Excerpt from IC-756Pro3 Service Manual p. 3-6 (Courtesy Icom Inc.)

3-2-10 TEMPERATURE PROTECTION CIRCUIT (PA UNIT)

The cooling fan (CHASSIS; MF1) is activated while transmitting or when the temperature of the power amplifier exceeds the preset value. The temperature protection circuit consists of Q10Q13 and R50.

While transmitting, Q10 and Q12 are turned ON, and provide a voltage to the cooling fan to rotate at medium speed.

The thermistor (R50) detects the temperature of the final amplifier (Q5), and activates Q11 and Q13 to accelerate the cooling fan when the detected temperature exceeds 70C (158F). The cooling fan rotates at high speed at 80C (176F) or more.

The thermistor keeps the cooling fan rotating even while receiving until the Q5 temperature drops to 60C (140F) or below.