Any owner of TS-940 knows that this radio has very pour PA drivers, MRF485 from Motorola. In case of damage, to replace these transistors you have to follow some tricky steps. To use low gain transistors, or to use high gain transistors (different dot colors) and modify the bias, etc.
In any situation you have big chances to blow up again the drivers. The first reason is because this transistors are high voltage transistors (29V) We know that high voltage RF transistors has higher gain then 12V transistors, and the possibility of oscillations it is also bigger. In the same time the design of the PA layout is not one of the best, which help to oscillate.
What I did, I replaced MRF485 with 2SC2509 (or any equivalent 10 to 15W, 12V RF transistors in TO220 package). This transistor is cheap (used in most of CB radios). Be careful to connections, 2SC2509 has different pin configuration, BEC, (MRF485 has BCE). You have to lift the L7 inductor (Final Unit) from the 29V bar, and connect to the output of a 12V/1.5A regulator. This could be a LM7812 (TO3 type). The input of 12V regulator was connected to 29V, from the same point where you disconnected the L7 inductor. I installed LM7812 regulator on a small heat sink, outside of the PA compartment.
Now you have to adjust the bias of the drivers. Put low power in SSB mode and adjust VR2 (Final Unit) to have 0.7V on the base of drivers. Finally if the output power is not 100W you can adjust VR2 (Control Unit), and VR3 (Control Unit) for ALC.
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