Factory mod of the FT100 for turning on the the fans during VHF/UHF receive
First two images were taken prior to Factory Mod
Next two images were taken after Factory mod
These pictures were taken before and after a radio was sent into Yaesu for the factory modifications. The first two pictures were taken prior to the mod and the next two were taken after the radio was returned. Note that there was a lot of solder flux left on the board prior to the mod. Could this be the solder flux that was blamed on some early failures of the VHF/UHF finals? This radio had experienced a blown VHF/UHF final. It appears that they cleaned off the flux, replaced the output transistor, and added a thermal switch to turn fans on full speed whenever the output temperature gets above 35C.
The thermal switch mounted over the V/U output transistor (Q3505) is a Tokin OHD1-35M. It is connected to the collector of transistor Q3507 (via the purple wire) which is in parallel with the signal from the microprocessor that turns on the fans full speed. The fans come on full speed during transmit and will stay on for 7 seconds after the TX event is over. On HF receive, the fans come on at reduced speed whenever a thermistor on the PA board crosses some threshold. This reduced speed is controlled with a separate transistor that routes the low line on the fans through a 39 ohm 1W resistor that limits the current and hence the speed of the fans. However, it appears that Yaesu made a conscious decision to keep the fans off during VHF/UHF receive. This allegedly was done because customers complained that the fans were a distraction while receiving in FM mode. The reason the fans could be more of a problem on FM is because listening in the mode is generally done with the squelch muted so the radio would be very quiet except for the fans. On the other hand, while listening in HF, the squelch is generally open so the fan noise wouldn't be as audible. More importantly when operting in CW or digital modes, another concern would be frequency drift due to temperature change, which could also be reduced by keeping the fans operating during HF receive. There are exceptions to these examples, i.e., some hams use SSB and weak signal digital modes on V/U and others use FM on HF, but for the most part they would be a minority of users of a radio like this in a mobile environment.
Some owners became concerned because their rigs got quite warm without the fan coming on since the FT100 dissipates about 14 watts on receive. One report showed that the internal temperature rise was about 30 degrees F (17C). The thermal switch mod fixes this by turning on the fans at full speed whenever the temperature exceeds a certain threshold. If you have this mod, then while receiving on HF you may notice that the fans are off, then come on at reduced speed, and then on at full speed as the temperature thresholds of the thermistor and thermal switch are successively exceeded. On V/U receive, the fans will either be off completely or on at full speed, depending on the internal temperature of the rig. Both fans are wired in parallel so they operate the same way.
The Tokin switch must have availability problems since people who asked to have the rig modified by Yeasu have had various switches installed including those that have trip points at 35, 40, and 50C. Others have reported modifying the rig themselves by simply tying one of the black fan wires (they are in parallel) to ground via a current-limiting resistor. The resistors have ranged in value from 39-50 ohms with a power rating of at least a watt. This keeps the fans on during V/U receive at a reduced speed regardless of ambient temperature. This should reduce thermal stress on the electronics and does not make excessive noise. Yaesu doesn't feel the thermal mod is necessary but has been adding it at the customer's request if the rig is sent in for repair.