[Icom] IC-756Pro Heat Issues & Fan Servicing Information

Adam Farson [email protected]
Sat, 27 Apr 2002 22:11:10 -0700


For IC-756Pro owners who may be experiencing excessive heating of their radios,
here is the relevant excerpt from the IC-756Pro Service manual:

3-2-10 TEMPERATURE PROTECTION CIRCUIT (PA UNIT)
The cooling fan (MF1) is activated while transmitting or when the temperature of
the power amplifier exceeds the preset value. The temperature protection circuit
consists of Q10-Q13 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 detects the temperature of
Q5, and activates Q11 and Q13 to accelerate the cooling fan when the detected
temperature exceeds 70�C (158�F). The cooling fan rotates at high speed at 80�C
(176�F) or more.

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

If your radio does not conform to this description, it may well be defective.

Q5 is one of the PA power devices. On my 756Pro, the air leaving the vent above
the TFT screen is barely warm (perhaps around 30�C) after several hours'
operating.

Hope this helps.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/