FTM-3100r

The work detailed on this page requires a steady hand and an attention to detail. By opening the radio, you are assuming responsability for the work or failures of the work. The authors of these documents to not assume any responsability for the results or side effects. Please be careful.

The FTM-3100R is an analog 2-meter FM transceiver that can be easily modified to send and receive 9600-baud FSK packet signals. At the time of writing, this radio is widely available for about $150 retail. The radio shares its main board with its C4FM-capable sibling, the FTM-3200R. Because of this, there are convenient circuit traces on the main board that carry discriminator audio to the (unpopulated) DSP daughter card, and feed wideband audio to the two-point FM modulator circuit. A straight-forward modification can add a dedicated packet radio port, while keeping all normal features of the radio completely functional.

Precautions: This modification requires careful techniques and may damage your radio. You may render it inoperable. This guide is provided for information only, and the authors assume no responsibility for damage to personal property. Do not attempt to perform this modification if you are unwilling to risk your radio!

Ensure your radio is disconnected from all external power, accessories, and antennas. Prepare a large, flat, clean workspace. Use adequate lighting and eye protection.

Medium phillips head screwdriver
Fine flush-cut side cutters
Wire stripper
Small drill bit (~0.8mm) and pin vise
Drill and 1/8 inch drill bit
Temperature controlled soldering iron with fine tip, brass tip cleaner

Approx 13” 4-conductor wire, I used 24 guage solid-copper from CAT5 cable
DE-9 male connector and shell
Optional 4-6 stud ring terminal, I used: /Gardner-Bender 22-18-AWG 4-6-Stud Ring-Terminal-Vinyl-Red-10-Pack-15-101
Optional 1/8th inch heat-shring tubing, 8 inches total (I used two 4-inch sections)
Fine rosin-core solder
One small nylon wire tie
Approx 6” of 30 gauge solid wire-wrap wire, like Kynar
Electrical tape

Procedure:

Remove three phillips head screws retaining the top black plastic shell of the radio. There is one screw on the top, and one on the left and right side of the radio.

The plastic top cover of the radio is snap-fit in place over the aluminum mounting boss protrusions on each side. Use your fingers to pry the cover over one side of the protrusions and then the other.
Prepare your DE-9 connector and cable. Connect 4 wires to TXA (pin 1), PTT (pin 3), RXA (pin 5) and ground (pin 6). Slide the heat-shring tubing over the conductors and assemble the DE-9 shell. I did not shrink my tubing. Strip about ⅛ inch off the loose ends of the 4 conductors.

In the next step,l drill a ⅛ inch hole in the back of the radio to route the data cable. I chose to drill my hole in the recess near the USB connector marked “data”. BE EXTREMELY CAREFUL, use a sharp bit and low drill speed. Do not press hard on the drill bit, you don’t want it to burst through the drill hole and damage the internal components of the radio. There is a diode and electrolytic capacitor that can be damaged by the drill, about half an inch away from the aluminum housing that you are drilling through. Feed the loose ends of the DE-9 connector cable into the hole you just drilled. Secure the nylon wire tie around the cable with about 3-4” of wire free on the inside of the radio. Trim the nylon wire tie.

Now, study the following two pictures to identify the signal connection points. I’ll use “up” to refer to the PCB direction relative to the top of the picture, “left” to refer to the PCB direction relative to the left of the picture, etc. GND is connected to one of the central PCB mounting screws, using a stud ring terminal. PTT is connected to the bare test point in the buss circuits in the lower left part of the picture. TXA is connected to the via at the bottom of a short L-shaped trace from an unpopulated SMT transistor footprint. RXA is connected to the via annotated, at the top of a trace leading to a capacitor near the unpopulated SSOP footprint. Crimp and solder a stud ring terminal to the GND conductor from the DE-9 connector. Remove the highlighted PCB screw from the radio, then place the ring terminal over the hole and replace the screw.

Prepare the two via connection points by CAREFULLY removing the solder mask on the vias. I used a small drill bit to do this, approx 1/16”, held between my fingers. Don’t use a drill. A smaller drill bit would probably be superior. I recommend buying a pin-vise kit and using a sub-1mm drill bit to carefully perform this task. It will only take a couple turns of the drill bit between your fingers to remove the solder mask from the via. You will see shiny copper when successful. Don’t remove too much material, you may damage the copper trace. Study the next picture which shows the completed wire attachments. Note the relief turns left in all the conductors. We are attaching small wires to delicate points on the PCB. be careful to arrange your wires so they are not straining the solder joints on the PCB.

Tin the bare copper vias you just exposed with a small amount of solder. I just applied solder to the clean tip of my soldering iron, then touched the iron carefully to the via. Use the smallest soldering tip you can find, on a good-quality temperature controlled iron.

Strip the insulation off the 30 gauge Kynar wire, and cut it into two pieces about 3” long each. Tin one end of each piece with a small amount of solder.

Attach the prepared 30 gauge wire pieces to the marked points for TXA and RXA. I did this by placing the tip of my iron against the wire, about ⅛ inch from the end of the wire, as I touched the wire to the pre-wetted via. Remove the iron and hold the wire until the solder cools.

Attach the bare ends of the TXA and RXA conductors from the DE-9 cable to the fine wires you just placed on the PCB. It’s best to leave a good amount of relief length in the conductor as you make the connection. I twisted the wires together, then soldered over the twist. Then I covered each wire joint with a small amount of electrical tape.

Tin the PTT test point on the PCB with a small amount of solder. Tin the bare end of the PTT conductor from the cable. Then attach the conductor to the test point, applying your soldering iron to the bare conductor as you touch it to the prepared solder on the test point. Remove the iron and hold the conductor in place.

Now, cover the joints between the thin wire and thick wire on the TXA and RXA lines. Use about ¾” of electrical tape on each, folded. The goal is just to provide electrical insulation, not mechanical strength.

Replace the radio cover. Modification complete.

This modification will provide discriminator audio to the TNC at about 500mV to 1.0V p-p. This is works very well with a NinoTNC on 1x RXA gain. The modulator connection for TXA causes this radio to transmit at about 1.3 kHz of deviation per volt of TXA audio. So, for a 3.0 kHz transmit deviation, adjust the TX_DEV pot with the radio connected until the peak-to-peak voltage at the TXA test point is about 910mV. I’ve used this modification to send and receive 9600 baud and 4800 baud GFSK modulated signals with excellent results. It also provides excellent AFSK and DPSK signal performance.