DEC H7822 power supply

Peter Coghlan cctalk at
Tue May 3 12:42:18 CDT 2022

Hi Maciej,

Thanks for your input.

"Maciej W. Rozycki" wrote:
> Lying on a side would also permit leaking, I've seen an H7821 damaged in 
> storage that way.  Gravity only helps with the leads up.

I have lots of H7821s that were on their side for a few years and leaked a
lot but I also have H7822s that were stored the same way and did not show
any evidence of leaking until very recently.  I think I also have some
H7821s that were the right way up most of their lives and have leaked a
little.  It's getting hard to remember what happened to which :-(

> From experience Chemi-con SXF caps used with many DEC PSUs need to be 
> urgently replaced.  Other Chemi-con lines reported affected are LXF, SXE 
> and KME.  Products of the time from other manufacturers may be affected as 
> well.  I'd have to check what line were those that leaked in a Bel Power 
> PSU that I had to fix (I reckon you had a similar experience, right?).

I still have the leaky electrolytics I removed from the POWER-ONE PSU in my
Cisco IGS a while back.  I stored them with their leads up and goo seems to
be still oozing out of some of them despite their inactivity and orientation.
These ones are marked Nichicon PL(M) 4700uF/63V, 2200uF/16V and 330uF/35V and
also have markings like H8950, H9018 and H8946 - maybe these are date codes?
I also removed the smaller ones like 47uF/35V PF(M) H8952 for example but it
is less clear to me whether these were leaking too or just got leaked on by
the others.  There were only a few of them so I decided they were better out
than in.  They all have similar coloured brown sleeves like the faulty ones
in the DEC power supplies too.

There are also the leaky 10uF/35V axial electrolytics in my LK201 keyboard.
Those are in orange sleeves and marked "ESZ", whatever that is.  They have
date codes like 8612.  I thought this might be a widespread problem but
so far I have only found it in one keyboard.

> It was the composition of the electrolyte that was outright wrong, so I 
> doubt it's batch-related.

> Once the seal has broken I guess all odds are off.  I could imagine 
> capillary action to take effect.

Looks like I need to go back and recheck everything I thought wasn't leaking
last time I checked :-(

Thing is, to check them, they have to come out of the case and that involves
at least some change in orientation, except for contortionists...

> 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.

> As a matter of interest what capacitance/voltage are those?  Are they of 
> the Chemi-con/SXF type too?

All the leaking ones in DEC H7821 and H7822 PSUs I have come across so far
are 1800uF/25V Chemicon with brown sleeves.  I think mine are all SXF
but I am not 100% sure of that.  There are lots of other electrolytic
capacitors in these power supplies but I've only looked closely at the
larger ones.  All of the reservoir capacitors attached to the mains bridge
rectifiers that I have seen look fine.  Maybe I need to go back and check
the smaller ones though :-(

I think I came across some LXF ones that seemed to be ok, I can't remember
where though.  I probably need to go find these and check them again :-(

Here's a thought.  Apart from the keyboard, all the ones I have seen that
are leaking are filtering the outputs of switch mode power supplies.  I
wonder does the higher frequency of the ripple they are dealing with have
a bearing on this?

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?

Peter Coghlan.

>  Maciej

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