This is performed after the 2708 -> 2716 eprom conversion on cpu board as per:
Once that upgrade is done, you no longer need the +12v and -5v voltage rails that were previously required by the 2708 EPROM, and can begin the first part of the upgrade process.
For reference, the stock P5 Rev2 rectifier list:
u601 +15v lm317t
u602 +12v 7812 / 78m12c
u603 +5v 7805
u604 -5v 7905c
u605 -15 7915 / lm320t-15
To begin the upgrade, first remove u602/u604 and any other unneeded parts remaining after the 2716 mod. u605 can be replaced with a LM337, basically making this more like a P5rev3.3 power supply. The rev2 PSU PCB has available places for two resistors, a trimmer and the capacitor needed for the LM337. You just need to cut the trace from pin 1 of the LM337 (and two resistors) to GND. You can find the correct component values in the rev3 technical manual.
Of particular relevance from the rev3 manual:
Page 55 / 56 (3-22 / 3-23)
Page 109 / 110 (this schematic shows post-removal of -5 and +12 lines as per service update which updated rev3.0s to rev3.1, they similarly swapped with newer eproms, very similar to the mod we need to make for the rev2)
The second and more significant improvement is a complete transformer swap, which keeps back panel almost completely cool compared to original setup. With the stock configuration, the +5V rail is regulated from around 22V and the current consumption is around one amp, so the heat dissipation is roughly 15-20 watts. With the separate 5V transformer output (about 9V unregulated) the heat dissipation drops to around 4-5 watts.
A suitable replacement transformer is the Signal Transformer (now, Belfuse) DMT-8-15 which is physically same size as the original one. It fits perfectly on same spot as
original one for easy mounting and it’s available in single quantities from Digikey or direct from manufacturer:
There is often a significant lead time on these – and it is advised to order it as soon as you intend to do this upgrade. Alternatively. you can find a transformer with similar specifications, but it is not guaranteed to fit into the stock transformer mounting bracket. I have seen people use torodial style transformers, often mounted to the bottom of the case. This doesn’t look quite as “factory”, but does work in a pinch.
Only thing you’ll need to do is figure out how to mount the rectifier and smoothing cap for +5V supply because there was never a spot for these on the original PSU PCB. We soldered the rectifier and cap on the solder lugs of the transformer and ran wires from there to 7805 on the PSU PCB. See the pair of twisted blue/brown wires in this example photo below. The trace from the 2200uF cap was cut and the 2200uF cap was replaced with 47uF cap to keep the reset
/ nwram circuit timing roughly same.
Special thanks goes to Riku over at analog.fi for most of this info, and helping me get these mods done successfully to my prophet 5. I have gathered this information here to help anyone else looking to get their rev2 running well.