> On Oct 10, 2015, at 2:15 AM, tony duell <a...@p850ug1.demon.co.uk> 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
