Mainboard repairs

I have several mainboards of the old slot 1/ socket 370 combo type that still take an old style keyboard but can handle both AT and ATX power supplies. None of these boot up, some act dead, others will cause the PSU to jump in overload protection when powered up. None give a post screen. As i have several older pc's in from a school i would like to find a way to revive them. No bulged capacitors but considering their age is it possible the caps have high esr anyway, causing the board to fail? I have no esr meter at hand, any test i can do? Where does one buy computer capacitors anyway?

Bart Bervoets

Digikey, Mouser, B&D Enterprises are sources I've used. Watch diameters since many boards won't accept capacitors even slightly wider than the originals. Those companies also have a good assortments of schottky diodes and MOSFETs, and check those parts for shorts because bad caps can make them overheat.

If you have a scope, you can test for ESR by connecting the capacitor to its square wave calibration output with a resistor Rin in series (try about 10K). Ideally the waveform will look like a ramp that starts at 0V, but ESR will make it start at a higher voltage. Calculate the ESR just as you would calculate a voltage divider. A search for "99 cent ESR" will give details.