9 Jun
1997
9 Jun
'97
9:21 a.m.
On Fri, 30 May 1997, A.R. Duell wrote:
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
> 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.
<snip>
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 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.
<snip>
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
nervous.
--
-tony
ard12(a)eng.cam.ac.uk
The gates in my computer are AND,OR and NOT, not Bill
Doug Spence
ds_spenc(a)alcor.concordia.ca