ICOM IC-706MKIIG fan mod
Unlike the old Icom 706, the 706 MKIIG HF/VHF/UHF multi-mode ham radio transceiver do not keep its internal blower running all the time in receive mode. It is switched "on" only if the internal case temperature rise above determinated temperature (60ºC/140ºF).
That could be good for the winter season at Colorado but it is a dangerous project decision for Florida's summer... Not to mention that, as a mobile device, the radio can be exposed to worst and extreme temperature conditions. So, the radio becomes very hot after half-hour or more of operation, especially in summer.
High temp is know as the worst enemy of ANY electronic component, both digital or analog devices. A small 200 ohms 1 watt (1% or 5%) metal-film or carbon resistor connected in the PA board, between the switched +13.8 volts and the blower positive lead will make it run very quietly, dropping the case temperature to very near that one show by former 706.
This (difficult) resistor value can be easily replaced by two 100 ohm 1/2 watt connected in series and protected by a piece of small teflon tubing.The voltage applied by the resistor over the cooler is 4.7 volts and it is sufficient to start and keep it running without any perceptible noise.
Resist the temptation to increase this receive-mode blower speed (reducing the resistor value) or your radio will be transformed in a vacuum cleaner... The 200 ohm value is the perfect balance for noise, operating temperature and keeping the dust aspiration at minimum acceptable.
Anyway, it is recommendable to clean the radio internally every 6 months, specially the fan's blades to keep it running efficiently even if this mod is not made. After this modification, the measured radio body temperature after two hours of receive-only mode was 37 ºC (98.6 ºF) at room temperature of 25 ºC (77 ºF).
This mod is active only in receive mode. It will not change or disable the TX (transmit) blower rotation that is controlled by a temperature sensor. Therefore in TX mode, the ventilation system will operate as usual or even better due the 46 mA auxiliary current supplied by this resistor that will keep the blower transistor driver/controller more cooler.
Another benefit gained with the more stable internal case temperature is the reduction of the SSB (single side-band) frequency drift, more perceived at VHF and UHF bands. One of the resistor lead is connected to the positive of the blower connector and the other in the RF choke (L613) loop as show in the pictures below.
If you want added security, provide some isolation on the resistor leads. DO NOT short circuit the choke winding and be very careful when soldering the resistor lead over the fan's male PC connector pin. Be carefull. If too much heat is applied over the fan's connector pin, there is a chance of disconnect it from PCB, disabling all the fan operation! Also DO NOT short-circuit those pins. Use only a small and approppriate SMD iron soldering there. Please check you work twice before apply power to your radio.
To do this mod you don't need to remove any board or disconnect any DC or RF internal cables. It could be safetly done (or undone) in a matter of minutes. Only the radio bottom cover must be removed (5 screws) to access the correct place. More than 550 successful reports have been received about this mod, all over the world.