[sdiy] How do old electrolytics go bad?

Bob Weigel sounddoctorin at imt.net
Fri Feb 6 23:40:34 CET 2009


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

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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