The concept of converting a disused computer power supply to 13.5V operation was first mooted in the November & December 1998 issues of the no-longer-published "Radio and Communications" magazine. This article builds on that information.
The process is relatively straightforward and involves removing
all the components involved with the existing 5V and 12V outputs, rewinding the main transformer and then changing the feedback components to give an output of 13.5V instead of 5V.
First, a few words on selecting the power supply to be
It must not be an ATX type since they work quite differently to
the types under discussion in this article. Then check that it will maintain a constant output voltage under load, eg, one or two 12V 50W halogen lamps.
Reverse engineering and conversion to a new output is difficult
at the best of times and nigh on impossible if the thing does not work in the first place!
Second, consider your power requirements. If you only need
about 10A at 13.5V, you probably don't need to change the main transformer as the original +12V can be modified to deliver 13.5V. This means that only the output voltage control sense circuits need changing.
Third, choose a unit that contains the least amount of dust
(possibly had the least use!) and check the fan for free movement and lack of "end play" in the bearings.
Fourth, check if the unit uses two ICs in the control circuit:
a TL494 and a LM339. Their IC pins and functions are easily identified, making analysis of the circuit much easier. If you can't identify the ICs, you may still be able to modify the supply, but you will be you will be very much on your own and the information in this article may not be of much help.