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

Mattis Lind mattislind at
Sat Oct 10 12:32:52 CDT 2015

lördag 10 oktober 2015 skrev Josh Dersch <derschjo at>:

> > On Oct 10, 2015, at 2:15 AM, tony duell <ard at
> <javascript:;>> wrote:
> >
> >>
> >> 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:
> >
> > That doesn't surprise me, the H7140 is one of the most complex PSUs you
> are
> > likely to come across...
> >
> >> 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 :).
> >
> > That 300-odd volts comes from rectified (or voltage-doubled) mains. The
> > relay is part of the soft-start circuit, it should operate after the
> 'bias' PSU
> > has started up (it shorts out a resistor in the mains input circuit).
> With no
> > other load on the supply you will get the 300V with the resistor still
> in circuit.
> >
> > There are _3_ SMPSUs in that box. One for the logic, one for the memory
> and
> > one (known as 'Bias' in the DEC documentation) to power the control
> circuits,
> > PSU control logic, etc. The last one is a relatively conventional SMPSU,
> it sounds
> > like it isn't working. It's nasty in that almost all the circuitry is on
> the mains side of
> > the isolation barrier, and an isolating transformer is almost essential
> when working
> > on it. The chopper transformer is on the PSIU baseboard, the chopper
> transistor
> > and much of the control circuity is on the 'Bias/Interface' PCB
> (leftmost board in the
> > PSU box). Be warned, therefore that some circuitry on this board is not
> isolated
> > from the mains.
> >
> > I would start by seeing if the 12V (and 5V?) from the 'Bias supply' are
> missing.
> >
> Thanks for the background and the tips, I’ll do some debugging.  Any
> thoughts on the “missing" diodes (D1-D4) on the bias board that I mentioned?
>From a quick glance at the schematic I think that this part is related to
the fan driving circuitry. The diodes look like some kind of protection
diodes which is good to have when switching inductive loads.  I have really
no idea why they have been removed. But I don't think it is related to the
PSU not working at all.


> - Josh
> > -tony

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