DEC H7822 power supply

Peter Coghlan cctalk at
Tue May 10 09:01:25 CDT 2022

Peter Coghlan wrote:
> 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 checked it.  It showed 4.4V and also seemed to be running rather
hotter than the other H7821 PSUs I've been working with.  I confirmed
that the internal thinwire tranceiver is still not working.  I swapped
out the H7821 in favour of a known working H7821 which gave me 8.9V
and the internal tranceiver then worked again.

I put the faulty H7821 into the system that the good H7821 came from
and confirmed that the internal tranceiver in that system now didn't
work and the 9V supply was around 5V.  With the different layout in
this system, I was able to see that the rear fan in the power supply
was not rotating which could account for it's higher temperature.
On the offchance that this might also account for the low 9V supply,
I tried unplugging the seized fan but this didn't make any difference.

The 9V supply comes from a 78M09 which I hoped would be faulty because
I can't make much sense of the circuit that feeds it.  I pulled the same
component from another older H7821 and it turned out to be a 7809 which
I tried fitting in place of the 78M09 but the symptoms remained the same.
Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
be a transistor.  Then I am completely lost.  I can't find the rectifier
or the winding on the chopper transformer that is supposed to produce power
for this part of the circuit.  Making comparison resistance tests between
the faulty H7821 and another one showed up some discrepancies around the
78M09 / 7809 and the D45H8 however the former were probably because of
the differences between a 78M09 and a 7809 and in the latter case, the
D45H8 looked reasonable when tested out of circuit.

The faulty H7821 has seen orders of magnitude more usage than any of
my others but it still has it's original SXF electrolytics which are
not showing any signs of leakage yet that I can see without unsoldering
them.  It's been the right way up pretty much forever.

Thanks Maciej and Tony for leading me to the cause of this issue.
Another mystery (almost) solved.


> (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.)
> Regards,
> Peter.
> >
> > -tony
> > 
> > 
> > 
> > >
> > >   Maciej

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