lördag 10 oktober 2015 skrev Josh Dersch <dersc...@gmail.com>: > > > On Oct 10, 2015, at 2:15 AM, tony duell <a...@p850ug1.demon.co.uk > <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.
/Mattis > - Josh > > > > -tony > >
