DEC H7822 power supply

Maciej W. Rozycki macro at
Sun May 8 06:51:51 CDT 2022

Hi Peter,

> > Right, my notes indicate Nichicon PL parts might be problematic too, e.g. 
> > one at 4700uF/10V on 5V output of the H7826 PSU.
> >
> 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.

> >  The symptom is exactly like with my broken H7821.  Check the power-good 
> > signal (brown wire with the H7821, possibly likewise with the H7822).  It 
> > should be driven high at the TTL level.
> >
> The wire in the same position as the brown one on a H7821 is purple on a H7822
> and they both seem to have the same function.  I ended up temporarily fitting
> a two 1000uF/25V capacitors on the lower board of my H7822 and reconnecting it
> to the upper board.  This gave me +5.1V on the purple wire and the CPU then
> ran ok for test purposes.
> I have a VAX 4000/100A which has a Zytec Model EP 071181 power supply.  The
> only Digital reference on it is 30-35042-01.  It is the same physical form
> factor as the H7822.  The output connectors look the same, including the
> wire colours except +12V is brown instead of orange and this one has an
> extra connector for 3.3V (which is not used for anything in the 4000/100A).
> The reason I mention it is there is a list of output voltages on the label:
> +5.1V, +12.1V, +3.3V, -12V and -9V.  By a process of elimination, the -9V
> supply must be the grey wire (which gives me -8.2V when I put the meter
> on it.  All the other voltages are much closer to specification).
> I wonder if the grey wire in the same position on the H7822 is also supposed
> to be -9V?  When I try to measure it on my H7822, I get a negative voltage
> but it is wandering around -4V.  In contrast, the -12V supply on the blue
> wire is a steady -11.99V, probably due to the 7912 regulator.  The same wire
> as the grey on the H7822 is yellow on a H7821.  I measured the voltage on a
> working H7821 and found a steady -0.2V which seems odd.
> 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 

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


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