Story
I've got 5 or 6 of these Lexmark dot matrix printer power supplies that would be great to drive my 3d printer project but they output 40V, just a little too much for the stepper drivers I have (their limit is 35V).
Just by looking at the thing you can see that it's a flyback converter, mains is rectified and filtered, that high DC voltage is chopped by a Toshiba K2601 transistor and sent through a high frequency transformer, there's a diode on the output with big capacitors and a loop control circuit.
I've seen some hacks on ATX power supplies to raise the 12V rail to 13.8V, you just need to alter the reference voltage of some control chip and the output changes.
The trick was to find the control chip on my supply and hack the reference to get the desired output voltage.
PCB bottom
I took a picture of the bottom traces of the PCB, loaded it in Gimp, applied some filters and fills to get a black and white version of the traces (below).
Black and white PCB bottom
Hacking and testing
From there I got the HA17431 datasheet, this is the chip that regulates the output.
R29 and R33 form a voltage divider on the ref pin of the HA17431.
There's also a way to change the output by turning on the A1266 PNP transistor labelled Q5, you need to pull the Command line to ground for that (but the output is then 17V, I was aiming for something between 28 and 32V).
So all that's needed is to change R29 or add another resistor in series or parallel, I was quite lucky because the first try was to add a 100K resistor in parallel to R29 and the output is then 31V, just in the range I wanted.
There also seems to be some kind of overvoltage safety circuit, tied to the +5V rail and to the +40V, I'd probably need to change ZD4 for it to trigger at a lower value.
I'd recommand to go slowly if you're increasing the voltage and the supply doesn't have any overvoltage safety, on this supply the capacitors are rated for 50V, you want to keep a safety margin.
Also since mains is connected to these kind of supplies be extra careful around the high voltage section of the PCB.
See on the right the final hacked version of the PCB, I removed the unneeded transistor and soldered the 100K resistor directly on the PCB using leftover vias.
PCB
Getting the schematic
Then all I had to do is add a new layer in the Top pcb picture, paste the bottom pcb picture, apply some mirroring and rotation and 50% transparency and I was able to see how the components are connected together and draw the schematic (below).
Schematic
Final product
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