Yaesu FT-8800 cooling modification
I don't like transceivers that become very hot... Hot components may lead to mechanica stress that can cause (premature) failure of components like the final transistor/FET. But I also don't like too much cooling fan noise. Luckily there's a rather simple modification to keep your Yaesu FT-8800 much cooler during receive. My (previous owned) Icom IC-756 became very hot during receive and the cooling fan was not activated since the fan only activates during transmission. I modified the circuit so that the cooling fan runs always at minimum speed. If there's a minimal airflow, the heat is easily blown out of the rig. This minimal airflow prevents heat accumulation inside the rig. I used this simple trick for the Yaesu FT-8800.
The (simplified) schematic of the FT-8800 fan control is shown below. The idea is that the fan is permanently fed with 13,8 VDC. The 13,8 VDC is fed trough a current limiting resistor to pin 1 of the fan connector at the right. If the cpu sends the 'FAN-CTRL' signal to the transistor Q1067, pin 2 of the fan connector is 'pulled to ground'. The result is that there's a current flow trough the fan and the fan starts spinning. 33 Ohms Resistor R1363 is a current limiting resistor resulting that the fan operates on 12 VDC (design specifications) instead of 13,8 VDC. A simple calculation shows that the maximum current draw is 55 mA for the fan at maximum power. This fan switches on when the transceiver is transmitting of when the chassis becomes too hot.
By adding a 330R resistor, the transistor is 'bypassed' resulting in a small current flow trough the fan. This resistor valued is based on practical testing. I like the fan working on the lowest speed as possible to minimize the fan noise. 330 Ohms works for me, but a lower resistance value will work. The lower the resistance, the higher the fan speed/cooling/noise.
Since the smt components are very small, I looked for suitable resistor placement. I don't like to cut the fan wire en just solder a resistor too it, I prefer a more elegant modification. The best option for me is using two test pads. At the bottom of the main board is the switching transistor QR1376 placed. In the surrounding area are four test pads. I chose to solder a regular 1/4 Watt trough hole resistor to pads TP1048 and TP1042. Test pad/test point TP1042 is a ground pad. Test pad/test point TP1048 is connected to the negative side of the fan connector. This modification can be reversed by removing the resistor. The modification is rather easy since only one resistor has to be added. But remind that the mainboard must be removed completely and that new heat conducting paste has to be applied replacing the main board to the chassis. Also surface mount soldering skills/equipment are highly recommended by performing this modification. But as said, cutting the black wire of the fan and solder a resistor to the two wires and a ground point, the practical effect is the same. ;-) Before and after images of the modification are shown below.