If you remove the small plate on the back of the unit you will
find an easy way to mount an 9 pin sub d connector (at least on
the "E" version) which has enough room for rx, tx and ptt.
You can get the signal output from the discriminator unit, plus
side of C163 (on my rig this capacitor was mounted the wrong way
round, I corrected this). The voltage is very low, so you may need
some amplification - 10 in my case. C165 (22nF) on the output of the
discriminator has to be replaced by 1.5 nF or 1nF.
The transmission signal goes to diode D47 in the modulator stage,
a resistor of 10k in series. Check the output adjustment of
your modem by using a second receiver. Adjust it that the
modem noise is somewhat lower than the noise of the empty
channel.
Use shielded cable for these connections.
The easiest way to find the ptt line is by looking at the
microphone connector.
If you want to suppress "man made noise" over the possibly
connected microphone while transmitting packet, you can short
the microphone by using a reed relay. Then you have to
connect the packet-ptt to the rig-ptt by using a diode to
prevent shorting the mike on normal transmission.
Cause I experimented a lot with my rig I also replaced the
IF-filter with a wider one. The built-in type CFW455E may
work ok.
I didn't make a BERT, but there are very few retries using filter
characteristic #1 on the 9600 modem of my TNC3S.
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