DEC H7822 power supply

[Peter Coghlan]

"Maciej W. Rozycki" wrote:
>On Mon, 2 May 2022, Peter Coghlan via cctalk wrote:
>
>> Thankfully, it doesn't seem to have been there long and didn't get a chance
>> to spread around the board.  Bizarrely this capacitor has it's legs pointing
>> upwards and managed to leak while there are similar capacitors on the other
>> board with their legs pointing downwards which don't seem to have leaked (yet?).
>
> 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 use always seems to be at the
bottom of the stack...)

>
>  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).
>

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.

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 the case :-(

I unsoldered the eight other 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.
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 but I left them standing on the bench
overnight with their leads upwards and they all have some signs of leakage
visible on them today.

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.
Maybe it carries greater ripple current than the others or something like
that?  I can see that the other four similar capacitors make up PI filters
for the +5V and +12V outputs along with a pair of smoothing chokes, however
I can't quite figure out what the more leaky one is responsible for.  It
could be associated with the -12V output which might also be the supply
for the fans.  This part of the circuit is very hard to trace.

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.

I am hoping that the leaky capacitors were responsible for the odd
intermittent failures I experienced with this power supply recently.

Regards,
Peter Coghlan.

>  Maciej