Modifying the IC22A for 9600 baud operation
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
by Mike Curtis, WD6EHR and Dave Shalita, W6MIK

This is one of the more difficult rigs to modify - most are even simpler!

This mod will make your IC22A into a dedicated 9600 baud packet radio.
Components with * are added to the crystal board.

   Remove the trimmer capacitor for the transmit crystal socket you'll be 
using for 9600 baud.  Replace with the above circuit.  The 4-20 pF trimcap 
should be a good quality ceramic or other low-drift trimmer.  (Sorry -
Radio Shack doesn't have anything suitable.)

   Connect 2 10K resistors to C-53 (both sides) and tie the other ends 
together.  This junction is where you'll pick off your RXA. Turn R-67 (dev
control next to P-1 and P-2) completely counterclockwise.  This kills
signals that may get into the phase modulator.

   Using a service monitor, or oscilloscope connected to the detector of a 
receiver, adjust the varactor bias for the best eye pattern.

Alternate bias adjustment:

Use the K9NG's 4800 Hz "idle" tone to adjust your modulator for best
linearity. Using a service monitor, or a receiver with a fairly wide
passband, adjust  varactor bias for the "best" sine wave.  Thanks to Brian
Kantor WB6CYT for this tip.

4.2        TNC MODS

   Your TNC will work better at 9600 baud if you speed it up.  For TNC2 
clones, change the Z-80 and Z-80 SIO to 10 MHz types, and change the clock 
speed by changing the jumper on JP1.  Older ones use the 2 OUTER pins, and 
newer ones use the center and (other) outer pin.

4.3        MODEM MODS

   Most rigs require a lot less TXA than the modem outputs, and a resistive
swamping network may be needed.  For example, the TEKK KS900 wants 50 mV.

   On the TEKK KS900/PacComm NB96 combo, I use 470 ohms series, and 39 ohms
across the TEKK's input terminal to ground.

4.3.1      K9NG MODS

   The K9NG modem's keying circuit can be omitted if you use the TNC's 
existing PTT circuit.  If you do this, remember to leave the PTT
connections uncut on the TNC modem disconnect.

   The K9NG modem's DCD circuit is terrible, and can be vastly improved by 
shorting R-31 and changing C-18 to 0.1 mF.

   You may find it helpful to widen the K9NG's receive filter.  Change C-13
to 2000 pF and C-16 to 1000 pF (half their present value for 9600 baud).  
This will widen the passband, and in the process will minimize group delay.

   Use the PTT from the TNC instead of the K9NG modem.  This  lets  you use
the TNC's watchdog timer, and makes things a lot  simpler.  Make sure to
cut only the following traces between pads on  your  TNC-2's modem
disconnect J4:

1/2     DCD
11/12   transmit clock
13/14   receive clock
17/18   receive data

   The TNC-2 manual tells you to cut other traces, but by doing it this
way, you'll be able to use the TNC's PTT circuit.  The K9NG's keying
circuit, which doesn't work all that well without modification (it's
designed for the Hamtronics FM-5), may now be omitted from the pcb, if you
like. This is most of the stuff on the lower edge of the pcb.

The G3RUH modem from PacComm already does this.