I have a FT980 I use only for SWL and the fan is not only noisy but runs 75 percent of the time. Do any past or present FT980 owners know the mod to fix that? Thanks
I agree with Lon that you should not just disconnect the fan. The radio needs air movement to get rid of the heat that builds up even on receive. One way to modify the fan is to simply run it at a much lower speed 100% of the time instead of listening to it switch on and off. If you do this, then the radio stays cooler than it does when you allow the heatsink to warm to a preset temperature and the switch on the fan to cool it back to room temperature. The constant on/off is annoying to a lot of people.
Google "FT-980 service manual" and the first result should be a page at dxzone.com that will let you download a free copy. Looking at the schematics on page 85 and 91, you will see that the fan is fed from +13.5. The fan is usually turned on by switching the negative fan lead to ground with a transistor (Q03 on the control unit). The switching transistor is activated by thermistors across J01 and J02 that control op-amp Q01.
Adding a 100 ohm 3-watt resistor between the motor negative lead (J03 pin 2 on the control board or J04 pin 2 on the PA board) and ground might keep the motor running at a low speed all the time while still allowing the motor to switch on at close to normal speeds if needed. If this were a squirrel cage motor I wouldn't try it because those need bit of a kick to get them spinning. Since this is a bladed fan it might work but it depends on the starting torque of the motor. You'd need to experiment a bit.
A different approach is to use a small 12v computer style fan and run it all the time if you can find a way to wedge it in there or tie wrap it to the back grill. A SileneX or equivalent fan is extremely quiet - even more so if you put a resistor in series with it. If an additional fan cools the radio enough, the original fan won't come on. It doesn't take a lot of airflow to keep the heat under control if you only use the radio for receive. You might even be able to remove the original fan and replace it with a computer fan as long as you don't transmit. If you want to try that, a 2700 ohm resistor across J01 (the thermistor jumper) on the control board would probably keep the fan running all the time at low speed. Keep in mind though that a computer fan won't move enough air if you transmit.
The standard caveat applies: Try this at your own risk. I hope it will give you a starting point if you decide to experiment.
Bob - K0BT