Does anyone have a King KMA20 audio panel maintenance manual or schematic? Mine is putting the pilot’s sidetone over the speaker when I transmit. Audio panels aren’t that complicated, but I need a manual for the connector pinout.
Carl Foster
Carl, are you using an intercom system? If you are, this is probably due to the intercom using an electronic reference to ground for the transmit key, instead of using a relay. The intercoms use an electronic circuit rather than a direct ground in order to independently control the microphones. The electronic reference often has too high an impedance to properly trigger the radio’s audio clamping circuit.
You can check this with a temporary circuit, and if that’s the fix, it is easy to incorporate. The intercom usually does have enough of a ground to activate miniature 12V relays. Here is what you do:
1. Pick up a very low-current miniature or subminiature relay from Radio Shack.
2. Use a jumper to apply 12V to one side of the solenoid circuit.
3. Connect the other side of the solenoid circuit to the Mic key lead. In other words, when you key the mic button, the circuit goes to the intercom electronic ground through the mic key ground circuit.
4. Connect one side of the solenoid’s load circuit to a good airframe/panel ground (ideally the radio stack common ground point).
5. Disconnect the audio panel audio suppression lead connection from the mic key lead at the intercom, and connect it to the other side of the relay load circuit.
With this in place, when you key the mic, the intercom operates the relay through its internal electronic ground. In turn, the relay provides a direct, low-impedance connection to ground, like a traditional radio system is expecting. That ground “signal” gets passed to the audio suppression circuit on the radio, which shuts down the audio output circuit.
If this does prove to be your problem and solution, I suggest buying a second relay (they cost very little), gluing them together, and connecting them up in parallel. They are very small and can be shrink-wrapped and simply tie-wrapped in place. With two in parallel they will almost certainly last the life of the radio stack. Please let me know how you make out.