DEC H7140 (11/40) power supply revisions and general advice?

[Josh Dersch]

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 
(http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp11/1144/MP00897_11X44sys_Dec79.pdf) 
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