[Kenwood] TS-940S CW Side Tone Not Working

Sat Oct 24 11:29:37 EDT 2009

Radio: TS-940S
S/N: 7030121
Problem: CW side
tone does not work, I can't hear anything
Comments: CW does transmit with VOX and in Send mode

Questions: I need help understanding how the CW side
tone works. I have the service manual
and have tracked two wires (STK and KEY) from the key jack to the control board
and to the side tone mixer. I have also
located the two signal sources CAR3 and CAR4 and while they have a signal I
have not tried to measure the frequency. I am not seeing a signal from the mixer.

First, I don't
understand how the voltages should behave when the key is down and
released. I've been able to measure both
DC and AC voltage changes along the STK and KEY lines. I did find the description in the service
manual for the STK and KEY lines, but it was sketchy and I am looking for more
information about voltages I can measure.

Second, where are
the switching components, and how should they function?

Third, could a
modification done by the previous owner eliminate the side tone? I have two radios TS-940 and TS-930 from the
same gentleman that have the same symptom, or problem. 

I think there are
two options for the problem. One, the
switching components are not working correctly. Or two, the mixer is not working.

Any help is greatly
appreciated. This is a very neat radio,
the receive is nice, the phone works well, and it seems to have the right power
output. When I get the side tone fixed
I'll start a more detailed check of the other parts.

Thanks in advance
Stephen Clark
WE5K
slclarktx at yahoo.com