9600 baud packet on a Kenwood TK-840

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.

Category(s): Hardware, Radio
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7 Responses to 9600 baud packet on a Kenwood TK-840

  1. You know, if you think it might be worthwhile, I can likely match that connector up and make the mod plug and play.

    • I’m sure you could! I needed this right away, so I just did it ugly style, but it would be cool to have something a little neater. The KCT-19 is the kenwood-supplied dongle that plugs in there. I imagine it might be possible to find the connector based on some specification of that…

    • Yep, and it’s linked in the post. Not sure it will have the info you’re looking for, but if you can find it, that’d be cool.


      • Yeah, I saw the link to it but had hoped that there would be a searchable pdf as that was a really bad scan. I found a supplemental that gave me what I need. *Sometimes* you can take the part number in the service manual and glean the original supply vendor part number from it (Icom is very good about that). That does not look to be the case with Kenwood, though. E40-5737-05 is the corresponding part number for CN1. The part number was a dead end. What I DID find was that apparently, Kenwood sells an option cable for these for about $15 bucks called the KCT-19 and it brings all of the signals for that connector group out of the radio.

        I found an old Repeater Builder mail list that describes them as follows:
        (All manufactured by JST Connectors, which ironically does the bulk of Icom’s stuff too)
        (CN1) ZHR-8
        (CN2,CN3) ZHR-3
        (CN4) PHR-3

        Digikey stocks all of these (even though you just need the ones for CN1 and CN4). If you want, I will make a couple of pigtail sets for you and ship them when I make my next order. I just need to know how long you need the wires to be.

        • Oh, sweet, thanks for those part numbers and the offer. I need to order some stuff from digikey anyway, so I’ll throw these in. I don’t have any more of the radios that I need to mod right away, so I’ll just squirrel the connectors away for the next time.

          Good detective work, thanks Eric!

One Response in another blog/article

  1. […] very cheap (< $100) on the used market. The TK-840 could make a nice weekend “mod” project for those that prefer that route to 9600 baud packet versus a more plug-n-play solution like a […]