I have a completely discharged (when I got it there was a paper clip between the posts) 120v capacitor. It is the same (physical) size as the capacitor my brother is currently using between his battery and amplifier in his car. I would like to know if I can charge this capacitor off my car battery and use it for my amplifier. I assume that it will only charge to the voltage I apply to it, but I would like to have an expert advise me on it before I try ( I'd just as soon not cook my amp). It says nothing about Farad capacity on it just the 120v designation. Thank you for any help you can provide.

Technically, it all depends. However, as a general rule, the answer is a very large no. Unless they are the same amplifiers, used for the same purpose, with the same sized (capacitive size, not physical size) capacitor, it could cause some severe trouble.

The capacitor itself would only store a potential up to the voltage that you put across it. However the capacitance of the capacitor (the Farad value) when combined with the other elements of the circuit cause different time constants and discharge at different rates.

For a series RC circuit (using your cap against a load);
VR = Vo e-t/RC
If C decreases, the time constant 1/RC increases.
This means that the voltage change happens much more quick. The passing of charge at a more quick rate means that more current is flowing through the rest of the circuit. This is liable to damage a circuit.

Simply,
1) Find out the capacitance required for your amplifier
2) Find out the capacitance of the cap you have
3) Only replace the desired cap with the one you have if you are SURE that they won't damage each other.

Is replacing an A/C capacitor a DIY job?

Can a diy'er replace an A/C capacitor?

In this case it should be fine. As sclark pointed out unless they are the same size in Farads (regardless of physical size), they will behave differently in a circuit in general. However, for your specific application, the capacitor is there simply to provide an extra "kick" when the amplifier tries to draw more current than the battery can supply at one time (in short bursts). In this case, it's simply a matter of bigger is better. If the capacitor is the same size (e.g. 0.5 farads) as your brother's, great! If it's bigger, even better! If it's smaller, the worst thing that will happen is you're stereo won't have quite as much bass "kick" as you brother's.

johngrace57, you'll get more responses if you post your question as a new thread, not as an "answer" to someone else's question.

Anyway, replacing an A/C cap is pretty straightforward. Just make sure you turn off the circuit breaker to the A/C unit before you do anything. Then, just to make sure it doesn't have any residual charge, place a metal screwdriver or wrench across the terminals to short them out. After that, it's usually just a matter of taking the old capacitor out, putting the new one in, and reconnecting the wires to the new one in the same configuration as the old one. If you're really paranoid about getting it wrong, take a picture with your phone or digital camera before you disconnect the wires so you can be sure to reconnect them exactly as before.

Definitely a pretty simple DIY job as long as you get an exact replacement for the capacitor.