In message , "Dave Plowman (News)"
writes
Can anyone give a link to a site which covers modifying an ATX PS 12 volt
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.

Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take you
into the realms of voltage dividers and the TL431.

Basically, the PSU will regulate the 5v or 3.3v rail and all the other
rails are set by the transformer ratios. You need to alter the voltage
divider for the 5v or 3.3v rail and all the other rails will change
proportionally (with the exception of the +5v SB as this is not
generated by the main switcher and maybe the -12 and -5v as these are
often provided by 79xx linear regulators). Don't forget, you may need
to add some load to the 5v and/or 3.3v rails to make the PSU stable and
don't mess around with it unless you're sure the main reservoir
capacitors are discharged.
--
Clint Sharp