This is from mods.dk
"Some FT-100 owners may have experienced what is widely known as the blown SRF 7043 VhF/UhF output transistor or Mosfet final syndrome, which frequently occurs for no apparent reasons. The problem is contributed by Yaesu?
??s lack of fundamental design and forethought.
The blowing SRF 7043?
??s problem are caused by the following reasons.
1. Earthing and shielding problems causing self-oscillation and SWR problems.
2. Insufficient or inadequate air-cooling system for the SRF 7043 heatsink, thereby averting the device?
??s heat sink to ventilate heat dissipated by idling bios current while in standby operation.
3. Yaseu either completely ignored or never considered the possibility of Mosfet RF thermal runaways, by not designing prevention or monitoring heat-sensing circuits for the SRF 7043 output stage.
Vertex or Yaesu?
??s engineers, by adopting some
?? modifications, have managed to get around most of these technical problems. These various Yaseu modifications are available on this web site. However, over a long period, the distressed Yaseu engineers have not managed to find the real solutions to these technical problems, which do raise a few questions.
1. Why did Vertex or Yaesu?
??s engineers use SRF7043 for output finals when other cheaper and better quality devices such as
?? SK702 and Phillips BLF549 devices were available?
2. Why did Vertex or Yaesu?
??s use Aluminum instead of Copper material for the finals heat sink, when Copper has greater mass and heat conductivity, and unlike aluminum does not produce the phenomena known as hot spots.
3. To prevent thermal runaways why did Vertex or Yaesu?
??s engineers not include a negative coefficient thermal sensitive zenner diode bios circuit, attached to the SRF7043 heat sink? There are numerous university papers and thesis?
??s on RF MOSFET thermal Bios circuits.
4. Why did Vertex or Yaesu?
??s engineers deliberately design the SRF7043 Mosfet RF final stage to be continually running in the Class AB bios mode operation, while operating during standby mode? Taking into the factor of a bios idling current of .4 amps, multiply by 13.2 volts, this equates to approximately 5.2 watts of constant heat sink dissipation. Hence, there is a definite need for a modification to the air-cooling fan system to be continually running all the time. I question the reasons why the Yaseu engineers would design this output stage to be running in Class AB Bios mode continually all the time. Hence in Class C Bios operation, there would be no heat and no blow-ups, because there would be hardly any quiescent current flowing through the TX output Mosfet RF stage. Since the SRF 7043 Mosfet RF device operates in the VHF and UHF bands, mostly in FM mode. There is no technical reason (other then for a small reduction of power output or unless using SSB modulation ) why the SRF7043 cannot operate in Class C Bios operation, all the time. Even in SSB modulation when you have to use Class AB mode operation, you can still use Class C Bios operation for standby mode.
I cannot understand why Vertex or Yaesu?
??s engineers did not do the following.
1. Why did Yaseu not make the FT 100 computer programmable? All they had to supply was an inexpensive computer cable and some software. FT 100 can easily handle extra bands (16) and memory; and if you can program them, wow. This computer programming would also bypass the frequency limitations and restrictions, able to set mode and repeater offsets, all within technical limitations.
2. Why they did not include an inexpensive AM filter and a stable oscillator, making it very saleable item for number of other applications?
The Vertex or Yaesu?
??s engineers in my opinion did know what they were doing.
The major problem for a number of FT 100 users is experiencing the problem of replacing and obtaining the SRF 7043 MOSFET finals.
To buy a direct Yaseu replacement SRF 7043, costs about $203 in US dollars from Ham outlets. I understand that the S as in SRF stands for shit.
However, you can use the following substituted devices
all quoted in US Dollars.
BLF549 80 watts available from Phillips, Arrow and other outlets $153.
MRF166W 40 watts available from Richardson Electronics $83.64
SK701 40 watts, available from Polyfet.com, guessometer price $84
Sk702 80 watts, available from Polyfet.com, guessometer price $98
WARNING, always wear a breathing mask and have a vacuum cleaner handy when removing damaged MOSFET RF devices. You can always use F2 or a rubbery glue, to hold things together when taking out damaged devices. Barium Oxide is a very poisonous substance to inhale. Never dispose of it in landfills.
Stray electrostatic voltages caused by incorrect handling techniques can easily damage these devices.
If you do not know what you are doing, always pay somebody else who does.
After installing the new device, always check the Bios current flowing through the device. The idling current in AB operation should never exceed 0.4 amps*. *0.8 amp in SK702. Use these figures at your own risk. Of course, you can always operate in the safer Class C operation mode. Make sure the cooling fan modification is working all the time. Do not forget Yaesu?
??s earthing modifications.
For the newbie?
??s, a thermal runaway occurs when a Mosfet RF device say normally running at 0.4 amps with no ventilation or cooling system, goes above it operating temperature parameters. Consequently, the devices Bios current increases, producing more self heat, and then more bios current, and so on, to point when it will self destruct. By increasing the Mosfets gate into the positive region, increases the current of the device. They have similar characteristics to Pentode tubes, but they are current devices.
Have not looked into availablity of the replacements mentioned. I am taking very good care of my original.