Using a Variac to revive power supplies

[Tony Duell]

> 
> 
> Nasty things exploding capacitors. Engineers dictum "If in doubt chuck =
> it

Although modst modern capacitors (read : anything you will find in a 
classic computer) have vents or deliberatly weakened sections of can so 
that they will fail in a relatively safe manner. Still unpleasant.

> out" ie replace. We used to reform supposedly new electrolytics if they

I don;t normally replace working components, but I did replace the pair 
of mains smoothing capacitors in my HP120. They tested fine, but the tops 
of the cans were bulging. The position of the PSU board is such that they 
were pointing straight at the CRT neck. Had one failed, I suspect it 
would ahve broken the neck off the CRT. Now, while that is unlikely to 
cause a violent implosion, new CRTs are much harder to find than 
smoothing capacitors, so I repleaced the latter before any harm was done.
 =
> had
> been in the stores for a while.

If a cpacitor refroms and has a low leakage, etc, then it should be 
perfeclty OK to use. After all, reforming is just redepositing the 
dielectric oxide layer, and that's how they were 'formed' originally. 

Perhaps I have just been very lucky (or only work on well-made machines), 
but I do not find smoothing capacitor failure to be as much of a problem 
as some people seem to think.

-tony