The Kenwood TK-840 is a nice commercial UHF radio that is starting to go for $50-$100 on eBay due to the fact that it is not narrow-band capable. It is happy in the ham bands, has a good screen, excellent rubber-covered buttons, and is quite small and rugged.
While not frequency-agile or field-programmable, it is more than adequate for a fixed installation, such as a remote base or digital mode transceiver. However, not much is available “out there” on how to interface it to a high-speed TNC. While you could use the well-documented mic and speaker jacks for 1200 baud, 9600 baud and faster require low-level access to the radio’s internals.
This rig is similar to (but much newer than) the oft-used Kenwood TK-805, for which there are documents available about general interfacing. This one is pretty common, but it actually only describes high-level audio connections, which aren’t suitable for high-speed stuff. However, you can follow those instructions to remove the speaker jack, jumper the proper traces to enable the internal speaker, and route a cable through the resulting hole in the case for interfacing.
The service manual can be found on repeater-builder, which shows the various boards and the signals on each of the inter-board connectors. In order to make high-speed packet work, you need access to the modulator for TX audio, the detector output for RX audio, ground, and of course PTT to transmit. In the manual, these signals are listed as DI (external modulator input), DEO (detector output), E (earth) and PTT respectively. If you want to power your TNC from the radio, you also need SB (switched battery).
On the main TX/RX board of the radio, on the left side (if facing the front panel), there is a small group of three connectors, two small and one large eight pin socket labeled CN2. The pins on the large connector are numbered from right to left, with the right-most pin being #1 and the left-most being #8. DEO is pin 1, DI is pin 4, and PTT is pin 7.
Since the pins aren’t exposed on the bottom side of the board, I carefully soldered to the top of each as they leave the board and enter the socket. It takes a steady hand and a good eye, as these pins are tiny. The nice thing about the older TK-805 is that all the components are larger and easier to solder to.
To the left of CN2 (above, in the picture) is the external alarm socket, which contains labeled pins for E (ground) and SB (switched battery). I soldered to the top of each pin here to gain access.
With everything buttoned up, I adjusted the TNC for the appropriate amount of drive to get about 3kHz of deviation. This took quite a bit of drive compared to the amateur radio I had been using with the same TNC for testing, but the Kantronics KPC-9612+ has plenty of oomph to accomplish the task. The radio appears to perform quite well with minimal additional tweaking.