Re: power supplies, was Re: NetBSD/vax

["Maciej W. Rozycki" <macro%linux-mips.org@localhost>]

On Sun, 22 Mar 2020, sadness wrote:

> >   Out of curiosity, what was the failure mode of that power supply?
> 
> In my power supply the problem was caused by failed capacitor, there was
> four nichicon 1800uf 25v caps that has leaked all over the board and
> corroded some of the solder joint in the pcb, every other caps in the
> psu was fine except this four, replaced them, cleaned the pcb, redone
> the damaged solder joint and the power supply is now working as is
> should, before the repair it was totaly dead, no voltage on any rail, no
> power led, not even the classic ticking sound of a switching psu in trouble.

 NB this is the usual failure mode of the H7821 PSU used in these systems, 
which has capacitors suffering from a known issue with the quaternary 
ammonium salt system used in the electrolyte.  It causes the seal to fail 
eventually and the affected capacitor to leak wreaking havoc.  Thankfully 
the H7821 is mounted such that the capacitors are upside down in normal 
operation making the gravity prevent the electrolyte from interfering with 
the seal.  Consequently leaks I believe only happen if a machine using an 
H7821 is stored in a different orientation, such as on its side, and not 
while in use.

 The capacitors I have seen fail in this PSU were actually Chemi-Con SXF 
line ones (1800µF/25V as you say) and according to my notes there ought to 
be five rather than four actually.  I would highly recommend chasing the 
fifth one and replacing it too.

 Other reputable makes and lines of capacitors of the time also suffered 
from the said problem, e.g. I saw Nichicon PL parts leak in the H7826 PSU 
used with some DECstation 5000 machines.

  Maciej