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

[Josh Dersch]

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