NinoTNC and Kenwood TK-790

Thanks for info and photos by John N3XKD from the Kentuckiana TARPN, and for page updates and more photos by Don N2IRZ from the Atlanta TARPN

The TK790 is a 45 (or 110 watt) 2m transceiver made for commercial 2-way use. They are available surplus/used for under $100.

At this time we've established that they are useful for 9600 packet radio using the NinoTNC, but have not yet tested them at a great distance. That testing is being performed, and the results will update this page.

This page describes the following:

The configuration of the internal resistor jumpers to defeat the vehicle aux-power requirement
The configuration of the internal resistor jumpers to enable Data input mode
The connection points needed for 9600 Baud operation
Wiring for the DB-25 Male to DE-9 Male wire between the radio and the NinoTNC
Some programming details to enable PTT on the DB-25
Converting the power connector to Anderson PowerPoles
Connection details for the (optional) N2IRZ RSSI Board (not yet a product, in Beta testing)

VARIANTS:
The TK-790 comes in a few different variants, all of which can be programmed for the 2 meter band:

TK-790/(B) F1 [148-174Mhz] 45 Watt
TK-790(B) F2 [136-156Mhz] 45 Watt ← Rare
TK-790/(B) F1 [148-174Mhz] 110 Watt ← Uncommon

Kenwood also offered the TK-690 (6 meter) and TK890 (70 cm) models, which are likely to be similar enough for these instructions to work, but we have NOT tested any of this on those models. If you have either model, please let us know if there's anything we should add to this page, thanks.

1. Internal Resistor Jumpers - For Power On

No parts are needed, but you will need a Philips screwdriver, a temperature-controlled soldering iron with a needle tip, small-diameter rosin-core solder, needle tweezers suitable for picking up tiny surface-mount resistors, a very steady hand and good eyes or a decent magnifier. A cell phone can be used as a magnifier.

Many of these radios will seem to not power on, as they are usually set up for Ignition Sense for a mobile installation. If you adjust the Jumpers inside the top case as shown below, it will remove the requirement for Ignition Sense, allowing the unit to power on using the power button.

Remove the top cover by unscrewing the five screws.

Identify the two areas of interest on the board.

DETAIL #1

R504 should be present
R506 should be absent
R742 should be present
R743 should be absent

Unsolder and move the jumpers IF NEEDED. These are zero-ohm resistors, so anything that shorts out the right places is fine (e.g., wire)

2. Internal Resistor Jumpers - Data Input Mode

This jumper sets whether the DB-25 input is microphone audio or data audio. You want data audio.

No parts required, but the same tools as above

You can expect to have to move R640 to position R641.

DETAIL #2

This completes the internal jumper configuration

3. Connections Needed for 9600 Baud

This section explains where the four connections for packet operation are found.

Parts Required:

One 4-inch piece of #30 AWG Wire (wire-wrap or magnet wire)
Two short pieces of 1/16" heat-shrink tubing

You will also need a temperature-controlled soldering iron with a needle tip, some hot-melt glue, a steady hand and good eyes (or a magnifier).

Transmit Audio (TXA), Push-To-Talk (PTT) and Ground are already present on the rear DB-25 Female connector. (PTT requires radio programming; see further below)
Receive Audio (RXA) must be taken directly from the circuit board; while there is a DEO (Detector Output) signal present on the rear DB-25 connector, it has some unacceptable distortion making it unsuitable for our purposes.

Start by identifying the area of interest: A small through-hole on the circuit board (called a Via), which is the pick-off point for RXA.

This image shows the front left of the top circuit board. Note the white ribbon cable. The insets show the board layout and a close-up of the via with the wire placed inside it, ready for solder.

Strip one end of the #30 wire to 1/16", and the other end to 1/4". Tin these ends with solder.
Place the 1/16"-stripped wire end into the via hole. Hold it in place, and secure it with some hot-melt glue an inch or so from the via. Hold it until the glue hardens.
Solder the wire into the via. It takes a very fine-point soldering iron and several seconds for the heat to melt the PC Board coating and for the solder to stick. Note that the inside of the hole may solder before the pad does, and that's good enough. As an alternative, you can solder the wire to one end of either of the capacitors (C531/C533) that are right next to the via.
Unplug CN505 from the circuit board (its wires lead to the DB-25 connector) and identify the 3rd green wire (RSV2). This wire connects to Pin 16 of the DB-25 connector.
Cut the RSV2 wire about an inch from CN505, leaving enough to reconnect it if necessary some time on the future.
Strip the RSV2 wire from the DB-25 connector about 1/4" and tin it. Place a 3/4"-long piece of 1/16" heat-shrink tubing over the #30 wire, then solder the RSV2 wire from the DB-25 connector to the #30 wire. Slide the heat shrink over the soldered connection and shrink it in place.
This image shows the green RSV2 wire cut, stripped, tinned and the #30 RXA wire with the heat-shrink tubing on it, ready for soldering.
Place a short piece of 1/16 inch heat shrink tubing over the cut end of RSV2 near CN505 and shrink it in place, to prevent the wire end from shorting to anything.

