![ic-756 pro ii no tx power ic-756 pro ii no tx power](https://docplayer.net/docs-images/46/21338257/images/page_2.jpg)
The physical modification is rather simple. The 47 Ohms resistor turned out to be the 'perfect compromise' in this situation. I also tried other resistor values, but the airflow was too low or the current was too high. 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). But if the resistor fails, there's no problem. An 0,5 Watt resistor would reduce the load and is maybe a better choice. This is close to the maximum load, but within it's limits. The resistor is loaded at 97,6% of it's maximum. A 1/4 Watt 'standard' resistor should do the trick. The heat production in the resistor is calculated: P = I^2 * R. The current trough the 47 Ohms resistor is approximately 72 mA. Also the fan is heard to hear so there isn't too much noise. The result is a very small airflow which is enough to keep the rig cold. I tested some resistor values between 10 and 100 Ohms and 47 Ohms turned out to be great. My idea was to add a resistor 'over' Q12 so that during reception the fan is powered.ĭuring TX is the current trough the fan approximately 104 mA. I studied the schematic and my idea was to create a small forces airflow to reduce the heat. The result is that the rig gets quite hot after a while. The rig produces during reception more than 40 Watts of heat. The fan is never powered during reception and therefore there's no forced air cooling. But during receive, the temperature is not controlled! The result is that during TX the temperature is controlled. The result is that there's more cooling when the temperature rises. Since the resistance of this path is lower, a higher voltage is applied to the fan resulting is more airflow. 14 VDC is therefore fed trough the 10 Ohms resistor R52, trough Q13 to the fan. When the temperature rises, the resistance of Q13 drops. Temperature sensing resistor R50 is powered by 14 VDC during transmit and controls Q11 and Q13. When the circuit is activated by the TX signal, the temperature control circuit is also activated. Without resistor R51, the fan will draw 197 mA and will make excessive noise due to the air movement. Due to this 56 Ohms resistor, the current is limited to 104 mA. The electrical noise is suppressed by capacitor C97. Fans are well known for generating radio frequent noise (EMI). There are some capacitors for noise suppression (C97) and current buffering (C99). The resistance between the emitter and collector of Q12 is very low and 14 VDC is fed to the fan. During transmit (TX) is the TX line pulled up to 7,8 VDC. The TX input line is during receive connected to ground. During receive (RX) is the fan never powered. Ground, 14 VDC input power and an RX/TX signal.
#IC 756 PRO II NO TX POWER MOD#
Goal: Airflow during reception to reduce heat.Īdvice: Do it! It's a simple and cheap mod without unwanted sideffects. The schematic with my (green coloured) modifications and notes is shown below. The schematic is luckily also rather simple.
![ic-756 pro ii no tx power ic-756 pro ii no tx power](https://ae01.alicdn.com/kf/H8a5e6412f5d54aa796b262b533fbe431l/Zendure-Passport-II-Pro-61W-Power-Delivery-Auto-Resetting-Fuse-Travel-Plug-Worldwide-Conversion-Socket-USB.png)
The fan control schematic is located on the power amplifier board. Therefore I started looking at the schematic looking for a 'real' solution. But I don't like crude 'hot glue' and 'tie-wrap' 'solutions'. The red-neck solution is to add a fan to the back of the rig. I didn't like this phenomenon and started looking for a suitable modification. There is a fan installed, but the fan only turns on during transmission. The temperature increases gradually in time. When the rig isn't transmitting (where the heat generation is even more) the rig gets quite hot.
![ic-756 pro ii no tx power ic-756 pro ii no tx power](https://i.ebayimg.com/images/g/PpUAAOSwmPphscNN/s-l64.jpg)
Since the Icom IC-756PRO rig is equipped with linear voltage regulators, the heat generation is at least 40 Watts. 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. The rig has a spectrum scope and therefore the rig is still very nice to use. Although this rig is rather old, the rig is still very useful. Since this year I've got an Icom IC-756PRO. Informatie pagina voor experimenteel radio-onderzoekers Please support this (commercial free) webpage by donating a small payment: