Actual fan in my unit was made by Younglin Tech. Model DFB802512H. (DC12V 2.0W)
This fan contains circuitry to control RPMs based on thermistor connected across two control leads. PSU has two 20k thermistors (one under each heat sink) in parellel configuration, and fan seems to expect 10k thermistor across the control leads.
I got an idea to build small adapter that plugs into the fan connector on the main PCB and and has connector for standard (PC) 12V PWM fan.
I had Adafruit Trinket (5V) laying around and it has just enough I/O pins to handle this.
I used pin 0 (digital input) to read the pulses coming from PWM fan speed (tachometer) pin, and pin 2 (analog input) to read the thermistor.
For output I used pin 3 (digital output) as serial (TTL) output to be able to monitor fan speed/temperature for debugging purposes and pin 4 (digital output) to generate 25kHz PWM signal to control the fan speed.
I put in nice (quiet) 80mm PC CPU fan (Noctua NF-A8) and cant believe how quiet this thing is now
I'm running fan at about 600rpm when heatsinks are in room temperature and then linearly adjust fan speed to maximum when heatsink temperature rises....