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Icom IC-756PRO fan modification

introduction permalink: http://www.amateurtele.com/index.php?artikel=312&id=#1417
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Since this year I've got an Icom IC-756PRO. Although this rig is rather old, the rig is still very useful. The rig has a spectrum scope and therefore the rig is still very nice to use. Newer rigs are equipped with a 'waterfall' spectrum and the screen resolution is much higher, but even in the year 2020 the IC-756PRO is considered a very good rig.

fan modification permalink: http://www.amateurtele.com/index.php?artikel=312&id=#1418
introduction
Since the Icom IC-756PRO rig is equipped with linear voltage regulators, the heat generation is at least 40 Watts. When the rig isn't transmitting (where the heat generation is even more) the rig gets quite hot. The temperature increases gradually in time. There is a fan installed, but the fan only turns on during transmission. I didn't like this phenomenon and started looking for a suitable modification. The red-neck solution is to add a fan to the back of the rig. But I don't like crude 'hot glue' and 'tie-wrap' 'solutions'... Therefore I started looking at the schematic looking for a 'real' solution. The fan control schematic is located on the power amplifier board. The schematic is luckily also rather simple. The schematic with my (green coloured) modifications and notes is shown below.

Goal: Airflow during reception to reduce heat.
Complexity: Low; Just add a resistor.
Advice: Do it! It's a simple and cheap mod without unwanted sideffects.


circuit description
The schematic is 'fed' by three wires. Ground, 14 VDC input power and an RX/TX signal. During receive (RX) is the fan never powered. The TX input line is during receive connected to ground. During transmit (TX) is the TX line pulled up to 7,8 VDC. This signal switches transistor Q10. The base is Q12 is therefore pulled low. The resistance between the emitter and collector of Q12 is very low and 14 VDC is fed to the fan. There are some capacitors for noise suppression (C97) and current buffering (C99). Fans are well known for generating radio frequent noise (EMI). The electrical noise is suppressed by capacitor C97. The current is limited by resistor R51. Due to this 56 Ohms resistor, the current is limited to 104 mA. Without resistor R51, the fan will draw 197 mA and will make excessive noise due to the air movement. When the circuit is activated by the TX signal, the temperature control circuit is also activated. Temperature sensing resistor R50 is powered by 14 VDC during transmit and controls Q11 and Q13. When the temperature rises, the resistance of Q13 drops. 14 VDC is therefore fed trough the 10 Ohms resistor R52, trough Q13 to the fan. Since the resistance of this path is lower, a higher voltage is applied to the fan resulting is more airflow. The result is that there's more cooling when the temperature rises. The result is that during TX the temperature is controlled. But during receive, the temperature is not controlled!

receive fan modification idea
The fan is never powered during reception and therefore there's no forced air cooling. The rig produces during reception more than 40 Watts of heat. The result is that the rig gets quite hot after a while. I studied the schematic and my idea was to create a small forces airflow to reduce the heat. My idea was to add a resistor 'over' Q12 so that during reception the fan is powered.

testing and measuring
During TX is the current trough the fan approximately 104 mA. I tested some resistor values between 10 and 100 Ohms and 47 Ohms turned out to be great. The result is a very small airflow which is enough to keep the rig cold. Also the fan is heard to hear so there isn't too much noise. The current trough the 47 Ohms resistor is approximately 72 mA. The heat production in the resistor is calculated: P = I^2 * R. P = 0,072 mA^2 * 47 Ohms results in 244 mW. A 1/4 Watt 'standard' resistor should do the trick. The resistor is loaded at 97,6% of it's maximum. This is close to the maximum load, but within it's limits. An 0,5 Watt resistor would reduce the load and is maybe a better choice. But if the resistor fails, there's no problem. The fan would stop in case the resistor fails and that 'risk' is an reasonable calculated risk I think (and I had no 0,5 Watt resistor available). I also tried other resistor values, but the airflow was too low or the current was too high. The 47 Ohms resistor turned out to be the 'perfect compromise' in this situation.

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physical modification
The physical modification is rather simple. After removing the top cover and second cover plate, the power amplifier board is located at the left of the rig (seen from it's front panel). Close to the outside wall of the rig is Q12 located as shown below. I soldered the resistor with the board still in place. Maybe the legs of the resistor could be shortened when the board was removed an the resistor was placed. But I don't think longer wires would have negative side effect and the resistor cooling due airflow is now even better. Note: The resistor shown on the image isn't the right value as it should be. This was a test resistor which is replaced later in the process by the final 47 Ohms resistor.

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results
I'm very pleased with the results! The rig stays cold during reception and the airflow can be heard hardly. I recommend performing this modification. The modification is cheap and simple and the result is very good. And the modification can be removed easily to get back in it's original state.

future modification?
If the rig is used for high TX duty cycles or high load conditions like 100 Watts digimode (which I don't recommend due to the heavy load!), the airflow could be too low. It's thinkable of replacing chip resistor R51 by a lower value. This will result in a higher current and higher fan voltage thus more airflow to improve cooling during transmission. Since I'm more a listening kind of operator, the modification isn't worth trying for me. Remind that the current trough the 'new modification resistor' will increase also and therefore a 0,5 Watts resistor is mandatory.

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