Reviving DEC RX01 power supply
ard at p850ug1.demon.co.uk
Mon Sep 3 18:20:34 CDT 2007
> I've got a H771-A (120V/60Hz) that's acting up on me. Part
> of an old PDP-11/03 that I'm trying to get back on its feet.
> As I've done with 4 or 5 other RX units in the past couple
> of years, after cleaning it up and reforming the capacitors,
> but before plugging everything together, I wanted to check
> output voltages. I'm sure glad I did because this guy is
> not normal!
> Where you usually see 24V, I'm getting 42V! And the so-
> called 'Raw DC' which is normally around 10V is 17V. At
IIRC, the 'raw DC' is unregulated, just rectified and smoothed from the
transformer. As is the 24V line. Both are about 1.7* what they should be.
It's gettign late, so I'll not find the prints tonight. But IIRC, the
transformer in this supply is a ferroresonant one, and that's what
stabilises the 'raw' and 24V lines. What happens if the capacitor hung
off that is defective?
> least the +/-5V lines are more or less correct at +5.1V and
> -5.3V, respectively.
> I need some help here. I've checked the printset on this,
> and frankly, this analog stuff is so much voodoo (to me).
> I get lost pretty quickly unless the circuit is simple,
> say, like a flashlight. Anyway, I've checked a number of
> voltages and voltage drops at various places that I
> thought might lead me to something, but I can't say that
> I'm any wiser now. I'm sure this is fixable, even by me,
> IF I can locate out the faulty component(s).
I suspect the last part is very ture. Actually replacing the faulty part
is the easy bit :-)
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