The above circuit was developed by Bill, W3DUQ and Tom, K1JJ for use in the Yaesu FT-102. But it can be adapted for use in almost any rig that produces its modulation (be at AM or SSB) at an intermediate frequency, that is then mixed to the desired output frequency and amplified in linear fashion. The resonant frequency of T-1 (8.2 MHz in the FT-102) would need to be changed to the appropriate IF.
Adjustment for the FT-102: Set carrier level pot for about 150 mA plate current with the front panel drive control (upper right) set at around 12 o cock. The audio level should be set to be just below flattop (using the RCA RF out jack on the rear panel, as a scope monitoring point with filaments off). This should coincide with full output on SSB, allowing switching modes without changing audio levels.
Additional Notes from K1JJ
First, you need the FT-102 service manual to locate the plugs and connections. Order one if you don't have it yet.
Look at the AM/FM board schematic. Plug 60 (P60) uses pin 3 as the input and pin 5 as the output. Pin 4 or 6 are ground. (this plug usually goes to J01 on the AM/FM board),
Using this old P60 I.F. input cable plug... the I.F. pin 3 to the old AM/FM board now goes to the bal mod board input.
And the old I.F. output cable pin 5 which used to go to the AM/FM board output now goes to the output of the new bal mod board. Just like replacing the old board, but without the plugs.
To access the plug connections, I just made up two short shielded cables with tiny 1/2" solid, stiff wire leads soldered on the end and slipped them into the existing female inline connector that usually goes to the AM/FM board. Connect to the hot I.F. and ground. Tape it up. I used 1/2 watt resistor wires clipped off.
Input audio to the new balanced modulator board from the old existing FT-102 audio board or do an external audio input from your audio gear. The latter yields the best fidelity.
I have four FT-102's modified this way and they work great. Other's have done the same.