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

tony duell ard at
Sun Oct 11 12:23:36 CDT 2015

> 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...)

I will give page references as for the .pdf of the printset on bitsavers.

The bias supply is a switcher. On page 162, T1 is the chopper transformer,
D6 the output-side rectifier, C3 the smoothing capacitor

On page 168 (Bias/Interface PCB) Q1 is the chopper transistor, E2 (555) the
control oscillator, etc.

E1 on page 167 produces 5V from the 12V supply, but if the latter is missing it
can't do a lot. Get the 12V back first.


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