One bad thing with the 991 is the fan noise. It’s loud!
So I order some new Noctua NF-A6x25 fans to replace the original Panasonic fan. My thoughts was to have two fans, on top of each other
to get better airflow and spin it more slowly. So I checked the service manual about the fan controller, and yes, it should go. There
is two transistors that controlling the fan. One can handle 2A, and the other 500mA. These fans take 120mA each, so
totally 240 mA. No problem!
The right ring is the fan connector. I will come to this later.
But, I tried with one fan first, and yes.. much quieter and better airflow. No need for two fans here..
This is the original Panasonic fan.
And here is my swap. It’s so easy so no need for any explanations.
This is the connector. The Panasonic fan used pin 2 and 3, but I used pin 1 and 3. In the schematic above you see that pin 2 going through a
resistor, I did’t like that. I want full speed.
I used small pin holders that I just pressed down over the pins and put some hot glue over it. Fast and easy!
And if I want to replace to the original fan later, I can do that. Nothing destroyed or broken.
Here is the new one, much quieter and better airflow.
Updated July 2018
After my fan change, I redesign the thing. As you may have read in Yaesu FT-991 as a 2M station
I have problem with high temp and overheating. Not because the fan change, I have more airflow now than with the original fan. The climate here is a big
reason to the overheating there we have around 30 degrees celsius (86 degrees Fahrenheit) indoor, sometime even more. So the radio get very hot if I use
it for some time (TX) and sometime the radio just flip the frequency down, and then it tried to compensate the same amount with the clarifier!
So I trying the double fan. The noise... well.. it starts to sound like the original fan, but know I have extremely high airflow and the radio is very cold!
I didn’t glue the fans if I should remove the second one later on.
If you don’t have a warm climate and high temp indoor, then you don’t need to take these actions bellow. Replacing the original fan works grate.
One other thing that I need to investigate. When I open the radio, I removed some cards and noticed that there is no thermal compound on some of the
transistors on the heatsink. That could be a big reason to this problem.
This is the heatsink under the HF finals. The 2M/70cm heatsink is under the tuner... it’s flat.