Modifications for the Kenwood TM-731

TM-731 frequency and cross band repeater


The TM-731a will transcive between 136-174 MHz and 400-500 MHz as well as perform cross band repeter function when the following modifications are performed.
  1. Remove both the top and bottom cover. Disassemble the front panel such that the control unit (X53-3250-XX) is accessable.
  2. Remove R121, R123 and R25. While R125 snd R124 must be present.
  3. If the time-out timer function is not desired, remouve R122 as well.
  4. Reassemble the unit and perform a microprocessor reset before operating the set.
  5. Preserve your license by operating whithin the bands it is cover for.

TM731 & 9600 Bps


The TM731 is different enought to cause problems.
What need to be done is to add another RX Mute transistor. (look the schematics and see how the TX deviation is shorted to ground during receive.)

Use a plain NPN and a pair of 10k resistor (copy the one used on VR3 in the schematics.)

Take the TX DATA and feed it to the collector through a series 1k resistor, and then feed that through another 1k resistor to the wiper on the deviator pot.
This was a result of Kenwood changing the deviation pot value from 10k to 100k (it's wasn't broken, but they fixed it anyway.) It is full of surface mount devices.

Article of Jeff WA6FWI @ WB6YMH.#SOCA.CA
TM-731A Hooking to TNCS


Marty Goodman KC6YKC
June 1992

Home packet BBS: WD6CMU Richmond, Northern CA

I recently acquired both a PK88 TNC and a Kenwood 731a dual band mobile transceiver. I wanted to hook the Kenwood to the PK88. What I found was that the audio output on pin 6 of the eight pin microphone connector on the front panel was not the same as the audio that comes out of the speaker or out of the rear speaker connector.

What you get out the audio from the front panel mic connector on pin 6 is an UNSQUELCHED audio. Kenwood does this, apparantly, to support their RC10 remote controller for the unit, to allow the RC10 independent control over the squelch on the audio at the RC10's speaker. This causes a problem, for when you feed that into the audio input of the PK88, its DCD light stays on ALL THE TIME. The PK88 will receive data just fine when hooked to pin 6 of the TM731a mike connector, but it will refuse to key the transmitter because it perpetually thinks the air is "busy".

The simple approach to hooking the PK232 to the Kenwood involves using a separate cable with a 1/8 in mini phono plug at each end, and run that between the audio phono connector on the PK88 and the speaker output jack for "main audio" on the TM731a. This will allow packet operation, and is no doubt the approach that virtually everyone uses. There are some drawbacks to this arrangement, tho.

Using two cables is a tad clumsey. Worse, when you plug in the cable in the rear speaker connector, there is no way to enable the speaker to audibly check your signal. It also makes it impossible to make a switch box to switch the TM731a from packet to audio operation at the flick of a switch, for as long as the phono plug is plugged into the back of the TM731a, you CAN NOT enable its internal speaker. Finally, this arrangement requires you to adjust the volume control to the right level and leave it there.

What follows is a mod that give you a SQUELCHED, LINE LEVEL audio signal available on pin 6 of the microphone connector. With such an arrangement, you can turn your speaker volume up and down as you like, and it will not affect the audio going to the packet box. You can use a single cable to connect the packet box to the transceiver, and it is easy to rig up a switch box that switches between the packet box and the hand microphone. The only disadvantage of doing this mod is that your TM731a will NO LONGER be compatible with the RC10 remote control device.

The Modification:

This modification requires skill at fine soldering, a good ultra fine-tipped pencil type soldering iron, and a short piece of 28 gauge stranded wire. You really should have at hand a Kenwood service manual for this rig, with complete schematic and board layout diagrams, before you attempt it, tho I HAVE tried to write my description so that you can do the mod without a service manual at hand.

  1. Turn off power and disconnect the rig from its source of power
  2. Remove the BOTTOM cover of the rig.

  3. Lay the rig on your work table with the front panel pointing away from you, and its rear heat sink pointing toward you. On the right will be the main volume and squelch control. Look at the little connectors that hook the front panel of the radio to the board you have exposed and are looking at. On the extreme left is one connector with eight pins in a row, with the third pin from the left unused. To the right of that is a five pin connector with the middle pin not used. To the right of that connector is an electrolytic capacitor, to the right of the cap is a two pin connector, and to the right of the two pin connector is a phillips screw that is one of the screws that hold the board you are looking at inside the rig.

