Not for computer use, but I need a high current power supply that can deliver 15+ amps at 13V or above (ideally 14V, but no more than 15V). All the high current power supplies are expensive, so I was wondering if anyone knows of a way or could point me in the right direction to modifying an existing ATX power supply to output 14V on the 12V rail. The great thing about switching ATX supplies is that they are cheap and capable of high currents on the 12V rails.

I've come across some power supplies that when taken apart, there is a variable capacitor that you can turn to increase/decrease the voltage in a tiny increment.

Also, here is a project of exactly what I want to do, except I dont really know what is going on:
http://www.webx.dk/oz2cpu/radios/psu-pc1.htm

Ideas would be great.

Is not to complicated to get 14V you just need to find the voltage feedback where are usually two resistors acting as a voltage divider (some time you may find a variable resistor if you are lucky) you just need to change the value of one of the resistors in order to get a different voltage on the 12V line. Probably you will need this 14V to charge batteries .
If you don't already have the power supply then you can buy from ebay something like this link they already have a variable resistor so you can set to 14V you can buy the 12V or 15V version and set your desired voltage.

The feedback comes from +5V, too. The divider probably consists of 3 resistors. Plus, some (many?) PSUs contain overvoltage protection. "Variable capacitor" ?? Think again.

What do you mean? I understand the higher quality PSUs have voltage stabilization features, but Ive taken apart a bunch of cheap power supplies where there was a variable capacitor that you can change with a screw driver, but I was only able to get the +12v rails to fluctuate +/- 1 volts.

The power supply on the eBay link looks good but there's not much details on it. I see there is a variable capacitor also on this power supply, but whether that will allow the voltage to go up to +15v is uncertain.

The one you think is a variable capacitor is actually a variable resistor to fine tune the voltage output.
In one of the ads on eBay I fond more details and they say that you can get about 20% voltage variation using the variable resistor they also have a 15V power supply so you can get that one and reduce to 14.4V or you need (probably 15V is closer to your needs).

Oh, my bad then. Thanks for the suggestions though. I ended up picking up a switch mode 400w 15V supply (10% voltage regulation) for about $50. That should do it, saves the hassle of modding an existing PSU.