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

[Jay Jaeger]

On 10/9/2015 10:42 PM, Josh Dersch wrote:
> 
> 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
> 

I have an HP drive (RK05 type packs) whose startup power supply had been
tampered with in very odd ways - missing parts, etc. (and maybe even
blown out circuit board traces - I forget - I haven't worked on that
drive in years).  I seem to recall that it looked like it had failed
catastrophically at some point.    I ended up wiring in a 24V stand
alone supply temporarily as a startup supply and got the rest of the
supply going that way.  Maybe someone did something similar with your
supply, and then ran out of time / gave up.  Perhaps they clipped the
diodes out to test them.

I have an 11/40 and an 11/45, so I probably have an H7140 (but I don't
seem to have a spare anywhere, which surprises me a little, because I
got parts of another 11/40 at one point), but I recall that the things
are nightmare-ishly heavy to mount, and there is only one of me, so
unless the area in question is visible without taking the supply out of
the rack, I doubt that I can help very quickly.

JRJ