AutoCom and electronics expertise needed

[daveinatlanta]

My problem: I have a Kenwood TK3131 hooked into my AutoCom Active Plus with the Kenwood to AutoCom interface cable (part 1430) and PTT (Part 1457). However I ride quite a bit with some old geezers who still use CBs. So I bought a Cobra 38WSXT and would like to be able to remove the Kenwood and plug in Cobra CB. I thought I could just buy some Radio Shack jacks to accept the male end of the AutoCom cable the currently plugs into the Kenwood, and wire those jacks to two plugs that would plug into the Cobra.

 

Before soldering up the adapter, I looked at the male end of the AutoCom cable that currently goes into the Kenwood. There are two jacks – a 3 conductor mic jack and a 2 conductor speaker jack. Using a continuity tester, I determined that the PTT button for the AutoCom cable on the handlebar simply ‘connects’ the sleeves on both jacks together.

 

The Cobra’s jacks are both 2 conductor – a tip and ring. After doing some preliminary wiring, the receive path from the CB to the Autocom and helmet works fine. However the PTT button on the handlebar does not trigger the CB’s transmit circuity and the Mic does not work. Undoubtedly this is because I don’t know how to wire an adapter up properly or I need to use some sort of RC circuit rather than just plain wires??

 

Attached is the schematic for the transmit side of the CB. I was hoping our resident AutoCom Engineer (we have one don’t we??) could provide some guidance.

 

As info – I did look at the AutoCom parts list and there are several cables that allege some connectivity with a Cobra (not sure if the Cobra is FRS or CB) but I wanted to make up something with parts I already had.

[Scarver]

Have a similar setup with a Kenwood TK3101 instead of the TK3131. Also have a Kenwood ham radio (TM-D700A) in the mix.

 

Had to build a special isolator to make the Cobra work with my Autocom Pro-7 Sport which is bike powered. Have a bike mounted CB antenna. Problem was that the Autocom PTT switch connected to Autocom ground, NOT Cobra ground.

 

Attached schematic shows the parts of my interface. Used half of a 1512 cable to connect between box with parts and the Autocom. 1433 (think this is longer version of 1430) cable plugs in box.

[daveinatlanta]

Thanks Herman. If I interpret your schematic, and I'm guessing here, I think you built the parts in the lower dotted box - which I concluded from finding RS part numbers for the transformers and the relay.

 

Your Autocom 1433 cable is different from mine. Your cable "Connects universal dual pin units including Icom, Yaesu, Cobra and more. Designed for routing through bike." per the AutoCom brochure. Mine just says "Connects Kenwood Portable Transceivers. Designed for routing through the bike."

 

I had hoped to build whatever interface I needed so that I could just attach it to the two plugs on the end of the Autocom cable - which normally plugs into the Kenwood TK3131. So I'd just take the Kenwood off, plug on the home-built interface, then plug it into the Cobra. I didn't want to have to remove the cable from the Autocom unit.

 

Do you know if I can do this??

 

Thx

[Scarver]

Yes, you can.

 

Built a box with two 4-pole relays to select between FRS/GMRS, ham radio, and CB. Box is located in tail of bike and has three 5-pin DIN sockets to accept plugs from each of the transceivers.. Have a 3-position switch mounted in front where I can easily reach it while riding; a RS shielded stereo cable connects between it and the box.

 

1430 vs 1433

Autocom recommended 1433 for the Cobra. 1430 may have different components inside plug (or none). Have you been able to determine if there are any inside this cable?

[daveinatlanta]

I have not looked at the cable from the AutoCom unit, to the PTT switch back to the Kenwood to see if there are indeed components inside. However, there are only three places components could be hidden: Inside the male and/or female DIN (Gray) plugs of the two autocom cables. Both of these ends look the same physical size as the cable that connects to my helmet. I tried to peel one of the gray covers back but I don't think I can without splitting it open with a razor blade. The other potential hiding place is the box for the PTT button and switch on the handlebar. It is a black box about 1" by 1" by 1/2" and it contains the PTT button and a three position switch for either 'always on' or to activate the Kenwood via VOX. I don't use either of these positions and just leave it centered where it defaults to the PTT button's control. I have not taken the box apart but to see if there is anything in there besides the button and switch. If there is anything inside, it would have to be really small.

[Scarver]

Dave, I measured the various connections of my cable with a meter and came up with the 2.7 K resistor and 22 nF capacitor. These will easily fit inside the grey plug before they encapsulate/mold it.

 

During the problem solving process I bought a now discontinued speaker/mic with PTT button (RS 21-1834) that had the right twin plug on it. Opened that unit to better understand the circuitry needed to operate the Cobra mic with PTT. My guess is that the 1430 cable has a very similar , if not identical, construction to the 1433 because it has to interface with the same Autocom driver circuitry. Perhaps the 1430 only has a tip and shell connection on the 2.5 mm MIC plug while my 1433 has a tip, ring, and shell connection but the ring has nothing connected to it. The RS speaker/mic only has tip and shell also - no ring.

[daveinatlanta]

Thx for the details. I'll measure my cable. Maybe there is a simple solution???