H786 troubleshooting

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Sep 27 16:06:09 2004

> Jumped into my first serious PSU repair.. :-)
> It's a dead H786 (from a BA11-N), tripped my house's CB initially..
> Traced it to a shorted bridge rectifier...promptly replaced that (with a
> higher spec 35A/1000V, original was 30A/600V).
> Put it a dummy load (a 12V/1A fan), fan turns.. YAY!
> But to my dismay, 12V o/p measured to just 3.5V while th 5V o/p measured
> around 100mV (!).

That 100mV output makes me wonder if the crowbar has fired (an SCR across
the 5V output that shorts it out if said output goes overvoltage).

I don;t know if it's safe to remove the crowbar on this PSU. Normally it
is, but, of course, only on a dummy load. If you can then get the PSU to
run, you want to look at all outputs with a 'scope to see if there are
any high voltage (typically around twice the nominal output voltage)
spikes on said output. And if there are, you've got a high ESR smoothing
capacitor there. I've seen this more times than I care to remember.

> Throughout all these, a high-frequency whining/buzzing is being heard..
> Any clues to go forward?
> Do I start checking from the output to input or do I move forward from
> the choppers?

Since there's something on the outputs, the choppers are working. I'd
look at the output and regulation sides first.

One nice thing about most DEC PSUs of this vintage is that they have
pulse transformers to drive the chopper(s). Which means the chopper drive
circuitry, the oscillator, etc, are on the isolated side of the supply
and you can probe them with a 'scope without having to worry about an
isolation transformer.

Received on Mon Sep 27 2004 - 16:06:09 BST

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