Kenwood TR-7920, Instruction Manual

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TR−7930 Tone Mod
KE7HR Paul Jorgenson
The Kenwood TR-7930 (and TR-7950) had an optional sub-audible tone generator board that
selected one of three different possible tones, which was not installed in the used radio I
bought. Researching the tone generator, I found a “works alike” replacement for more than
twice the price that I got the radio for. Digging through my junk box, I found an old Communica-
tions Specialists SS-32 tone generator. With some clip leads and another junk box connector
that mated with the pins on the radio board, I set up a successful test.
The SS-32 has 5 DIP switches to set the output tone. By removing the DIP switch assembly
and replacing it with switching diodes and three NPN general purpose transistors, I now had a
three tone switcher, just like the original.
The 1N914 type switching diodes are run from the one side of the old switch pads to the collec-
tor of the transistor. The base of each transistor is switched by a voltage from the radio when
the front panel tone control switches to the different tones. This causes the diodes to conduct to
ground (emitter of the transistor), setting the tone output.
I mounted the transistors and diodes in the place where the DIP switch had been. The tone
board was mounted to the inside of the radio frame with double sided foam tape.