    Locate that five pin connector mentioned above, the one that has its middle pin unused. The SECOND WIRE from the LEFT of that connector should be a RED wire. That spot on the connector will be labelled on the main board you are looking at as "RDM".
  4. CUT this red wire about three quarters of an inch from where it leaves the connector. Tape up the portion that goes to the connector.
  5. Strip off about a quarter inch of insulation from the portion of the red wire that dives into the rig and heads toward the front panel.
  6. Locate "IC 8" on the radio. This is actually NOT a real integrated circuit, but rather is one of six little vertical circuit boards that jut up like fins from the main board. IC 8 is the left most of the six such boards, and is right next to IC 9, which is a zig zag in line pin integrated circuit. IC 8 is located just toward you from the five pin connector with the four wires... the one from which you cut the red wire. IC 8 is labelled as "IC 8" in silk screening on the circuit board.
  7. Solder a short length (2 or three inches) of stranded, insulated, 28 gauge wire to pin 9 of IC 8. Pin 9 of IC 8 is the next to last pin of this single in line pin "IC". The "last" pin of IC 8, or pin 10, is located closest to the rear of the rig (closest to you, given the orientation of the rig that I suggested).
  8. Now solder the other end of this wire to the stripped, cut portion of the red wire that you prepared in step 5 above.
That's it! you now have modified your rig to provide line level, squelched audio on pin 6 of the microphone connector.
For those technically inclined, you should note that this audio is leaving an audio amplifier, and going thru both a 1K ohm resistor and a .1 mfd capacitor before it arrives on pin 6 of the microphone connector. Thus the signal is DC-blocked and reasonably protected against minor nastiness that might appear on pin 6.

I'd appreciate any feedback from those attempting this modification, tho of course I must note I cannot take responsibility for any harm that may come to your radio due to either proper or improper implimentation of this modification. I CAN say honestly that I HAVE done this to my radio, and it appears to work quite well.
Kenwood Cross Band Repeat: How to USE it


Hardware information for modifying the Kenwood TM-731A for cross band repeat is widely available on many packet bulletin boards. However, information on how to impliment cross band repeat is often not present along with the description of the hardware mods. What follows is KENWOOD's documentation on how to USE the TM731a after it has been modified for cross band repeat:

Text of Kenwood Sheet on using Cross Band Repeat on the TM-731A:

The repeater corss-band operation allows the TM-731A to receive on one band and re-tramsmit the signal on the other band. An incoming signal will automatically be switched to the sub-band. The TM731A will then re-transmit the signal from the main band. Each band may contain shift information. Only one band can contain sub audible tone information (the optional TSU-6 ... currently called by Kenwood the KQT8... is required for tone decode). If one band requires a sub-audible tone, either select the TONE function for encode or the CTCSS function for encode / decode. DO NOT turn both TONE and CTCSS on in the one band.
  1. Select the first frequency in the main band. If required, select the offset.

  2. Press the BAND key to move the contents of the main display to the sub-display.

  3. Now select the second frequency and offset.

  4. If Tone or CTCSS is required for one of the bands, continue with this step. If Tone or CTCSS is not required, go to step 5.

    1. Place the operating frequency that requirs TONE or CTCSS in the main band (use the BAND key).

    2. Select the sub-audible tone by pressing the F key and then the T.SEL key. Rotate the main encoder knob until the desired tone is displayed. Press any front panel key to display the operating frequency.

    3. If only the TONE function is required, press the TONE key (the [T] indicator will light in the display).

    4. If the CTCSS function is required, do not turn the TONE function on. Instead, press the F key and then the CTCSS key (the [CTCSS] indicator will light in the display. If the CTCSS arrow is not pointing towards the main display, press the F key and then press the CTCSS SEL key.


  5. Adjust the main and sub-band squelch controls to the threshold point. The TM-731A will transmit in the repeater cross band mode if the squelch controls are set too low or a signal is received. (comment: setting both squelch controls properly is VERY important!)

  6. Note:
    The next step will place the unit in the repeater cross-band mode

    Press the F key and then press the A.B.C. key. Three dots should be lit in both display frequencies. The unit will now operate as a cross band repeater as described above.

  7. To take the TM-731A out of the cross-band repeat mode, repeat step 6.

Kenwood TM-731A firmware bug


The Kenwood TM731A dual bander has a firmware bug that breaks the 2m VFO scanner in certain circumstances, that is, the 2m VFO scanner won't work properly if certain PL tones are stored into memory 1. So far, all tested 731A's have had this problem. The highest serial number tested was 72713. To demonstrate the problem, perform this experiment:
  1. If not already done, load search bounds into memories A and B of the 2m unit. Verify that the VFO scanner works.
  2. Change the PL tone for memory 1 to (for example) 91.5: select memory 1, press F VFO to transfer it to the VFO, then press F TSEL and select 91.5 Hz PL tone, then store back into memory 1 by pressing F MR.
  3. Set the VFO to a frequency outside the range given in memories A and B, then start the VFO scanning (by pressing VFO for a few seconds).
  4. If your radio has the bug, it will alternate between the frequencies stored in memories A and B. If you start the scan while the VFO is within the search range, it will scan up until the top limit is reached, then begin alternating between the two limit frequencies.
  5. To restore 2m VFO scanning, program memory 1 with one of the several PL tones acceptable ... the default of 88.5 Hz works fine.
  6. The 70cm unit does not appear to have this problem.


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