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buildersRadios & Wiring for Radios ➜ Yaesu FTM-3100r

The work detailed on this page requires a steady hand and an attention to detail. By opening the radio, you are assuming responsibility for the work or failures of the work. This guide is provided for information only, and the authors assume no responsibility for damage to personal property or side effects. Do not attempt to perform this procedure if you are unwilling to risk your radio! Please be careful.

Yaesu FTM-3100r wiring for 9600 baud

This procedure is new as of April 2, 2023. Modified May 16, 2023. Please send success/failure/problem/correction/hints to tarpn@groups.io.

The FTM-3100R is an analog 2-meter FM transceiver that can have a TNC data-connector added, enabling the radio to participate in sending and receiving 9600-baud FSK packet signals. At the time of writing, this radio is widely available for about $150 retail.

The FTM-3100R 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 procedure can add a dedicated packet radio port, while keeping all normal features of the radio completely functional.

This is quite a boon to our project as it scores above our other prime 9600 candidate, the Tait TM8105, in several ways. The radio is more powerful, 65 W, has an S-meter, easy frequency selection, monitor speaker with volume control (does not affect Rx packet data). Even the squelch affects the monitor speaker without muting the receive data. This radio is also available new at these vendors for only a little more than the 25 W Tait was available on eBay:

—— Procedure by Nino KK4HEJ with modifications by N2IRZ and KA2DEW

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

Tools required:

Supplies required:

Remove Top Shell

Note the two black tabs that go off the back of the top shell, one over the power cord, and one over the fan. These tabs make it easy to identify the back vs the front of the top shell.

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.

Drill hole for cable

In the next step, you will drill a 1/4 inch hole into the side of the black cover about 1/2 of an inch forward of the back left side of the shell and about 1/2 of an inch down from the top edge. You'll do this with the cover removed. Make sure you don't drill into your workbench. That's embarassing.

The hole in the black cover is going to line up with a pre-existing hole in the cast-aluminum chassis. The hole is going to be behind the left rear mounting bracket screw.

This photo is of a new unmodified cover sitting on top of the radio chassis, sitting on top of a re-assembled, modified, radio.
click to embiggen

Test penetration of the CAT5 wire into the chassis. If your cable doesn't fit, pick the next larger drill bit and enlarge the hole. Put the top cover and CAT5 wire asside until a later step.

Study the PCB and identify the solder points

Study the following several pictures to identify the signal connection points. There are three 30-gauge wires to attach.

This photo is looking down on the top side of the radio, having removed the cover.
Notice the brass spacer to which the center of the top cover would be screwed. Also, in the front left of the PCB, notice the set of pads for an integrated circuit which is unpopulated -- I think these are actually pads for a surface mount connector used by a daughter card in the FTM-3200R model. These two locations, the region around the unused pads, and the region around the brass spacer, are important to the modification, as you'll see in following images.
click to embiggen

This annotated image shows attach points for PTT, TxAudio, RxAudio.
The TXA and RXA spots are pre-tinned.
The PTT spot needs to be tinned.
click to embiggen

This photo shows the wires attached, black to PTT, red to TxAudio, blue to RxAudio. Note that my choices of color were not ideal. I'll try to do this mod over again at some point with idealized colors.
PTT is connected to the bare test point in the buss circuits in the lower left part of the picture.
TXA is connected to a pre-tinned pad.
RXA is connected to one side of a existing surface mount capacitor.
click to embiggen

Doing the job

I recommend setting up your magnifier as described below and then manipulate a cold soldering iron to demonstrate to yourself that you are steady and able to see the points in question while holding the iron and the wire.
Strip each 30 gauge wire just enough to attach to the three spots - 1/16th of an inch.
Tin the 1/16th inch bare lead.

Using some magnifier (I used an iPhone clamped above the radio, in the camera app, and at 3:1 zoom), touch the end of one of your wires to the right hand end of the capacitor. Move the soldering iron tip (hot) up against the wire and give it just enough heat to melt the wire's solder to the capacitor's solder. After you solder the wire, rerouting it with a 1/2" turn radius should not pull the wire off the capacitor.

Send the other end of the wirewrap wire over the side of the radio and out of the way. Do this with each of the wires as you complete the job.

Tin the top of the PTT pad. Make sure you do the correct one. Carefully tack your wirewrap wire to the PTT tab.

Carefully solder your wire onto the tinned via for the transmit audio.

Now the hard part is done.

