Icom IC-706MKII Manual page 28

KB2LJJ Radio Mods Database
more loss and some tilt in the pass band, but it's shape factor is 1.5 and it's bandwidth is 2125 Hz similar to the #110.
IC-706mkIIG fan mod
Author: Sergio PY1BEK
Unlike the 706, the 706MKIIG do not keep its internal blower running in receive mode. So, the radio becomes very hot after half-
hour or more of operation, especially in summer.
A small 200 ohms 1 watt 1% metal-film resistor connected in the PA board, between the switched +14v and the blower positive
lead will make it run very quietly, dropping the external body temperature of the MKIIG very near to that found in the 706.
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 to temptation of increase the receive-mode blower speed (reducing the resistor ohmic value) avoiding to transform your
radio in a vacuum cleaner...
After this modification, the measured radio body temperature after two hours of receive-only mode was 37 at room temperature of
25 ºC (98.6/77 ºF).
This mod will not alter the transmit behavior of the ventilation system that will remain operating as usual or even better because
the 46.5 mA auxiliary current supplied by this resistor will keep the blower drive transistor more cooler.
Another benefit is the reduced frequency drift in the VHF and UHF bands, SSB mode*.
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.
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 male PC connector pin.
In this mod, there is no necessity of board removal and could be done in a matter of minutes. Only the radio bottom cover must be
removed (5 screws).
73, Sergio PY1BEK
User comment
This mod is really a very nice solution to the excessive heating problem of the 706MKIIG. Have done it and it works great. But
when making the mod care should be taken on the location of the 200ohm resistor. Between the fan connector and the RF choke
(L613) there's a relay case and a tape-type wire and connector. Be careful not to let the resistor (that becomes very hot during
operation) touch the relay case or the tape-wire as they can melt under long term exposure to the resistor heat.
But, overall, this is the best mod i've seen on this subject as it don't change the regular operation of the fan in the xmit mode.
Rafael Sa
PY4RGS
http://www.kb2ljj.com/data/icom/ic-706-MKII.htm (28 di 30)23/08/2009 23.05.07
Care with the resistor location
From: Rafael - PY4RGS