I had started this project a couple of years ago, but I honestly couldn't remember what kind of shape the radio was in. At the time I lacked any test equipment to properly check it. Any tests I would have done would have been over the air with an antenna and a second radio. To further complicate things, I apparently had dived straight in and ripped out the channel selector knob and attached wires to the appropriate pins on the PLL so that the Arduino could change channels. I even have a sketch that drives the HHCH and the PLL to do just that. But back to the present, I was still unsure about the state of the radio.

With the radio connected to a service monitor I attempted to test it, but even with no audio into the proper pin on the mic jack I was getting 100% (or more, the monitor maxes out at 99.9%) modulation with all sorts of tones and chirps. Inspecting the radio and my test setup, the first thing I noticed was that several of the wires for the PLL channel selection had pulled out of the breadboard. At 30 AWG, they don't stay in very well. I soldered them to pin headers to prevent this from happening again. The problem persisted. The radio most likely remained on the selected channel (40) when the wires pulled out because it is selected by pulling all pins low. I can only assume that is the default state of the pins on the PLL.

The next logical place to look was the mic test cable. I removed it, and shorted the connections on the back of the mic jack manually and the radio keyed with a silent carrier as it should. So the problem was in my cable or my keying setup. I dismantled the connector on my mic cable, and in the process accidentally broke one of the wires off the connector. Now too short to reach, I had to completely redo the end of the cable, while I was at it I rearranged the wires so the colors would make a little more sense and made sure the mic audio was using the shielded coax inside the cable. It should be noted that the other end of this cable terminates to pin headers that plug into to breadboard.

With the mic cable redone, I was able to use it to key the radio with a nice silent carrier. When I applied a tone to the mic input it came across beautifully. I started to get a hunch as to where I went wrong, to confirm my suspicion I tried connecting the TX line from the mic cable to chassis GND instead of mic GND and sure enough, the audio went nuts again. My problem was that when I read "Grounded to Pin 1" on the mic datasheet I assumed it was the same as GND for the rest of the radio. This proved not to be the case.