As I wrote recently, I have been tackling the low-frequency instability problem in the TS-930S PA unit. My first attempt was to add several bypass capacitors to from the supply side of L7 to ground (through a lug on the Q7 mounting screw a short distance away). This should tame the drivers, although the problem is not there.
The hum came back when I pushed the power beyond about 50 watts. The push-pull MRF-422 final amplifier circuit in the TS-930 comes from Motorola EB27, with a few small modifications. However, it is not clear to my untrained eye where additional precautions could be taken. I have a hunch that my next target will be this portion followed by the 2SC2075 pre-driver stage if I come up empty on the finals.
