DEC H7140 (11/44) power supply revisions and general advice?

Josh Dersch derschjo at
Fri Oct 9 23:22:59 CDT 2015

And as I just noted after Jay's response, the subject line should say 
"11/44" not "11/40."  Corrected...

- Josh

On 10/9/15 8:42 PM, Josh Dersch wrote:
> Hey all --
> Once again, I find myself in over my head debugging a power supply, 
> this time an H7140 from a PDP-11/44.  Here's the skinny:
> I examined the supply physically before experimenting and found a 
> capacitor on the Bias/Interface board that was leaking, bursting and 
> rather burned-out looking (not a great sign) -- this is capacitor C4 
> in the printsets on Bitsavers 
> ( 
> Everything else looked OK physically; I replaced the obviously bad 
> capacitor at C4.
> When power is applied (plugged in, breaker switch flipped to "On") the 
> relay does not click - based on my readings of the manuals this should 
> happen after the bias voltages are up to spec.  I measure 308VDC on 
> the lugs on the top of the memory board, so that's at least something 
> working.  Getting to other points to test voltages is a bit more 
> difficult, especially with those high voltages in the way, what a nice 
> design :).
> Switching the front panel switch to "Local" (or any other position) 
> has no effect -- no fans, no LEDs, nothing.  I've double-checked all 
> the wiring and everything looks OK.
> Capacitor C4 looks to be involved with the START-UP DRIVE signal 
> circuitry (which drives the relay) so the behavior I'm seeing makes 
> sense if C4 died and took a couple of things with it (or if something 
> else died and took C4 with it).
> Here's where it gets kind of odd -- I spent some time testing diodes 
> and transistors in the related area near C4 and while doing so I 
> noticed that there are four diodes (D1-D4) listed on the schematic 
> that are missing from my board. "Missing" as in someone clipped them 
> out at some point -- there are just nubs of the leads left.  I'm not 
> sure why this would have been done, but there were a number of ECOs 
> applied to this board (a few wires and resistors added) and I don't 
> want to assume that if I just put four new diodes in that it won't 
> cause other problems.  Anyone know if there were other revision levels 
> of the bias/interface board that would have done away with these 
> diodes?  Anyone have an H7140 they can easily crack open to compare?  
> (It's actually relatively easy to get to, if you can get to the 
> supply...)
> Thanks as always,
> Josh

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