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

tony duell ard at
Sat Oct 10 04:15:34 CDT 2015

> 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.


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