So what I ended up doing is probing around the 3 voltage regulators, reading resistor values near them and searching for continuity to out voltage rails and ground. I decided one of them is 12V regulator by resistor values and connections and measured resistance from pin1 to ground to be around 3k1. Took a 50k potentiometer and thought 40k-45k would be a good start to run in parallel to ground not to disturb the value too much. Fired up the PSU and 12V rail read 12.8V. Tweaked it a bit until it showed 13.8.
I have no idea if this is the right thing to do but it seems ok.