Buzzing PS?

From: Doug Spence <>
Date: Mon Jun 9 16:21:00 1997

On Fri, 30 May 1997, A.R. Duell wrote:

> > 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.


> > 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

OK, here's a question from an electronics-know-nothing (which hopefully
isn't in the FAQ, else I'll look like a complete idiot :) ): How do I
tell the difference between a switching supply and a linear supply? Just
from looking at it.

I get the idea that linear supplies have honking big transformers in them,
and switching supplies don't.

> 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.

I don't remember there being any very large capcitors. (Nothing like in
my PET 2001, anyway... hooo boy! ;) ). I'll take another look inside to
see if I can spot the ones you're talking about.


> 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.

Well, the ][ and /// use different connectors, but I noticed that the
///'s supply has info on the PCB.

The supply works, BTW. As I mentioned, the system powers up and runs as
if nothing were wrong. The noise the power supply makes just makes me

> --
> -tony
> The gates in my computer are AND,OR and NOT, not Bill

Doug Spence
Received on Mon Jun 09 1997 - 16:21:00 BST

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