(KDJ advises you to read this entire page before beginning any mod.)
This mod locates a signal on the circuit board where the voltage stays in excess of 3V when the radio is squelched. When the radio receives a signal in either trunked or conventional mode, this voltage goes to less than 0.2V.
The idea is to feed this signal to a transistor switch which picks a relay or whatever. I would not try to sink a relay directly on this pin. Put as little load on it as possible. A little transistor switch is really nothin'.. a couple of 2N2222s, a couple resistors... if you can't figure it out then you shouldn't even think about attempting this mod.
Now of course I make no claims of warranty or fitness for any particular purpose, and I am certainly not responsible if YOU fry YOUR radio. That said:
You Need: Phillips screwdriver, soldering iron & solder, hookup wire, small sharp X-Acto knife, SMT board repair skills, etc...
NB: to avoid dislodging the connector pin, do not allow the soldering iron to linger on the connection; do this quickly. Tin and trim the end of the wire first; now apply just a touch of fresh solder to the pin/PCB junction & allow to cool; wipe the iron tip; touch the wire end to the connection and heat the connection; as soon as the solder melts, get the iron away; wire should be securely soldered to the junction and the pin must be straight and in-line with the other pins.
Did you know that the earphone jack on the 235 is actually a stereo (tip-ring-sleeve) jack?
Don't know about you, but I don't normally use stereo headphones to listen to the thing, so I might want to use that extra wire out of the radio for something else (like the COS mod above) to avoid drilling a hole.
I couldn't build this small enough to fit inside the case, and it's too much drain on the battery, so that's why it's "external". I am using this circuit to drive a wireless (carrier current) intercom; could also be used on a modified 49 MHz. baby monitor or to kick a tape recorder, etc. Have fun!
Ken Johnson KC4YOZ