source for large capacity electrolytics

[Brent Hilpert]

On 2010 Sep 8, at 1:01 AM, r.stricklin wrote:
>
> The 5V rail in my Sun 2/120's PSU recently failed suddenly while the 
> system was running. No pop, but the rail measures out around 2.5VDC. I 
> believe I have tracked the fault down to one of a pair of 26000uF 
> electrolytics which are in parallel across the 5V output studs. These 
> are screw-lead, rated for 7.5V (12V surge), 85 deg. C, -10 +75% tol. 
> They are 1.375" dia, and 2.125" tall. Mechanically, there is no room 
> for anything even slighlty larger.
>
> Mouser stocks a part which is physically too tall---at 3.125", too 
> tall by an inch. This is a 10V (12V surge) part. They also stock a 50V 
> part from a different manufacturer which is even taller (>4"). There 
> are 10V (12V surge) parts that fit, but have too low a capacitance 
> (18000uF). There are 7.5V (9V surge) parts which will fit, but have 
> too high a capacitance (49000uF), and I'm not totally sure how to be 
> certain that a 9V surge rating is adequate in this application.
>
Forgot to mention: considering the caps are on the output of the 
regulator, I would expect the 9V surge rating to be fine. If they were  
before the regulator it might be a greater concern.

What is the basis for suspecting the caps? If they are dragging the 5V 
down to 2.5 they should be sucking a lot of current through the 
regulator and dissipating a lot of power themselves (heating up).