost certainly on the line
side of the switching supply, the output voltages are probably
still fine, but the supply's power ratings are reduced because of
 the decreased line-side capacitance.

The non-working machine may be similar, but the capacitor
that opened is not paralleled by a working one, or it may have
failed with a momentary short (quite common), blowing the
internal fuse as well.

-michael

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