Re: Power supplies

[<JCA31@MAINE.MAINE.EDU>]

Thus far i've received several helpful emails (Thanks all!) that seem
to point the finger at the oscillator section that drives the main
switching transistor. I should have mentioned straight off that these
are, i believe, the Astec supplies (A2M0030 w/Apple markings).

The driving oscillator theory is supported by the evidence that a) most
of my working supplies emit a high pitched whistle not unlike a monitor,
and b) all the dead ones are as quiet as .. the dead.

A number of components have been pointed out, in that section. A question
that comes to mind is, "how many different board revisions were there?".
I ask this because it would be handy to be able to say "R3" as opposed to
"the 1M about an inch behind and to the left of the transformer". All my
supplies seem to be the same, but then, practically all of them date from
1982 to 1984.

So, having said that, there's a half-watt 150k resistor right next to the
main transistor. Said resistor is marked "R3". There's another marked "R2",
just like it, between the big input filter caps. These are the only thing
in that area remotely close to the 100k culprit that several people have
mentioned.

A closer visual inspection turned up a large, green resistor, marked
"R4", value apparently 27ohms, which is probably >= 2W, right next to
a 10V 220uF electrolytic (C7), that looks like it's been running
extremely hot. (The board is cooked brown where R4's leads enter it.)
Some of the email pointed the finger at the  cap. I'm wondering if the
cap tends to fail from the heat source right next to it..

I guess it's time to dig out the soldering iron. (and, to answer a
prior post, my dvm has a junction checker.) Apologies for the
rambling post.  Thanks again!

-j