10 Oct
2015
10 Oct
'15
2:15 a.m.
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.
-tony