Wiring in a CTCSS or PL tone board into an older radio

[dewdude]

So the thing about CTCSS is it's basically just an audio tone; and all you have to do is insert it in to the transmission somehow. The trick is you have to insert it *after* any filtering but before modulation.

My guess would be on the VCO board after Q357. The block diagram shows the microphone as entering the VCO board, running through a filter, then in to a modulator. The center leg of RT351 before it goes through C365 should be where you'd want to inject the CTCSS.

As far as the tone input on the accessory plug...typically this is for hooking up a TNC. I mean..I guess you could try inserting tone on to Pin 1, it feeds the VCO board the same place as the microphone..so as long as the mic filtering on that board doesn't filter below 300hz it's fine. They label the line as tone-burst...I have no clue why it's tied to pin 5 through a cap...there may have been a plan for that at one point. That will probably work. Won't hurt to try.
73,

-KM4JOJ

[Kamakiri]

That's exactly how I had it wired, center leg of RT351. It turned out to be correct, but when I wired it the first time the insulation rubbed off the wire when I reinstalled that part of the chassis and it had grounded out.

It's working!!!!

These pics show the testing and the final product. For power I just wired it right to the on/off. Not sure whether I should add a tiny toggle or not yet. It's not the prettiest mounting, but there's really not a lot of choice here except installing a ribbon cable and connector and making some sort of artsy box for it. But I'm a mechanic, not a woodworker

Things that the instructions don't tell you:

1. For the grounds, you just tie them all together and ground them to the chassis.

2. Compare the positions of the molded dots on the part to the picture in the instructions and make sure you have it correct. The resistor blocks just below and to the right of the LED display have no polarity.

3. Only the wires shown in the photo are needed. The 4 pin connector by the IC isn't used.

You end up with 4 wires coming from inside the radio.....ground, PTT, Tone, and +12V. That's it!! This is an outstanding kit to order because there's no jumpers or guessing your PL frequency or any of that garbage.