[rescue] Power supply repair (was: Tektronix XP350 & IRIX questions)

[Tim H.]

On Sun, 14 Jul 2002 23:40:02 -0400
"Torquil MacCorkle, III" <torquil at rockbridge.net> wrote:

> So, I am wondering. Just in general, when repairing power supplies,
> what are the basic steps i go through. I am familiar with basic
> electronics terminology....

It's been a while, so trust this only for concepts, disclaimer, blah
blah....

Assuming switching powersupplies, as opposed to heavy iron supplies, the
concept is you charge a capacitor to the required output voltage, then
turn it off until it drops below output threshold, then charge it up
again, repeat ad-nauseum.  The switching is done with a transistor, the
regulation is usually done with a custom analog chip but sometimes with
off the shelf standard parts. 

Any math needed should be easy, because you are working with DC, so
everything is straight multiplication and division with actual values. 
Transistors should be on or off, there shouldn't be any significant time
spent below saturation.  It's really almost a digital circuit with
analog feedback. 

You will probably need an oscilloscope to troubleshoot, unless you are
real lucky and there are blown fuses.  I always start troubleshooting at
power in, tracking forward through the circuit, what you may find is a
power good signal that runs off the feedback section, and shuts the
whole power supply down if one of the output voltages is out of spec,
you may have to fudge that one temporarily to do your troubleshooting.

  Oh, and it is a lot easier to troubleshoot a switching supply while it
is working, so I would hang a resistor load on all the outputs, like use
a ten watt resistor appropriately sized to pull a couple watts(always
check that the supply is supposed to supply a couple watts, if there is
some kind of idle supply for a power save mode it may not, but generally
pulling a couple hundred milliamps is pretty safe).  Otherwise the
output capacitors charge up and the supply sits there doing nothing.

I generally dislike working on powersupplies, at least in PeeCee
supplies there are tendencies to use great gobs of hot glue to hold
components still, and they weren't made with ease of troubleshooting in
mind, hopefully yours are nicer to work on. 

Tim