Wiring inside the box

The CAT5 wire should be stuffed from the outside of the black cover such that about 8 inches of the wire is inside the cover. You'll be pulling some of that back out once the radio is ready to be sealed up.
  1. Strip 4 inches of the jacket off the CAT5 wire.
  2. Cut the blue/white striped wire to about 1" long and strip the wire to give us 1/4" of bare wire.
  3. Route the wire through the chassis in the left side (toward the rear) hole Yaesu cleverly provided us.
  4. Obtain your stud ring-terminal.
  5. Remove and discard the insulating jacket (if any) around the cylinder part of the ring-terminal
  6. Unscrew the PCB hold-down phillips head screw that is to the left and slightly to the front of the brass spacer (circled in this photo).
  7. Make sure your ring-terminal fits under that screw.
  8. Using a long nose plier, hold the ring-terminal by the ring-end and solder the blue/white striped wire to the outside of the top of the cylindrical part of the terminal.
  9. After the ring-terminal cools, grab your three wirewrap wires and thread them through the ring-terminal cylinder from the ring end and up.
    We will NOT be crimping this ring-terminal!
  10. Screw the ring-terminal down onto the PCB (carefully not on top of the 3 wirewrap wires!) such that the cylindrical part points to the back of the radio.
  11. Dress the wirewrap wires by pulling on them through the ring-terminal cylindrical section and such that the already soldered wirewrap wires lay across the front-left of the board. Do NOT pull the wires tight. You'll want nice 1/2" radius turns and do not stress the joints. They are not going to take much tugging and you could pull parts and PCB traces right off the board. Use maximum care.
  12. Tie a thin plastic tie around the brass spacer in the center of the board, such that it is relatively firm and up near the top of the spacer. Do not cut the end off the tie.
  13. Wrap the remaining wires of the CAT5 around the spacer, between the PCB and the tie-wrap, such that one single complete wrap occurs. This plus the friction of the wire through the chassis, and with the blue/white soldered to the ring-terminal, should keep your CAT5 wire from ripping out of the box. This wrap also protects the wirewrap wire from stress.
  14. Trim the white/brown, white/green, and white/orange wires back to 1/2 inch from the completed wrap around the brass spacer. Do not strip these.
  15. Trim the solid-brown wire back to 1/2 inch as well and, again, do not strip the end.
  16. Cut the solid-orange, solid-blue and solid-green wires to about 1 and a 1/2 inch and strip the ends back to expose 3/8" of wire.
  17. Take the ends of your wirewrap wire, and identify which wire goes to the PTT pad.
  18. Route the PTT wirewrap wire to the solid-orange CAT5 wire.
  19. It is ok to have some slack on this wire.
  20. Strip 3/4" off the PTT wirewrap wire and wrap it around the stripped end of the orange wire.
    If you have a wirewrap tool, use that.
    If not, keep the winding tight.
  21. Orient the joint between the two wires so the insulated wires come off the joint in the same direction allowing you to slide a shrink-wrap tube over the joint after it is soldered.
    Don't solder or shrink-wrap yet.
  22. Repeat for the TXaudio wire, wrap it around the blue CAT5 wire.
  23. Repeat for the RXaudio wire, wrap it around the green CAT5 wire.

  24. Put a small piece of cardboard or a couple of layers of paper underneath the three wirewrap/cat5 joints and then solder the 3 connections.
  25. After the soldering cools, take a 1/2" piece of heatshrink and cover the each joint.
  26. Shrink the shrink (using heat gun, coming close with soldering iron, or careful use of a lighter) and make sure the heatshrink tube will no longer come off the joint.
  27. Bend the three joined wires to the left of the brass spacer such that they are away from the coils on the PCB.

DE-9 connector and cable

While pulling on the CAT5 wire gently from outside the black cover, place the cover on top of the radio and snap it into place. Put the three screws in place to attach the black cover to the chassis.

Cut the CAT5 to length so it reaches the TNC.
Strip the ends as appropriate for a DE9 connector.
Remove the brown and brown/wite wires.
Solder the orange, blue, green and blue/white wires onto the DE9 male as shown in the image.

Procedure complete

This procedure 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 3.3 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 radio, with the wiring as described, to send and receive 9600 baud and 4800 baud GFSK modulated signals with excellent results. It also provides excellent AFSK and DPSK signal performance.

A TxDelay setting of 40mS seems adaquate.
The receiver AFC is stable 40mS after unkey if there is a signal present.
Receive audio looks good 20mS after unkey.

- Conception, proof of concept, analysis, initial test, declaration of success, by KK4HEJ
- PCB connection locations, chassis penetration, more analysis by N2IRZ
- Cable dressing, web page and photography by KA2DEW

© Tadd Torborg, 2023 -- all rights reserved