Re: Power supplies keep crapping out on me...solved!

["Michael J. Mahon" <mjmahon@aol.com>]

Scott Alfter wrote:
> (Yes, this is a reply to my own post.  Yes, the post to which I'm replying
> is about six months old.  It took me this long to get around to tracing down
> the problem and fixing it.)
>
> I had a problem with Apple II (IIe/II+) power supplies quitting on me.  The
> computer would run just fine for months at a time, then after I turned it
> off one day, it wouldn't switch back on.  Some possible solutions were
> offered, but I didn't get around to trying them.
>
> I wanted to get my beer fridge running again after my recent move.  After
> cleaning up the little bit of dusty mold that had accumulated inside the
> fridge, I plugged my IIe back into the fridge (a cable connects the computer
> to a temperature sensor and a relay that controls the compressor).  Of
> course, it didn't start up.
>
> I looked up the previous messages on this topic and set about to take the
> computer apart so I could get at the power supply.  While doing this, I
> noticed that the computer would start if the floppy controller was removed. 
> As soon as the floppy controller went back in, it wouldn['t start.  Removing
> the 1-meg RamWorks II, the LocalTalk controller, and the Super Serial Card
> didn't make any difference; the computer would start without the floppy
> controller, but wouldn't start with it.

This behavior is completly diagnostic of "marginal power supply
insufficiency".  Over time, the electrolytics in the line side of
the supply decrease in capacitance, reducing the current capability
of the supply, until, one day, it doesn't have the ability to operate
with full load.

Removing any load (by pulling a card) will fix the problem until more
time passes and the capacitance (and current capability) is reduced
enough for it to fail again.

Also, a rise in AC line voltage can allow the supply to operate (until
the mains voltage decreases).

> Someone else might've thought that the floppy controller was bad.  I figured
> that maybe it was trying to pull more power than the power supply could
> handle.  I could've tried hotplugging the floppy controller and seen what
> happened, but I didn't want to screw things up more than they already were.
>
> One suggestion I had been given was that maybe a small-value (1-4.7 uF)
> electrolytic capacitor in the power supply's startup circuit had gone bad. 
> I didn't see anything in that range on this particular supply.

That's possible, but I wouldn't expect a supply with that problem to
start with slightly reduced load, like yours.

> After a bit of googling, I ended up at the page in the Apple II FAQ that
> covers power supplies.  It described my problems exactly--works for a long
> time, then has difficulty starting up again.  I tried the suggested fix, and
> now my IIe is running again.
>
> If your power supply was built by Astec, there's a 220-uF electrolytic
> capacitor near the transformer in the middle of the board.  It's rated for
> 10 WVDC, which apparently may have been under-specced.  The recommended fix
> is to replace it with a same-value capacitor rated for at least 25 WVDC.  My
> parts bin had some suitable capacitors rated for 35 WVDC, so I desoldered
> the old part and soldered in the new part...a ten-minute fix that cost me no
> money.  Cool. :-)

Great!

I have a few Aztec supplies, but most of them are not.

I'll look for the equivalent cap on one of them and see if it gets it
going. It would seem that there is more than one way for these supplies
to become marginal.

> (One bit of unrelated weirdness I noticed when testing the fixed power supply
> (prior to fully reassembling the computer) was that if you fire up a IIe
> motherboard without the keyboard plugged in, the red LED near the power
> connector will light up, but the speaker won't beep.  Plugging in the
> keyboard got it to start up.  Is this behavior normal?)

Yes, the computer is running it's self-test.  You should hear that when
it completes the test and beeps repeatedly.  (Or if no speaker is
plugged in, the LED on the other side of the rear of the board will
flash.)

> The computer is now up and running, and now I'm ops-checking the beer
> fridge.  It's getting cold like it should.  It's still ramping down to the
> setpoint I gave it, but if the computer starts cycling the compressor on and
> off after that (and I'm 99.9% certain it will), then I'm good to go.

Happy brewing!

-michael

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