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

[Josh Dersch]

On Sat, Oct 10, 2015 at 2:38 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:

> >
> > 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
>
> Yes.
>
> > 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
>
> Remember that said diodes do slow down the decay of the current (and thus
> the magnetic field) when the transistor turns off (this can be a problem
> with
> solenoid drivers for paper tape punches, etc). It is possible that with
> the diodes
> in-circuit the fan drive waveforms were not right.
>
> > PSU not working at all.
>
> Correct. The fan driver is powered by the 36V output of the 'memory' SMPSU,
> Even if it is defective, the bias supply should run, the logic supplies
> should come
> up in the ON position of the key, and so on.
>
> That 'bias/interface' PCB contains (at least) the bias supply chopper and
> control
> circuit; the fan driver; and the control logic to start the 2 main PSUs in
> the correct
> posiitions of the keyswitch,
>
> -tony
>
>
>
> /Mattis
>
>
>
> > - Josh
> >
> >
> > > -tony
> >
> >
>

Thanks for the explanation.  I tacked on some wires to the bias board (to
bring them out for safe probing); the wires are connected to the 5V logic
supply and the +12V bias supply and I get nothing at all out of them when
the H7140 is turned on.

I'm slowly coming to terms with the schematic but I'm not quite clear where
these two voltages are being generated.  I note from the block diagram that
the +15 and +12 are rectified on the motherboard (backplane) -- which
should make it GREAT fun to probe.  So it looks like some careful
disassembly is now required (unless there's a trick to probing things on
the backplane...)

Thanks,
Josh