jer@peora.UUCP (J. Eric Roskos) (01/09/86)
I asked this question in here awhile back, but haven't gotten an answer yet; I'm hoping by now someone may have found out the answer... It's now fairly well known that some subset of the Macintoshes experience a power supply failure if left turned on for an extended period of time. The symptom of this failure is that a transformer on the analog board becomes shorted and overheats. However, this isn't necessarily the actual problem; it could be that some other part, e.g., a capacitor or transistor, has a manufacturing defect or was not used according to the part's rated specifications, and that this part fails. For example, the failure might cause the oscillator in the switching power supply to stop running, causing a DC current to run through the transformer for an extended period, causing it to overheat. So, my question once again is, what part is failing? If it is some small part, it could probably be replaced with a part with a better rating, thus avoiding the failure. I would rather replace a 30 cent capacitor or $2.00 transistor, than have to replace the whole board, now that my machine is out of warranty. If you know what part is failing, or even just have a rumor about what part is failing, please mail to me, and I will summarize... Thanks... -- UUCP: Ofc: jer@peora.UUCP Home: jer@jerpc.CCC.UUCP CCC DNS: peora, pesnta US Mail: MS 795; CONCURRENT Computer Corp. SDC; (A Perkin-Elmer Company) 2486 Sand Lake Road, Orlando, FL 32809-7642 "Oh, is he your friend? Ask him his name!"
jer@peora.UUCP (J. Eric Roskos) (01/27/86)
This is a summary of responses received to my query regarding what part
fails in the Macintosh power supply. Alas, the results weren't too
conclusive, as yet.
I received only two responses to the query (I guess that reflects the
different user-demography of the people who buy Macintoshes, i.e., more
software than hardware oriented).
One person said that, in Macintoshes that are carried around a lot, a
board or connector comes loose, "unloading" the power supply. (I assume
this means that the power supply becomes disconnected from its load, which
in certain designs of switching power supplies will indeed cause the
power supply to fail.) He said that this causes a resistor to fuse open
on the power supply board, and that replacing the resistor fixes the problem.
A second person said that an electrolytic capacitor on the analog board
shorted, causing the flyback transformer to burn out. However, he wasn't
sure which capacitor it was that failed.
So, the problem is narrowed down a little now... in at least some cases,
there should be a visibly burnt resistor on the power supply board which
can be replaced to correct the problem.
The other case would be much harder to fix after-the-fact; if it was possible
to identify which capacitor was failing, I suspect it could be replaced
before the failure occurred.
If anyone experiences such a failure and takes their Mac apart to see what
went wrong, look at the electrolytic capacitors and see if you can see any
that appear to have gotten hot, or look like they have a greasy substance
dripping out, or have the rubber stopper in the end sticking out. This
should give some clue. I'll look in mine if it develops the failure (which
I expect will happen eventually).
Thank you to "smh" at AT&T Murray Hill, and Bart Massey at Reed, for
responding to my query.
--
UUCP: Ofc: jer@peora.UUCP Home: jer@jerpc.CCC.UUCP CCC DNS: peora, pesnta
US Mail: MS 795; CONCURRENT Computer Corp. SDC; (A Perkin-Elmer Company)
2486 Sand Lake Road, Orlando, FL 32809-7642 LOTD(2)=a
"I didn't watch the Super Bowl."