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Re: Turn on Apple to preserve it?

Paul G. wrote:

On Mar 12, 9:00 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:


This will re-form the electrolytics in the 120v section of the supply,
which is the biggest source of problems.




That's very useful info, Michael. I had a weak power supply that would
click for a while but eventually come up and boot the computer.  I
don't have a variac, so I tried leaving it on all day in the hopes
that would reform the electrolytics. It seemed to make things worse.
Plan B was replacing the caps. I keep a box of PC power supplies to
scavenge for parts, and was able to find suitable replacements.


Usually, if an electrolytic has de-formed, it will leak (electrically)
like crazy, causing it to run hot, and consequently leak liquidly or
even explode.

If you got past the first hour without damage, then the caps were
probably formed properly.

It sounds like they had dried out or otherwise increased their
Equivalent Series Resistance or decreased their capacitance.
Either or both of these problems will cause the line supply to
have lower output voltage and worse load regulation, so when
the switcher tried to supply a load, it went out of regulation
and "clicked".

If the low-voltage filter caps in the other end of the supply
were de-formed, their electrical leakage could have increased
the load on the supply incrementally, pushing it into "click"
mode--but I'd be much more likely to suspect drying out of the
line supply caps.


Replacing the electrolytics in the 120V section did the trick.  It
boots right up now.    I have an ESR meter (effective series
resistance) which I built from a kit back when I was repairing
monitors. That is useful for quickly finding dried out electrolytics
in circuit, but it did not identify these as bad. They only measured
about twice as high as the replacement parts- typically bad parts
measure 10 times higher.  So your suggestion that the weak link was
the 120V caps was a great help.


Excellent--I'm glad it worked!

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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