Dave,
the internal mic circuit has audio HF & LF filtering. This means that if you
were to apply a signal below 300Hz or above 3,400Hz into the mic
circuitry,then the level of this signal will be greatly reduced, or even
removed.

The CTCSS signals centre around 100Hz and therefore cannot be injected into
the mic socket.
In fact you have to travel all the way through the mic circuit until you get
to the microphone op-amp (IC1) See circuit as per Paul.

The amp IC, IC1 has a resistor/cap feedback circuit which provides the req'd
filtering. The CTCSS encoded signal output should be injected around Pin 1
(output of IC). There maybe voltages at this point ,so a capacitor must be
used to block this voltage. You will see two varicap diodes D1/D2 and it is
the varying AF voltage feeding these diodes that produces the modulation of
the carrier.

I have carried out this mod on my IC-290E, by using an external CTCSS
encoder chip (FX315) mounted in a small aluminium box with DIP switches for
selection.

I connected a small piece of screened wire from Pin1of IC1 and connected it
to a spare pin on the nine way facility socket on the rear panel. At this
point I also connected the aforementioned cap.

A transmit switched voltage for feeding the external CTCSS unit supply line
was found on this facility socket. Please check the circuit for your radio.

Remember that the CTCSS encoded signal should only be activated when you
transmit.

Dave, you must have a good look at the schematic provided by Paul and come
up with a plan of attack.

You will of course understand that you attempt these modifications at your
own risk. It worked for me on my IC290.

Do a search for FX315 encoder/decoder chip circuitry on the net. I got my
chip(not cheap) from Mode Components.(UK)

Regards and good luck and thankyou Paul for schematic.

Roger, G7JAQ


http://www.btinternet.com/~g4kqu/Mod...nts/index2.htm




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