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

[Josh Dersch]

On Sat, Oct 10, 2015 at 8:44 PM, Josh Dersch <derschjo at gmail.com> wrote:

> 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
>
>
>
And a quick correction, it's -15V, not +15; I need to proofread more :)

- Josh