OFNA Ultra GTP / MBX conversion

[BrianG]

"electrical genuis background"??? lol, yeah right! Just a hobbyist...

Anyway, to make one out of discrete components would be fairly easy:

Maybe a Schmitt trigger (op-amp or comparator based), with a thermistor v-divider on one side and a fixed 5v regulator as a reference on the other. Drive a small NPN transistor configured as a switch to turn on the fan.

Or, see if you can find an all-in-one thermal switch IC. Most of the ones I know of are thermally controlled fan speed controllers, not on/off controllers.