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

Josh Dersch derschjo at
Sat Oct 10 10:18:35 CDT 2015

> On Oct 10, 2015, at 2:15 AM, tony duell <ard at> 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?

- Josh

> -tony

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