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

tony duell ard at
Sat Oct 10 16:38:32 CDT 2015

> 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

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,



> - Josh
> > -tony

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