Re: Electrolytic caps?
From: Pooh Bear (rabbitsfriendsandrelations_at_hotmail.com)
Date: 02/28/05
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Date: Mon, 28 Feb 2005 00:20:09 +0000
John Woodgate wrote:
> I read in sci.electronics.design that Joerg <notthisjoergsch@removethisp
> acbell.net> wrote (in <Kb6Ud.7462$OU1.5066@newssvr21.news.prodigy.com>)
> about 'Electrolytic caps?', on Sat, 26 Feb 2005:
> >Hello Watson,
> >
> >>Yes and yes, but it depends on how old those caps are. Over five years
> >>old, and they're not 'fresh'; ten years and they're 'stale'.
> >>
> >>
> >
> >Nah. I have some 40+ year old radios where the electrolytic caps are
> >just fine. A couple times I actually did a resistor charge and leakage
> >test just to see. They were nearly as good as new.
> >
> >But sometimes when they had been in storage for decades they may have to
> >be "formatted" slowly to get them used to the job of "being a capacitor"
> >again.
>
> It took a long time to discover that sodium chloride (common salt)
> contamination was responsible for short life of aluminium electrolytic
> capacitors, simply because traces of salt are almost everywhere. So if
> your old caps have, by chance or design, a low salt content, they may
> well still be good.
Talking of salt messing things up....
Do you recall that GEC did a deal with Nat Semi IIRC to make a UK cmos fab
back in the 70s ?
One of the lecturers at UCL had been there and taught us semiconductor
physics. They could never get the process fixed. The chips would work - but
'go bad' after some months.
It was finally found that the the nichrome wire that they were using in the
ovens was quenched in a salt solution during the manufacturing process.
Sodium ions were 'boiling out' of the wire and entering the chips. Initially
this caused no problem until the sodium ions migrated into the active region
and threw the threshold voltages of the fets all over the place.
Sadly the problem was diagnosed too late and the project was dropped.
Graham
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