[sdiy] How do old electrolytics go bad?

[Roy J. Tellason]

On Friday 06 February 2009 05:40:34 pm Bob Weigel wrote:
> I typically see old electro's drift higher and higher..even to 10X the
> original value and then they'll plummet often to dead or they'll even
> short sometimes.  This in observing Fender and other amps of various
> ages largely.
>
> The *source* of this phenomena, I assume, is that the electrolyte
> gradually degrades and in doing so some functional distances may be
> altered..and of course closer plates mean higher capacitance.  Shrinkage
> of the dielectric material.   And then as it gets too close it punctures
> and leaks sometimes.  Or something totally disconnects in other cases.
> Not sure but rambling possibilities anyway :-)
>
> Anyway that phenomena is common whatever the case, let me assure you.  -Bob

The actual electrolyte isn't the stuff that's between the plates,  which is 
conductive BTW,  but a thin layer of oxide (?) on one plate,  I forget if 
it's the positive or the negative one.  This is applied when the cap is made,  
and the thickness of the layer determines both the capacitance and the 
working voltage.

As time goes by,  and with no voltage applied,  the oxide will start to 
dissolve,  giving a thinner layer,  higher capacitance,  and lower working 
voltage than what's on the label.  This happens even with brand new caps that 
have been on the shelf for a while.

The solution is to apply current-limited voltage to the cap,  over time,  
which re-forms the oxide layer.  This is an especially common practice with 
folks who are building antique-style gear or trying to restore old radios,  
where the required parts may simply not be available any more.

Google "electrolytic re-forming" for more info on this.

The other thing is that most electrolytics have fairly wide tolerances,  most 
often either -20%/+80%,  or -0%/+100%...!

I got a cap checker a bunch of years ago,  a B&K unit.  What got tossed out 
was a bunch of caps that were all reading *way* low that I'd salvaged out of 
assorted gear over the years.  Stuff that read on the high side never 
bothered me all that much.

> Ingo Debus wrote:
> > Hi,
> >
> > we all know that electrolytic capacitors often go bad when they are
> > old. In the past I had replaced some that had a significantly lower
> > than rated capacitance. But now I got two that show a higher than
> > rated value on my DMM: one is rated 100uF/3V and shows 270uF on the
> > meter, the other is rated 16uF/16V and shows 23uF. Of course I
> > desoldered them from the circuit before measuring.
> > Is this normal? Or did they have the higher values from the
> > beginning? Or are they leaking and thus fooling my meter?
> >
> > I estimate these caps are about 40 years old.
> >
> > Ingo
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