DEC H7822 power supply

Peter Coghlan cctalk at
Mon May 9 04:55:02 CDT 2022

Hi Maciej,

> > 
> > What is H7826 used in?  I don't think I have any of those.
> DECstation 5000/1xx systems and TURBOchannel Extender boxes.  Not sure if 
> anything else.

Ok.  I have VAXStation/MicroVAX and Alpha systems but no DECStations so that
explains why I haven't seen them.

> > Do you know what the white wire is for?  I originally thought it might
> > be power-good but only because the white wire on a H7816 seems to be
> > power-good.  On the 30-35042-01, it is slightly above 0V.  On my H7822
> > it is about 5.6V and on my H7821 it is 8.8V.
>  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.

Aha - that explains the odd and slowly varying different voltages I've
found on these leads in different machines.  It didn't dawn on me that
they could be isolated from ground.  I should have noticed that there
were two adjacent leads which behaved like this.

This should help clarify some of the puzzles I came across when I was
trying to trace the circuit of the H7821 and H7822.


>  This voltage is also present with the H7819 PSU.  It's not clear to me 
> what it is used for.
>  Maciej

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