17 Jul
2015
17 Jul
'15
12:45 p.m.
On 2015-Jul-17, at 11:42 AM, tony duell wrote:
Could you, please, explain why? And how often should this be done? Every
week, every month, every year, or what?
FWIW, the number PSU elecrtrolytics I have replaced can be counted on the fingers of
one hand -- in unary. Well, perhaps both hands. But it's <1% of all the PSU
electrolytic
capacitors I own.
Only 2 cases spring to mind :
The PSU in my 11/44 had a high ESR capacitor on the +36V rail (all other caps in the
machine
were fine)
I changed the 2 mains smoothing capacitors in my HP120 not because they were electrically
defective (they tested fine) but because one was bulging a little on top and had it
exploded it would
have hit the neck of the CRT with all the problems that would be likely to cause.
I do find this witch-hunt against capacitors to be curious, given how few I've found
to have
failed. I suspect a lot of it comes from audiophools who think this is the way to fix
anything...
This is something Tony and I are quite in agreement on.
Similar to Tony, (and as mentioned in discussion on this topic a couple of months ago): in
the solid-state category, of the many pieces of 1960s & 70s and later equipment I have
or have serviced, the vast majority are running with their original capacitors.
If you're dealing with a 1936 or 1952 tube radio, a knee-jerk "replace the
capacitors" is warranted.
If you're dealing with a 1970s computer, it isn't (IMHO). Esp. when they're
screw-terminal 'computer-grade' caps.
My own perception of the concern is that it has been perpetuated over the years from the
vacuum tube / antique radio arena. The issue of capacitors "drying out" dates
from the days (1920s,early 30s) when electrolytics actually were filled with an active
liquid which actually did dry up.
"Dry electrolytics" were developed in the 1930s, and while early dry
electrolytics also warrant replacement, the chemistry and techniques have seen a few
improvements in the many intervening years, and solid-state equipment is not placing the
same stresses on caps as tube equipment.
In other arenas it's a real issue, in a modern arena it is largely lore.
The point of electrolytic caps is to form an oxide to be the dielectric, formed (in part)
out of the electrolyte, and while I'm no expert on the chemistry, I will point out the
oxidised state is 'the' or 'a' low energy state, and hence relatively
stable. Rust doesn't normally undo itself.
It is generally a good idea to re-form electrolytic
capacitors in power
supplies, and to bench check the power supplies (under some kind of
load) before actually applying power to the whole unit.
It is always a good idea to replace electrolytic capacitors in power supplies.