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