To use the ICOM IC-970 on the UOSAT's (9600 baud), I made the following
modifications:
Transmit
Solder a shielded wire (I used RG174) to the anode of D16 (1SV50E).
This is at the junction of R52 (470K) and C44 (.0047). Make sure to
solder the shield to the adjacent PC board ground plane.
Receive
Solder a shielded wire to pin 9 of IC11 (MC3357P), again make sure
to solder the shield to the PC board ground plane.
Both transmit and receive are on the main unit circuit board. Make sure that
you use IC11 as this is the sub band receiver. If you plan to use the IC-970
for terrestrial packet (ie. simplex on either 145 or 440) you will need to add
another shielded wire to IC5, which is the main receiver discriminator.
I then brought the shielded wires out the rear of the radio.
I did this by removing the plate which covers the two holes located below ACC(1). In order to
disconnect the wires from the modem, I installed two phono jacks in the center
of the holes on the metal plate. I have used this setup with the G3RUH modem,
PacComm NB96, and the MFJ-9600.
I found that it was not necessary to add the 10K series resistor to the transmit
audio/deviation line. On all three, with
the center of the 10K pot range as 12 o'clock, I found that about 1 o'clock to
work best on for me.
Note:
Make sure that the DC power source for the TNC and 9600 baud modem are on
a clean supply (no ripple). Several users have found this to be the
reason they could not get their modems to work. Also make sure that the
TNC modem switch is set to 9600 baud as this provides the 153.6kHz to the
modem. The modem documentation is not very clear in this area. I run the
terminal speed at 19,200.
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