Buzzing PS?

From: A.R. Duell <>
Date: Fri May 30 14:44:49 1997

> Hi,
> The Apple /// I picked up last weekend has one annoying flaw, and that is
> a *very* loud power supply, that seems to get louder the longer the
> machine is powered up.
> I think this thing becomes even louder than our 1950s-era Westinghouse
> refrigerator. (OK, perhaps an exaggeration, but the sound seems to
> carry farther.)
> There is nothing obviously functionally wrong with the computer - it boots
> up fine, there's no wavering or glitching in the display, nothing to say
> that something might be wrong.
> Except the buzzing.
> It makes me nervous.
> Is this noise normal, or is it a sign that something is about to go
> kablooie? And if something's wrong, is there any way of knowing which
> part is about to go?

No it's not normal. The Apple uses a switching type supply, and there
shouldn't be any 60Hz waveforms anywhere after the mains
rectifier/smoothing cap. If it's buzzing there is a problem

It _may_ be nothing more than the mains filter coil (if it has one), or it
may be a dried up smoothing capacitor (the 2 big ones rated at about 300V)
that needs replacing. You could try those first.

> It looks like the power supply should be easy to repair, but I don't know
> what I'm doing with these things. I know it's been stated here that power
> supplies should be checked before powering anything up, but I don't really
> know how to do that, and sometimes I'm just too impatient to see my new
> toys running.

You'll regret it. It takes _1_ dry joint to break during shipping for all
the chips in your new toy to be killed at switch-on. I've almost seen it
happen (a colour monitor blew up after being turned upside-down since a
dry joint opened and removed the regulation from the PSU - guess who's got
that one to sort out!). Please check the PSU before going any further.

Apple PSUs are easy to check. The 6 way cable unplugs from the main board,
and the wiring is given in the Apple ][ reference manual. Just connect a
voltmeter between the +5V output and the ground wire (red and black?) and
power up. I don't think you need a dummy load. If you don't get 5V when
you power up, you need to sort out that PSU.

> Doug Spence

The gates in my computer are AND,OR and NOT, not Bill
Received on Fri May 30 1997 - 14:44:49 BST

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