30 May
1997
30 May
'97
7:44 a.m.
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
--
-tony
ard12(a)eng.cam.ac.uk
The gates in my computer are AND,OR and NOT, not Bill