DEC H7140 (11/44) power supply revisions and general advice?
derschjo at gmail.com
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...
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
> Thanks as always,
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