DEC H7822 power supply

Peter Coghlan cctalk at
Mon May 9 05:50:53 CDT 2022

Tony Duell wrote:
> On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
> <cctalk at> wrote:
> >  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> > wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> > across the suspicious connections as any reference to ground may not be
> > indicative.
> >
> >  This voltage is also present with the H7819 PSU.  It's not clear to me
> > what it is used for.
> It's only a guess (this is rather more modern than the DEC hardware I
> have), but it may be a supply for a thinwire ethernet transceiver
> circuit. This would be insolated from system ground of course, and
> I've seen something similar in an X-terminal.

Hi Tony,

After years of rediscovering the same failures over and over again,
I finally managed to get in the habit of making notes while I am
trying to diagnose faults.  Now I just need to get in the habit of
reading my old notes when I start to work on a new fault.  I did
that just now and I found that I have a MicroVAX 3100 model 76 whose
internal thinwire tranceiver failed last summer.  Not finding any
obvious reason for the failure, I replaced it with an external
tranceiver plugged into the AUI port and then forgot all about it.

I should check that this floating 9V output from it's H7821 PSU is
working correctly.

(I wonder why it is described as -9V on the label of the 30-35042-01 PSU
in my VAX 4000/100A?  If it had been shown as +/-9V for example or even
just plain old 9V, I would have been less inclined to assume it was
referenced to ground.)


> -tony
> >
> >   Maciej

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