If you look at the schematic of most older PC power supply units you will find that they almost
all use the same PWM IC to control output voltage. There should be two resistors (one from +12V
and one from +5V, that join with a third resistor to ground at a pin on the PWM chip. The combination
of the 12V and 5V inputs generate usually 2.5 volt potential at this PWM input pin-1. That is matched
with a 5V / 2 = 2.5V reference connected to the opposite input of the error amplifier at pin-2.
Matching of these two input voltages is used to regulate the PWM output and thus the 12V and 5V
The PWM chip is usually a TL494 or something very similar. The three resistors connect to pin-1
and control output voltages.
By re-computing the value of the 12-volt resistor and disconnecting the 5V resistor it is usually
possible to make the 12V output be close to 13.8 or something similar and remove regulation
of the +5V.
You may notice that the 5V AC windings and the 12V AC windings on the transformer are in series.
This means that your total current output can be close to the original 5V rating as long as the 12V
rectifier diodes will stand that amount of current. In some cases it is possible to interchange the
5V diodes with the 12V diodes to provide more current handling capacity. Info from the diode
It may be necessary to add more filter capacitance to this new 13.8 volt output in order to minimize
switching transients and LF ripple.
PLEASE BE VERY CAREFUL WHEN WORKING WITH PC POWER SUPPLIES. THE AC LINE
INPUT IS RECTIFIED TO 360 VOLTS DC WITH NO GROUND ISOLATION FROM THE AC MAINS.
IF NOT VERY CAREFUL YOU COULD EASILY ELECTROCUTE YOURSELF.