In my Icom, I modified the fan power according to Karsten Brinck - OZ1DB. This one adjustment will cause the fan to rotate at low speed when receiving. Without this modification, it did not spin. I was not satisfied with the function because these low fan speeds varied according to the supply voltage. In addition, if I set the minimum speed at 13.8V by selecting the zener diode, the fan did not spin at all at 13V. Therefore, I replaced the zener diode with the LM78L05 stabilizer. The diode is Schottky, just because of the low voltage drop. In my case, there is 4.7V on the fan in the entire range of supply voltages. The fan spins reliably and is very quiet. If you need to set a higher voltage for the fan, use the dashed variable resistor, measure it and replace it with a fixed resistor.
This modification does not affect the original function of the control circuit, this means that when transmitting or overheating the transceiver, the fan receives full voltage and has maximum speed.
You can get to the regulator after removing the upper black cover, then you have to remove the gray shielding plate with the speaker. The regulator circuits are on the far left when viewed from the front. Good luck.
Dodo, OM1AVL