DEC H7822 power supply

Peter Coghlan cctalk at
Thu May 5 15:44:06 CDT 2022

Hi Maciej,

> 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.

> I can confirm now Nichicon PF 47uF/35V parts to be the source of an issue 
> with my Bel Power.  All four leaked.  Thankfully I was able to fully 
> revive that PSU (now in 24/7 operation).

I got replacement capacitors for my POWER-ONE unit but I haven't managed
to obtain a replacement IRFPE50 chopper transistor yet.  I found a sort-of
local supplier that lists them but they have a EUR 30 minimum order and
they don't stock much other stuff I need or want :-(

> These are likely standard type parts.  I've yet to come across a low 
> impedance axial capacitor type.  I may have missed something of course.

I couldn't find any suitable axial replacements so I ended up using radials
which were roughly the same height as the width of the axials.

> >
> > > Heat dissipated by the cap itself under high ripple current never helps 
> > > and will surely speed up cap deterioration.  After all its service life 
> > > halvens with each 10°C temperature rise even with non-faulty parts.
> > >
> > 
> > When I was shopping for replacements, I was a bit alarmed to find that
> > the maximum specified "endurance" (whatever that is) I could find was
> > 5000 hours.  This isn't much more than a long life incandescent light bulb.
>  Mind that it's at 105°C.  If you keep such caps operating at 65°C (which 
> is still rather hot), then endurance raises to 80000h (~9 years continuous 
> use).
>  Anyway try to chase replacements specified for at least 10000h at 105°C.
> Nichicon HE/UHE and Panasonic FR seem suitable replacements for Chemi-con 
> SXF, Nichicon PL, surpassing old parts in terms of ESR/impedance/ripple 
> and dimension-wise.

When I was trying to order them, I found it hard to get anything suitable.
I ended up ordering 15 (out of 16 in stock at Mouser) Panasonic FC series
to do just three PSUs.

> I don't have any H7822 PSU.  Your experience with the H7821 is the same 
> as mine though (and I still need to figure out what's wrong with one which 
> still doesn't drive its power-good line active after recapping).

There is a 464 or 560 Ohm resistor near the capacitor in middle of the H7821
which has been damaged by leakage in several of my PSUs.  It is in series
with a thermistor mounted on a heatsink and as far as I recall it is part
of the fan control circuit.  It might also contribute to power-good though.
Other components could be damaged too or there could be goo lying under
some components.

> I used 
> Nichicon HE P/N UHE1E182MHD as the replacement for those.  There does 
> appear to be COVID-related shortage of this part (600 expected at Mouser 
> 15/03/2023, ugh!), which used to be readily available in large quantities 
> several years ago.  However Panasonic FR P/N EEUFR1E182 is available in a 
> small quantity (and is better).

I should grab some EEUFR1E182 before they disappear.

> I came across LXF parts in one H7826 only and they were clean, but I 
> chose to replace them as a precaution anyway as I've got stuck with trying 
> to repair a couple of broken H7826 PSUs already still not working after 
> cleaning the mess and replacing broken caps (mind that I'm a software 
> engineer with enough hassle to sort out on the software side already).

Sounds like it was better to not take a chance on them.

> I've seen leaks from SXF parts on the primary side too with the H7826, so 
> it is not that they only fail on the secondary side.  Also it is lower 
> frequency ripple that's more problematic, see frequency correction factors 
> for ripple current in datasheets, because impedance is higher at lower 
> frequencies.  That's an inherent property of capacitance.

That makes sense.

> > I replaced the five leaking capacitors on the upper board in my H7822,
> > disconnected the input to the lower board and moved the LED connection to
> > the upper board.  The machine powered up nicely afterwards, the fans spun
> > and the green LED came on after a short delay.  However, the diagnostic
> > LEDs all come on and stay on so it appears the CPU is being made aware that
> > the lower board is not functioning, even though I don't need it.  I should
> > to do some comparisons with a H7821 and see if I can work around this.  Or
> > maybe I could fit some not quite to specification electrolytics from the
> > junk box on the lower board, just to keep it happy?
>  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.


>  Maciej

More information about the cctalk mailing list