DEC H7140 (11/40) power supply revisions and general advice?
derschjo at gmail.com
Fri Oct 9 22:42:30 CDT 2015
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
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,
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