DEC H7822 power supply

[Maciej W. Rozycki]

On Tue, 3 May 2022, Peter Coghlan via cctalk wrote:

> > Would the system have been possibly stored upside down sometime?
> >
> 
> I don't think so.  It may have spent some time lying on it's side due to
> deteriorating rubber feet and for ease of access and but I can't see any
> reason for it ever being upside down.  Once I discovered this issue a few
> years ago, I checked all my power supplies, removed any leaking capacitors
> and changed to storing the machines the right way up in a vertical stack,
> with newspapers between them in place of the rubber feet.  I hoped this would
> prevent any capacitors which hadn't shown any signs of leaking from starting
> to leak.  (Now whichever machine I want to work on always seems to be at the
> bottom of the stack...)

 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.

> >  These nasty caps do leak even while in storage (and even if never used, 
> > not even soldered ever, according to one source, a repair professional).
> >
> 
> They are nasty and devious.  In my case, the ones that have been stored in
> any orientation but not used much seem to have fared better.  I only started
> using this machine with the H7822 for extended periods for the first time a
> few months ago.  One of the reasons I started using it more is because I
> thought it was immune to the leaky capacitor problem!  It never saw any
> serious use before that, even when it was new.

 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 have at least two machines with H7822 power supplies.  Even though they
> have capacitors that look the same as the ones in the H7821, the ones in
> the H7822 power supplies didn't seem to be showing any signs of leaking
> when I examined them some time ago so I thought they might be from a batch
> that was unaffected by the problem.  It seems that this was not true :-(

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

> I unsoldered the other eight similar capacitors (four one each board) from
> the H7822 yesterday evening.  I found a small amount of leakage under most
> of them but it was was only evident after they were removed from the board.

 Yes, it's been a common case.

> In general, there was less damage visible under the ones on the lower board
> with leads facing down oddly enough.  I thought one of the capacitors from
> the upper board had not leaked at all.  I left the removed capacitors
> standing on the bench overnight with their leads upwards and they all have
> some signs of leakage visible on them today.  It's hard to draw any
> conclusions.

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

> The capacitor from the upper board that leaked enough for me to notice it
> might be under greater stress when operating than the others.  I think the
> same capacitor in the H7821 power supplies seems to leak more in those too.

 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.

> I don't have enough spare capacitors to replace the four on the lower
> board.  I am going to leave those out, leave the input lead to that board
> unplugged and plug the green LED into the upper board.  I hope it will then
> behave just like a H7821.  There won't be any power to the front disk drive
> connector but I am not using that so it doesn't matter unless the "power good"
> output is affected, in which case I will have to think of something else.

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

  Maciej