The FT-847 has two fans inside for cooling. One fan runs all the time; it's the one on the back that you can see. There is another fan that runs at a low speed during receive, and at a higher speed on transmit. Some people find the fan(s) to be too loud for their comfort. Others don't find it so loud, or find that other fans in the shack make as much or more noise so it is not an issue.
Recent postings (April 99) have identified a wide variability in the loudness of fans, sometimes for fans from the same manufacturer. Apparently these small inexpensive fans are sometimes loud, sometimes not. Add in human preferences, and it's difficult to say what is too loud and what is not. Yaesu has recently replaced some of the fans under warranty when requested. Sometimes those replacements are quiet, sometimes not.
There have been a number of fixes suggested on the web page, identified here as the Zener Diode Fix, the Gang the fans fix, the Disconnect fix, and the Replacement Fix. Before you undertake any of these fixes, you should consider the design of the cooling capacity of the radio. The fans were designed to provide cooling under a wide variety of conditions. Cooling requirements for a black radio mounted in a vehicle are substantially higher than for a rig in a typical ham shack.
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Zener Diode Fix
This fix involves adding a low voltage zener diode in series with the power lead to the fan on the back of the rig. It involves simple surgery of cutting the wire and soldering in a zener diode. A 3.9 volt zener diode (available from Radio Shack) will drop the 13.8 volts down to 10 volts. This slows down the fan considerably and makes it more quiet. One person noted that the fan is a "12 volt" fan being run at (typically) 13.8 volts, thus accounting for the loudness.
Incidentally, the information in the FAQ is wrong - the fan is not running directly from the input power supply. With 13.8V in I see about 11.2V going to the fan, which is in-spec for a 12V fan. I did try fitting a fan from a 486 CPU but found it still noisy and the airflow was reduced quite a lot. There is a 470 ohm SMT resistor near the fan that seems to be responsible for dropping the voltage to 11.2V. I will try adding the zener or resistors in parallel with that resistor and report back. (Info by Hamish VK3SB)
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