DEC H7140 (11/44) power supply revisions and general advice?
mattislind at gmail.com
Sun Oct 11 02:02:27 CDT 2015
2015-10-11 5:44 GMT+02:00 Josh Dersch <derschjo at gmail.com>:
> On Sat, Oct 10, 2015 at 2:38 PM, tony duell <ard at p850ug1.demon.co.uk>
> > >
> > > From a quick glance at the schematic I think that this part is related
> > > 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'
> > 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
> > 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.
Then I would check the circuitry on page 3 of the schematic on the bias
board. Use a insulation transformer (and possibly a variac) and check
voltages here. Does the 555 has supply voltage?
Does the 16.5V zener operate correctly? Is Q6 ok? If yes, is there a
waveform on 3 of the 555? What is the signal like on base Q1? Collector Q1?
(probably nothing since you don't have any bias voltage)
> 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...)
The bias voltage is generated by the chopper on page three in conjunction
with T1 and some rectifying diodes on the motherboard. Then it is feed back
to the bias board on J2:2 to produce the 5V. (page 2)
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