Bill & Megan Buhler

Kenwood TM-V71a - Extensions

Over a year ago I lost the RJ11 - RJ45 control head cable that comes with the radio. Without it of course the radio is worthless. So I searched online for a pinout so i could create a replacement. No dice. Every article I could find simply says to use a RJ45 to RJ45 coupler to connect to the radio, and then connect to the jumper, or buy the very expensive remote head cable. I didn't find this at all appealing.

Finally I got my hands on a service manual and decoded the pinout.

Kenwood TM-V71a Removing Wood Pecker Sound From Mic Extension

My father in law, K7DRB, came up from Phoenix in July to witness my daughter get baptised. While he was in town I setup his truck for his TM-V71a by adding power cables from his battery to the back section of his king cab for power. Then I ran double shielded cat6 cable to between there and the drivers area / center console for the control head and microphone.

This took a lot longer than I had anticipated but I felt look pretty good. Our first test was with the doors open and David calling on the radio, while I listed to my HT. It sounded really good, but there was a lot of background noise. So we closed off the cab and tried again. This time I could hear a constant clicking sound.

After sleeping on it Sunday after church we tried again with David reving the motor while not talking. I could hear the clicking steadily without a change in frequency or pitch. After searching online I discovered that the TM-V71a was described as getting a woodpecker sound if the mic cord was extended. This is blamed on a clocking signal sent to read the DTMF keys.

David was out of time so I promised I would research a solution for him. My first approach was to see if running a balanced line between the microphone and the radio without the coiled cord made a difference. On my radio the woodpecker was there in force. So I scratched that as a solution. It did make for some good learning about balanced audio and what kind of amplification is needed to remove common mode interferrence. I find myself wondering why the radio manufacturers aren't using a balanced circuit to begin with...

So I decided I would need to convert from unbalnced at the microphone, to balanced for the extension, and the finally back to unbalanced. I could do that with transformers, but there will be losses, and at mic levels those could be substantial. But since the radio provides 8 votlts DC to the micrphone, I figured I could steal a little bit of that to do the conversions with some opamps. So I designed a opamp based differential amplifier with signal buffering and unity gainto go between the extension cable and the radio:

Initial tests with it still passed through the pulsing signal, and no other audio. I then added a transformer at the mic end and still no audio. My next step will be active amplification / balancing on the mic end. But I'm suspicious that the op amp I bought from Radio Shack isn't working correctly.

16 August 2016

OK, so after a lot more experimenting I think I found a much simpler solution. It just so happens that the serial data causing the pulsing noise is on the orange pair, white orange wire. The orange wire is not connected at the radio or the microphone. So I built a quick inverting buffer circuit to go at the microphone end, and terminated the radio end through a resistor to the ground wire. Now I can faintly hear the pulsing if I turn up the receiving radio almost all the way, but it's pretty faint. Without a oscilloscope I don't know how much the reduction is, but it's dramatic.

So at this point I'm debating whether I should borrow a oscilloscope and measure both signals, or if I just move forward. I do know that David and Sherri have left Phoenix for their house in Alberta, and will be by in a few weeks. So that is my firm deadline to solve the issue.

Icom 706mkIIg Microphone Repair Project

When the spring clip on my microphone broke I figured it would be a very easy repair since I've installed hundreds of rj45 crimp on connectors in my lifetime. This has ended up not being the case.

I started by chopping off the end and stripped back a little more insulation. Comparing the end I had removed with the wires revealed a problem. I had two black wires on the end but inside I was missing a wire! Instead there was shielding wrapped around the white microphone wire (which shielding I think also acts as the drain wire). Thus the microphone signal has its own shielded path along side the other microphone wires.

Feeling confused I searched online for a input and found this very useful site: Wiring Color Code and Pinouts for IC-706MkIIG and HM-103 Mic which explains that the extra black wire is the shield around the white and is somehow bonded with a regular black wire in the factory

So I obtained a short length of black 22 guage stranded wire and spliced it with the shield and soldered it all together. But this proved impossible for me to stuff into the rj45 and have end up in the right spot. Also the wires on the joint broke... So I was staring at an even shorter mic cable and needed to repeat the whole process.

I known the solder joint increased the strain of the small straneded wires too much, so I needed another way to do this. I tried heat shrink, but I couldn't find any that was a tight fit before shrinking, and what I had became too think when shrunk. But then I remembered a product I bought years ago and never got around to using until it was spoiled, Liquid Electrical Tape:

Picture of Star Brite Liquid Electrical Tape

I couldn't buy some immediatly due to budget constraints, but a few weeks later I grabbed a bottle from the store and last night I began the repair for the second time.

First I twisted the braid wires together again into a single stranded conductor and coated it with a coat of black liquid electrical tape. Leaving it to dry for fifteen minutes.

After a second coat it looked pretty similar (if a little less smooth), to the other wires. With a little bit of coaxing I was able to insert the wires into the connector and verified my microphone is working. It's so nice to have my main station back in operation!

So in summary, if you need to repair a mic cord with a internally shielded wire, use liquid electrical tape to insulate the braid for a reliable and easy connection to the modular plug.