Re: power supplies, was Re: NetBSD/vax

[Dave McGuire <mcguire%neurotica.com@localhost>]

On 3/22/20 6:48 PM, Maciej W. Rozycki wrote:
>  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.

  Excellent information Maciej, I've made a note of this too.  Thank you
for the explanation.

  Of course now I remember that I've been storing probably 25
VAXstations and DECstations of that era up on their sides for twenty
years.  Crap!

              -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA