This is a cut and paste of a guide by Nold at BitBuilt found here. I found this while stumbling around one day.
Faking the FANs PWM-Signal
Why use Arduinos or 555's... damn... just use some solder!
[See attached "Image 1 - fake_pwm_hack.png "]
Some background: The DC uses IC601 (a multivibrator IC) as some kind of ADC (Analog-Digital-Converter), to convert the PWM-Signal from the fan (up to 2.5v), to 3.3v (= logical "true"). So when there's no fan, it'll put out 0v (= logical "false"). So all we need to do is pull-up the output signal of the IC & done! How nice Sega prepared this mod for us & gave us some free solder-pads.. (It might be safer to use a resistor here, but it should even work without the IC, anyway; also there is a capacitor, protecting a possibly connected fan.)
Faking the FANs PWM-Signal
Why use Arduinos or 555's... damn... just use some solder!
[See attached "Image 1 - fake_pwm_hack.png "]
Some background: The DC uses IC601 (a multivibrator IC) as some kind of ADC (Analog-Digital-Converter), to convert the PWM-Signal from the fan (up to 2.5v), to 3.3v (= logical "true"). So when there's no fan, it'll put out 0v (= logical "false"). So all we need to do is pull-up the output signal of the IC & done! How nice Sega prepared this mod for us & gave us some free solder-pads.. (It might be safer to use a resistor here, but it should even work without the IC, anyway; also there is a capacitor, protecting a possibly connected fan.)
