Originally posted by: mindless1
The card uses a reference design, you can see on a better picture like
this one it's a low esr, electrolytic, surface mount, 100uF, 25V.
Since I don't have one and can't measure voltage at that point in the circuit, 16V would be the conservative maximum value it would need be. The 25V is not important, manufacturers often use whatever they have stock of that fits - so long as voltage is same or higher than needed. There is no way it could need over 12V though next higher common voltage rating is 16V. To keep ESR lower I would either stay with 25V at 100uF, or use 16V at 220uF or higher instead.
If you look at a picture of the back it appears (you would have to confirm yourself, seeing a picture is the next best thing but still not as good as actually having card in front of you) that although it's a surface mount cap, the design can accomodate through-hole leaded caps instead if that's what you had or could find more easily.
picture of back
Those holes look like they would accomodate a roughly 5 to 8mm diameter cap (the common lead spacing one in this diameter size range would have), not 10mm diameter or larger. The white side of the silkscreen for the through-hole cap mounting corresponds to the negative cap lead.
You could use a cap off a spare/dead motherboard if you had one, it wouldn't need be electrolytic, could be solid instead (typically blue or purple colored top instead of black) but it should be low-esr for good life. The solid are low esr but with a *generic* electrolytic you can't assume so unless the manufactruer description or spec sheet plainly states it and it would as this is a primary selection criteria for a cap.
You shouldn't go lower than 100uF but a little higher shouldn't be a problem, I might go up to 1000uF in a pinch but if I had any to chose from would probably pick 220uF to get a little lower ESR and better life (all else being equal) without unnecessary extreme increase in capacitance. There is an art and black magic in cap replacement once you realise many are selected as much for budgetizing a design as any other factor, none of the caps on these cards are used for timing circuit values so none of the uF values are critical to the extent that you at least don't go too low in value or extremely high and cause too much current inrush, voltage depression when the circuit starts up. Bleh, probably more than you wanted to know but the main point is it need not be exact match to what it used to be, just falling within certain parameters.
If you were thinking about getting a cap locally like at Radio Shack, they aren't likely to have anything suitable in a low ESR, occasionally in their bulk mixed blister packs they had some brown Nichicon PM or PW which would work if the random assorted value on it happened to be right as mentioned above in uF, voltage, diameter, but don't count on it, usually their bulk packs are just general purpose and none of their individually packaged caps were particularly low ESR last time I bothered to check their bins. Digikey, Newark, Mouser are among a few online electronics houses that would have something suitable though Digikey I know has a minimum order limit else they charge an extra $5 fee making them unsuited for a cap. Maybe a local TV/etc electronics repair place would sell you one for a dollar if you asked nicely.
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