DEC H7822 power supply

[Maciej W. Rozycki]

On Sun, 8 May 2022, Tony Duell 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.

 That makes sense to me.  Both the VAXstation 3100 and the DECstation 3100 
systems (which are among the users of the H7821 PSU) and the VAXstation 
4000 system (likewise the H7819 PSU) have a built-in 10BASE2 interface.  
Where these PSUs are used for storage expansion boxes some voltages are of 
course unused.

 NB I have literally just recapped an H7816 PSU where I spotted a bunch of 
Chemi-con LXF parts starting to leak (both the DEC-usual 1800uF/25V parts 
and less common 2200uF/10V and 1000uF/25V ratings) as well as a pair of 
notorious Nichicon PL parts at 4700uF/10V (though these were clean and 
from the look of the closing cap a more modern variant; I've replaced them 
for extra safety anyway).

 Thankfully the PSU still was in a working condition and it was easy to 
get it disassembled enough to get at the parts, there was virtually no 
corrosion yet, and the PSU now continues working after the recap, so it 
was quite an easy task on this occasion.  The old parts, including the 
leaking ones, still showed in-range values of capacitance and ESR, not 
unusually, which is obviously why no damage to other circuitry has been 
caused.

 Lucky me, also to think to have a look inside!

  Maciej