Modifications for the Kenwood TM-431

TM-431A Wide Band Transmit Modification

This modification allows the TM-431A transceiver to transmit and receive from 420.000 MHz through 444.975 MHz with essentially full power throughout the band.
  1. Disconnect power
  2. Remove top cover
  3. Locate a green jumper wire sticking out of the front panel assembly (behind VFO switch)
  4. Cut the jumper in half and insulate the ends
  5. Assemble radio
  6. This will reset the microprocessor and you will have to reprogram all channels

9600 mod for TM431A

Connect TX data to MO input at PLL circuit board (X58-3480-00) through a 100 kohm resistor.
Connect RX data to DET output pin 12 at IC1 (FM IF HIC)

The only problem is that you must mute the TX data when you RX, because this way you modulate the PLL.

You can find these pins when you open the bottom cover.
Locate the four points where the PLL circuit cover is soldered to.
They are marked with 0 in the drawing.

                     FRONT PANEL

                0                       0
          MO> o
                0                       0

PS. I have tried this - it works.
Measure deviation using TM231,431,531


To all technical kenwood freaks,
All people clever enought to know that they have to do something about the tremendous amount of people constantly overdeviating, mainly all using off the shelf kenwood equipment, coming straight from the factory adjusted at 6,7 or EVEN 8 KHz Peak deviation !!!!

This article describes how YOU can use your kenwood transceiver TM231, TM431, TM531 and apparently also the TMX41 range to measure ON THE AIR deviation of repeaters and all other stations...

The principle of measurement is indeed very simple and DOES NOT NEED ANY MOD INSIDE YOUR TRANSCEIVER NOR EVEN OPENING IT !!!

Your microphone connector provides at pin 6 an audio output, straight from the FM detector. As all kenwoods use the same detector chip and coil, all give the same output voltage for a given deviation. The output at this pin 6 (and pin 8 as a ground) is linear up to 5 KHz deviation. As the IARU standard for amateur radio communication is 3 kHz peak deviation, this range from 0-5 KHz is perfect. Connect your oscilloscope to pin 6 and 8 as ground, 400 mV peak to peak voltage corresponds to 3 kHz deviation. So 200 mV is 1,5 KHz deviation and so on.
CTCSS deviation should be around 300 Hz although most kenwood transceivers only react well from 400 Hz.
Packet should be set to approximately 2 KHz.

And additional an extra tip for improving the modulation quality of your kenwood TMx31:

As the microphone amplifier is quite overdriven, modulation sounds very rough, due to extreme clipping of the signal, especially a nuisance in noisy environments, such as when mobiling in a not so luxury car.
This can become much better just by changing or in some cases adding the input resistor of the microphone amplifier. Differnt types are used depen- ding on the market the transceiver was designed for. As they still believe at kenwood that europe uses a 25 kHz raster and the states 20 kHz, europe types get lesser attenuation before the amplifier then stateside trx's.
Hey You there at the kenwood factory reading this: whole europe is changing to a 12,5 kHz raster, so will you please stop delivering your transceivers and portabels with a deviation standard dating from jurassic park time !!!

schematic info:
Deviation potmeter TM231 VR3 and decrease R64
Deviation potmeter TM431 VR2 and decrease R58
Deviation potmeter TM531 VR3 and decrease R60

So remember 3 kHz is the standard, do not discuss wether it is good or not, IT IS THE STANDARD FOR AMATEUR RADIO.
It is the only way to live peacefully and to prevent wars between adjacent repeater or simplex channels.
(and by the way, narrowing the receiver bandwidth extends your range...)

Im looking forward to all comments, and I hope that somebody of the kenwood technical crew also will read this and do something about the adjustment of the new transceivers...

73 from Pedro M.J. WYNS AA9HX, ON4AWQ, ON7WP,
full licensed transceiver doctor... HI !

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