After trying to solve this the easy way with no luck, by looking for docs
already posted, a few of us have decided to bug this out. A quick call to
ICOM put us on the rite track. This non-destructive mod for the IC229h
works well for 9600 baud packet. You will need your schematic that came
with your radio. However, our call to ICOM revealed some discrepancies on the
schematic in the FM detector stage, primarily the FM detector chip itself.
Earlier models had the TOKO TK10487M detector and the newer models have the
MOTOROLA MC3372M. ICOM, unfortunately, did not make this change in the
schematic supplied with the radio. This document will show the connection
points for both chips. Remember to use good quality shielded cable like
RG-174u, RG-178u or similar. Most of the points are going to require micro-
surgery, so get out your pencil point soldering irons and magnifying glasses.
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TXA:
On the schematic you will see a block labeled "VCO UNIT". At the
bottom of this there is a four pin connector. The third pin is labeled "MOD".
This is where you want to inject the TXA from the modem. The "VCO UNIT" in
the radio is shielded by a can. Removing this "CAN" to get to the circuits
can be a tedious endeavor. So you can find this "MOD" point on the underside
of the radio. It is located beneath a flat metal shield about 1" X 2". After
removing this, with the front of the radio towards you, the point can be
found in the right corner towards the front where the shield covered. There
will be four solder lands rite in a row from left to right. Trace these lands
with the schematic to find the "MOD" land. Solder your TXA there. Replace
the metal shield. This is as tuff as it gets. I promise!
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RXA:
Well this one is easier. If your radio has the TK10487M, solder RXA
to pin 11 of this chip. If your radio has the MC3372M, solder RXA to pin 9 of
this chip. Either chip can be found on a board in front of the "metal VFO
shield can", on the top side of the radio. You may have to tilt the board a
little, but be careful. Don't apply too much heat to the chip or you will have
a big problem. I followed the trace away from the chip and attached RXA.
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PTT:
This can be found on the mike jack pin 5. I made this an internal
connection for neatness. Neatness counts!
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GND:
If I need to tell you where to find ground, you shouldn't be doing
this mod yourself. Attach all the shields from the above wires together
outside of the radio to modem ground. Attach only one shield internal to
the radio, the "CAN" is a good spot. You don't want to cause any ground loops,
do you? Protect the other two shields from shorting to anything in the radio.
This is why they make heat shrink tubing.
You may have to add 5-10mF non-polarized capacitors in series with the TXA
and RXA lines. Some modems have included these in their circuit, so check.
Now you are ready to make the connections to the modem. This will have to be
left up to you. There are too many modems and tnc's out there to list all
of the connections to them. But you should have the docs with your modem, so
it should be easy. Disconnect the radio from the modem if you use the radio
for anything else. Well there you have it. This works great, it is what I am
presently using.
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