From: "Powermac" <[EMAIL PROTECTED]>
Date: Fri, 19 Aug 2005 01:43:44 -0400
----- Original Message -----
From: "Jeff Walther" <[EMAIL PROTECTED]>
To: "Vintage Macs" <vintage.macs@mail.maclaunch.com>
Although, as old as that machine is, anyone who wants to keep one
running probably needs to learn the basics of power supply repair--at
least enough to replace the capacitors every decade or so.
Jeff Walther
So what are the basics for PS repair (is there a PDF or book on the
subject)?
I wish that I knew. :-) I can replace most components skillfully,
but have few skills in diagnosis.
In most cases, it seems to be that power supplies fail from worn out
electrolytic capacitors. So a shotgun approach that often works is
to replace all the electrolytic caps in the thing. Also, look for
discolored (from heat) spots and bad solder joints.
I think that the way a technician would diagnose such a unit is to
develop a basic understanding of what voltage is expected where in
the PS either by having a schematic or working one up by examination.
Then start measuring voltages either from teh input towards the
output or vice versa.
If you start at the output, you'd measure the output to see if it's
correct. If not, move back behind the last stage of components and
see if the voltage has the proper form there. Keep working backwards
until teh voltage has the proper form for that stage. The failure is
probably in front of that stage.
If you start at the input, you'd follow the AC current in, make sure
it's reaching the first component properly and then work your way
forwward as in the above, until you find the stage where the voltage
is not as expected.
Of course, you need at least a multimeter that can handle AC and DC to do this.
Before the advent of switching power supplies, a typical power supply
would have a transformer as the first component. That is a bunch of
coils of wire around an iron core. 120VAC would go in and a lower
voltage such 18 VAC would come out the other end. Or possibly it
would have two or more taps such that 18 VAC and maybe 8 VAC would
come out.
Then there would be a rectifier stage to convert the AC to a rough
DC. This DC would have a lot of level variation in it.
Then there would be a filtering stage to smooth out the DC into
something useable. This was usually done with a bunch of capacitors.
There might also be a solid state DC-DC voltage regulator in there to
really nail the power output. And there could be filter elements
included in any of the stages.
On such old style power supplies its fairly easy to know what to
expect and to measure the levels to see if they look okay.
Switching power supplies are somewhat different animals and frankly,
I don't remember the little that I read about them. IIRC, they use
a solid state component that takes a higher than desired voltage and
switches open and closed very fast. The duty cycle of this switching
depends on the relationship between teh desired voltage and the input
voltage. Then the output is smoothed out with some filtering
(capacitors again, I think) and a lower voltage is achieved.
I'm not sure how you'd check that the switching component of such a
supply is switching properly. It's output should look something like
a fast square wave and I guess you'd need an oscilloscope to check
it. Of course, if the input to the switcher is good, and output is
bad, and you've already replaced the filter, then the switcher is
probably the place to look? Like I said, I just don't know enough
about diagnosis to be confident.
Jeff Walther
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