This image shows the completed work

4. Wiring for the TNC-Radio cable

This cable connects the NinoTNC to the Radio.

Parts Needed:

DB-25 Male solder-cup connector (parts-express 090-570)
DE-9 Male solder-cut connector (parts-express 090-550)
DB-25 hood (parts-express 090-705)
DE-9 hood (parts-express 090-695)
12"-16" length of 4-conductor stranded wire. CAT5/6 wire may be used if the wire is stranded. Solid wire will break eventually.
A soldering iron, solder and a screwdriver will be used.

Connect the wires as follows:

DB-25M ----- DE-9M

Pin 13 ----- Pin 1 (TXA)

Pin 7 ------ Pin 6 (GND)

Pin 15 ----- Pin 3 (PTT)

Pin 16 ----- Pin 5 (RXA)

This image shows the DB-25 M connector with all seven pins soldered, including the (optional) N2IRZ RSSI Board signals.

From left to right:

Red (Pin 13): TXA to TNC
Black (Pin 7): GND to TNC
Black (Pin 18): GND to RSSI Board
Green (Pin 16): RXA to TNC
White (Pin 2): PTT to TNC
Red (Pin 14): +13V to RSSI Board
Green (Pin 1): RSSI Signal to RSSI Board

5. Programming Details

The radio must be programmed to feed PTT to the DB-25 connector.

Using the KPG-44D software and the KPG-43 programming cable (both available from bluemax49ers.com), connect to the radio and select "Edit/Function Port". Set Radio 1 AI1 to EXT PTT as shown.

Now select Edit/Optional Features and check the box for "Ext PTT w/MIC Mute. Uncheck any other boxes.

(Note: You might want to set Minimum Volume to 0 and Timed Power Off to Off.)

After setting everything else as desired, load the program into the radio.

6. Converting the power connector to Anderson PowerPoles

This one is fairly simple. The original power cable ends in a Molex-type connector, which may be inconvenient and thus, if inconvenient, this can be converted to Anderson Powerpoles. The key is that the wires for the power connector have ring terminals held in place by screws inside the radio, as shown here.

Parts Needed:

A pair of red and black PowerPole bodies.
Two PowerPole contacts to suit your wire (the 15A size is good with #18 wire).
A 12 to 16 inch length of (at least) 18 AWG red/black paired wire. Heavier wire is not needed.
Two small ring terminals, suitable for the wire and the screws in the radio.
Crimpers for the PowerPoles, and for the ring terminals.
Philips screwdriver.

Remove the four screws holding the plate (containing the accessory connector and power connector) to the back of the radio.
Remove the four screws and take off the bottom radio cover.
Pull off the thin metal shield plate, and locate the two screws holding the wires from the power connector to the circuit board.
Unscrew both screws to disconnect the wires from the circuit board.
Remove the power connector body from the plate, then feed the wires through the rubber bellows to remove it completely.
Strip the wire ends and crimp on the PowerPoles and ring terminals.
Feed the ring terminal end through the plate and bellows, and fasten the ring terminals securely to the board with the screws.
Add some strain relief (such as a zip tie) to the wires where they pass through the plate.
Reinstall the rear plate, shield plate and bottom cover.

At this time, you might want to reinstall the top cover and anything else we've forgetten to mention.

7. RSSI Signal Connections

The TK-790 does not have an RSSI indicator, which would be useful when setting up a link. This section explains the connection details to obtain RSSI, Ground and +13V power from the radio, with which the N2IRZ RSSI Board can give a visual indication of signal strength. All of these signals are available on the DB-25 connector.

Parts Needed:

The completed Radio to NinoTNC cable described above, with the DB-25 shell removed.
A 20 inch length of three-conductor cable, ends stripped and tinned. CAT5/6 stranded cable may be used.
A four-contact female 0.100" header body with solder tails (for the RSSI board).
Soldering iron and solder.
Some hot-melt glue or silicone caulk.

NOTE: These instructions do not explain the setup and programming needed for the RSSI board. We are only picking off the needed signals!

Solder a wire to Pin 1 of the DB-25 male (RSSI), then solder the other end to Pin 1 of the RSSI Board connector.
Solder a wire to Pin 18 of the DB-25 male (Ground), then solder the other end to Pin 2 of the RSSI Board connector.
Solder a wire to Pin 13 of the DB-25 male (+13V), then solder the other end to Pin 4 of the RSSI Board connector.
Place a dab of hot-melt glue in the RSSI connector's Pin 3 cavity. Silicone caulk may be used instead.
On the radio's top circuit board, locate the two "SB" pads near CN505, shown below, and short them together with solder.

Note that F501 (500 mA) is there in case of an external short on the +13V pin. Exact replacements may be difficult to find, so take extra care to avoid a short circuit.

For more information, consult TNC and Radio Adjust and Test or the operations guide for your TNC.