[sdiy] How do old electrolytics go bad?

[Nathan M. Reeves]

I know in switching power supplies the heat getd 'em from being  
cramped in there so close to other components...
thats why they have fans but i guess it still gets too hot....i've  
replace caps on a power supply less than 3 years old!  (and just out  
of warranty) 8)
apple PC power supplies are notorious for this behavior...
cheers,
nate


On Feb 6, 2009, at 5:40 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
>
> 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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