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From: jer@peora.UUCP (J. Eric Roskos)
Newsgroups: net.micro.mac
Subject: Re: Power Supply Troubles?
Message-ID: <2177@peora.UUCP>
Date: Fri, 16-May-86 08:57:32 EDT
Article-I.D.: peora.2177
Posted: Fri May 16 08:57:32 1986
Date-Received: Sun, 18-May-86 15:01:41 EDT
References: <13507@ucbvax.BERKELEY.EDU> <3345@ut-ngp.UUCP>
Organization: Concurrent Computer Corporation, Orlando, Fl
Lines: 37

> My new board has a much brighter display; I used to keep the brightness
> control pretty much full up, I keep it at about 75% now.

There's a control on the analog board (as is the case on the video boards
for most terminals and also most TV sets) that controls the maximum
brightness available from the user-accessible brightness control.  In
most video boards (thus probably in the Macintosh too) it is actually just
another potentiometer in series with the brightness control (usually there's
another fixed resistor in series too, just in case you turn both to their
minimum-resistance position).  In some TV service manuals I've seen, you
set the internal control to give a particular voltage level when the
external control is set at maximum brightness, so it's probably best not
to haphazardly change it.

Regarding the power supply, though... for about 6 months I've been
periodically asking in here "what part fails"... well, mine finally failed,
and I found out: when it started giving the "jerking" display described
in the above posting, I showed it to one of our hardware maintenance
people next door, who said "oh, that means the insulation's breaking down
in the flyback transformer, usually, although also sometimes the magnets
on the yoke may be coming loose".  He said I could get a new flyback
transformer from one of the local electronics parts places, but finally
I took it in and got the board swapped, just to be safe.  I notice that
the new board has a different flyback transformer, and also it seems to
be farther away from some of the nearby components, so maybe it was arcing
to an adjacent component in the early boards... or maybe it was just a
defective lot of transformers.

Incidentally, I notice in my new power supply board that three of the
heat sinks have stubs of wires from rectifiers on them, where apparently
they took the rectifier + heat sink out, cut off the rectifier, and
reused the heat sink on a TO-220 transistor instead (so they didn't have
to reuse the hole where the rectifier lead was soldered).  This makes
me think those rectifiers must also fail frequently, which in turn makes
me think it really *does* get too hot in there...
-- 
E. Roskos