Yaesu FT-100/D HF/6m Final Replacement as published in Mods.dk

[n6...@aol.com]

From 'mods.dk'

>>>> "Due to the discontinuation and shortage of the MRF255 final transistors, I had to start looking for a replacement when a radio with dead finals was delivered.
Initially a perfect replacement could not be found and I needed to start looking outside the square.
Looking for a device with similar specifications was hard but I settled on trying the MRF141, identical package and similar specifications. As the device is designed for 28 volt as well as for VHF, it was going to be a gamble.

Please note: As this procedure may be confusing for some people it may be wise to take photos during the disassembly to enable the radio to be reassembled to its former glory.

The replacement proceedure

• Remove the bottom cover and identify with the aid of the service manual the various parts discussed during the procedure.
• Disconnect the common supply wire attached to L3008.
• Carefully disconnect the choke wiring from the drain connection of Q3007 and Q3008.
• Remove the fasteners securing the transistors to the heatsink.
  Without applying too much heat, remove both faulty MRF255 transistors.
• Clean the heatsink removing all residue of thermal paste.

Always use an antistatic wrist strap attached to a suitable earth point when handling static sensitive devices to minimise the chance of damage.

Note the length the MRF255 transistor leads as the new ones of the MRF141 are likely to be too long, trim each lead as required using a pair of needle nose pliers for support and a pair of cutters for the trimming.

Apply heat transfer paste to the replacement devices and secure the transistors to the heatsink, solder the transistor leads in place.

Re-attach the wiring of the choke to the drain connections of the replacement transistors and re-solder the common supply wire to the choke.

Alignment

When the replacement devices were fitted it was not possible to easily adjust the required Drain current, so a minor modification was done replacing resistors R3046 and R3047 (1K8) with 1K resistors.

Bias alignment is simple, connect a suitable current meter in series with the radio, set both trimpots VR3002 and VR3003 fully counter clockwise.

Switch the radio on, (14.2MHz, USB), with the microphone gain set to minimum, press the PTT button and monitor the current consumption (TX current).

Carefully adjust VR3002 to read TX current + 250mA, adjust VR3003 to read TX current + 500mA.

The end result is that the radio puts out 100W on all HF band and 6M.

This completes the replacement of the final transistors." <<<<