[ICOM] External Fans

[Bob DiPippo]

Just add a 82 ohm 2 watt resistor from the black lead or negative lead of 
the fan to ground. This will run the fan at half speed on receive and will 
run at full speed when needed by the fan circuit. Have done this on many 
706, 746, and 756's. All the fan circuit does is ground the negative lead to 
turn on the fan.

Bob, k1wyc

-----Original Message----- 
From: JK
Sent: Tuesday, April 10, 2012 2:27 PM
To: ICOM Reflector
Subject: [ICOM] External Fans

Placing a small 12V muffin fan directly behind the PA area exhaust slots on
my IC756 PROIII resulted in keeping the radio quite a bit cooler.

The fan pulls air out continuously. I picked up the 12V from one of the two
accessory sockets on the rear panel. The internal fan only comes on when in
transmit mode or when the thermistor mounted next to one of the final
transistors gets above a specified temp. Then the fan will continue to run
even in receive mode until the sensor cools down.

If you do place a fan in the back rear, make sure it pulls air out as the
internal fan is pushing air out the back when it runs. Pushing air in from
an external fan in the rear would be counterproductive.

The radio consumes about 40Watts in just receive. That amount of wattage
will bring the heat up inside the cabinet. After a few hours with power on
the radio, place your hand on the left side of the cabinet and you'll find
this seems to be the area that gets warmest. The cabinet top will also get
warm but not as much as the left side. The weak link in all electronic
circuits are the electrolytic capacitors that have a tendency for the liquid
or gelled electrolyte to slowly evaporate over time. The warmer the
electrolytics become the faster this evaporation occurs. Reducing heat in
most electronic devices will usually prolong their lives.

I monitored the temps with an infrared temperature device and found the
addition of the external fan to be quite beneficial.

I have not checked the schematic to determine if there is protective
circuitry that will prevent transmitting if the internal fan does not
activate in transmit mode. I did have the fan plug disconnected and I was
still able to transmit. I suspect by this experience that the radio does not
have a fail safe mode if the internal fan indeed fails. If the fan does
fail, one or both final transistors in addition to other amplifier
components may be destroyed. My IC-751 fan failed
and both finals blew out. Obviously ICOM did not have a fan failure
protection circuit. Since the fan can fail over time, each time you operate,
look down into the top ventilation slots to make sure those blades turn when
you press the transmit button. The fan blades are easy to see through those
vent slots.

Probably not a bad idea to also open the radio every 3-5 years and inspect
the fan and the PA board components for dust. Remove that dust with a soft
brush or an air compressor set at low PSI.

Jay....NE2Q


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