Kenwood TS-770 VHF-UHF transceiver
A friend and radio ham brought this nice little transceiver into the play pen complaining of a faulty knob. The transceiver that is.
Well made, dating back to the late ‘70s and quite desired within the radio ham community. All seemed to work, except for the rotary tuning knob which had no affect. A quick look at the service manual showed that there is an encoder, LED and photo transistor coupled affair. The actual encoder can be tested in situ with the top and bottom covers removed and scoping around showed no digital signals coming out of the unit. So, out it had to come. To do this, you have to tilt the front control panel down. Now a word of warning if you have to do this.. Unscrew the 3 earthing connections and the two RF connectors feeding the meter assembly. I suggest you leave the ribbon cables in place as these are likely to be fragile due to years. With the front down, you can unbolt and remove the encoder.
To test the encoder away from the main body, will need 5V for the logic and 7.5V for the analogue detection side. Luckily, the LEDs were found to be okay. Testing the photo transistors is a bit awkward as the outputs go into virtual earth inputs of the detection OP amps. Whilst there were outputs from the OP amp for each photo transistor, the voltage swing did not make the TTL levels required by the Schmitt invert IC. The sensitivity of the OP amp inputs is controlled by two 1K potentiometers. Both these were open circuit. With these sorted and adjusted, we now had logic outputs.
With everything assembled and powered on we were greeted buy again, no function. After head scratching, it was found there was a high resistance connection providing the 7.5V from one of the main board feeding the OP amp. This supply is switched on and off by the main board to disable the tuning knob.
So, typical. Two faults causing the same condition. Just my luck.
