Setting up my TS-790A for Packet Radio

Packet Radio is a digital operating mode based on analog FM (frequency modulation). First of all, you can establish a direct link between your station and a digipeater, which is basically also such kind of a station. Most digipeaters are interconnected mostly with microwave links, making up a word wide network. The routing protocol used for it is AX.25, an X25 variant for the special amateur radio purposes.

For Packet Radio, you need a Terminal Node Controller. I am operating the TNC2S for this purpose. Since 9,6 kBit/s require rather fast TX/RX switching times, the Kenwood TS-790A needs a small modification which I am going to describe here.

I decided to use the following modification:

The Kenwood TS-790A makes a wonderful 9600 BPS Packet Radio Transceiver as it has an excellent front end, powerful transmitter, and 144,440, and 1.2 GHZ 
coverage. The CFW-455F filter inside is not too narrow despite what you may have read and coupled with the low noise front end of the rig enables the G3RUH
modem to reliably decode valid data that barely moves the S-meter.

The modification  discribed below is simple, reversible, and does not impair normal operation or the appearance of the transceiver. This is a "no holes"
modification as it allows the G3RUH modem to be connected to ACC 4 jack on the rear panel.

Place the TS-790A upside down on a soft clean surface so as not to mar the cabinet. Remove the bottom cover to expose the IF. Board. With the front panel
facing you the work area is the upper left quarter of the IF board. Identify pin 9 of IC8 (MC3357P). There are two of these chips on the IF board but only the
main demodulator IC is in the work area. RX audio to the modem will be derived from this point. Next locate J31 a two pin connector near D81 and X2 (10.965 
OSC). TX audio from the modem will be supplied to the hot lead on J31.
Note the large aluminum heat sink that spans across the IF board near the rear of the chassis. A round lug will be mounted on it near the rear of ACC 4 jack.

Wire an eight pin din plug. Use shielded cable for TX and RX audio. No traces on the IF board need to be cut. Pins 3 and 5 of ACC 4 are not connected to
anything as the radio comes from the factory. Make sure the Din plug fits the jack properly before wiring it. 

PIN 2 - ground
PIN 3 - TX audio from G3RUH              
PIN 5 - RX audio to G3RUH
PIN 8 - PTT
  
Prepare two six inch lenghts of RG-174. Prepare one end of each cable so that the shield will reach the ground lug mounted on the large aluminum heat sink. The
cables will be routed over the top of the heat sink and will not be crushed when the bottom cover is reinstalled as there is adequate clearance.

Solder the center conductor of one cable to pin 3 on the rear of ACC 4 (verify with ohm meter). Solder the center conductor of the other cable to pin 5 on the
rear of ACC4 (verify with ohm meter). Dress the cable connected to ACC 4 pin 5 over to IC8 pin 9. Cut to size and solder (be careful- use minimum heat and thin
solder). The exposed shield is discarded and heat shrink tubing placed over the end to prevent any shorts that would otherwise occur from having exposed
strands of the shield touching something. Dress the other cable connected to pin 3 of ACC 4 to J31 and cut to size. Prepare the end in the same manner as the
other cable. Solder the center conductor to pin 2 of J31. This is accomplished by cutting the lead going to pin 2 about 1/2 inch back, stripping the insulation
back 1/8 inch on the two ends, and then joining the center conductor of the cable going to pin 3 of ACC 4 and soldering the three exposed ends together. Use
heat shrink tubing to cover the solder joint.

NOTE that the banded end of D81 is not the cathode of the  varactor diode ! There is no need to insert any additional capacitance in series with J31 pin 2.
The shields are not connected at both ends of the RG-174 cables in order to prevent ground loops. The Aluminum heat sink was used as ground as no other ground
was available that was close to ACC 4. Your sight may be better than mind and you may find one.

Adjust VR-1 on the MC-NB96 board for 75  mv p-p as measured on pin 2 of J31. This will give a good initial setting for injected transmit audio level.

This completes the modification. 9600 BPS Packet can now be enjoyed on each of the bands installed in your TS-790A. For non-packet operation either unplug the
modem from ACC 4 or turn off power to the modem. For 9600 packet operation unplug the  mic and turn the mic gain to minimum.

Thanks to Frank Anderson, W7ZTA and Jeff Angus, WA6FWI for their help. Please report any errors or suggestions to:
Mike Hooper
KF6PU @ WB6YMH.#SOCA

Please come back in a few weeks and you will find some pictures of the modification here.