Reviving DEC RX01 power supply

J Blaser oldcpu2 at
Sun Sep 2 21:10:40 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 
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[1] 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).

Hints or guidance on what else I should look at would 
surely be appreciated!

- Jared

[1] Here are voltage readings taken at a few locations:

- Transformer outputs (these look normal):
   blue: 27.8VAC
   purple: 11.2VAC

- PCB connector P1:
   2 (blu): -5.3v
   3 (blk): 0v
   4 (gry/red): 5.1v
   5 (blk): 0v
   6 (red): 42v (should be 24v!!!)

- PCB connector P2:
   1 (org): 17v (should be 9.5v!!!)
   3 (blk): 0v
   4 (gry/red): 5.1v
   5 (blk): 0v

- Voltage drop across:
   D5 15v Zener: 17v
   D11 6.8v Zener: 7v
   R12: 25v
   R13: 12v

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