Modifications for the Kenwood TM-531



19-07-1998 TM-531 & 9600 Baud

To use the Kenwood TM531 with a g3ruh 9600 Baud modem:

  1. PLL-unit modifiation:
    Change R56 to 3k.
    Change C56, 57 to 10uF/10v.
  2. Main board:
    Remove C55, 70, 118.
  3. TXA:
    TXA via 150k to MO (pll-unit).
  4. RXA:
    RXA to pin 12 ic2 (main board).
  5. Change C34 (ruh board) to 1uF (mkm).
  6. You'll need a eprom COA.
  7. Solder a ptc (siemens b30-0845-05 (60 Gr. Celius)), on the VCO-unit.
73's, Ed. PE1NAT
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I received your mail today morning,on Dec., 15.
I'm using TM-531 on G3RUH type 9600bps modem.
We ha local aria network using 9600bps modem.
It is working well,now. But, beginning of this network is not easy.

You can connect G3RUH type modem as follow.
1. TXA
Take off the shielding plate under the main PCB.
Search for VR3 on main PCB.
VR3 is indicated on PCB "MAX DEV" for adjust maximum deviation.
The slider of this VR is connected to MD pin of VCO through C55 capacitor.
Connect TXA to MD pin of VCO using resister(56k to 100kohm) and capacitor 0.1uF,connected in series,like follow.
     TXA ---II------///----MD(VCO)
           0.1uF    56k to 100kohm
If you get weary to take off shielding plate,you may connect the TXA to hot end of VR3, using the resister and capacitor as aforesaid.
TM-531 & 9600 Bps.

2. RXA
Search for IC2 on main PCB.
IC2 is a 16 pin hybrid IC for IF AMP and discriminator.
You can discriminate pin number of this IC from printed symbol on the main PCB, and grand pin.
Pin 2 and 16 is connected to grand.
pin 5 is output for ceramic filter(CF1),and pin 7 is input from the filter.
Connect RXA to pin 12(DET) directly.

9600bps on 1200MHz band is very difficult. Because,frequency stability is not enough. And, IF band width is not enough,too.
We are developing SMC transceiver for G3RUH modem on 144,430,1200MHz band.
It has high stability PLL OSC(about 1ppm) and good linearity modulator (voltage controlled X'tal OSC)
Sammy JE1RLA @ JE1RLA



19-07-1998 Measure deviation using TM231,431,531

From AA9HX - ON7WP @ ON7RC.BT.BEL.EU

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 !
ON7WP @ ON7RC.BT.BEL.EU



20-04-2000 TM-531A 20 MHz offset modification
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 956 (9-1-1990)

The TM-531A offset can be changed from 12 MHz to 20 MHz be performing the following modification. Once the modification is done, only 20 MHz can be selected. It is recommended that this modification be performed at the dealer level since they have the correct tools to disassemble the transceiver.

Required tools: 

Spanner wrench     (W05-T004)   for mic jack nut
11mm wrench        (W05-T009)   for rotary encoder nut
  1. Disconnect the power supply and antenna.

  2. Remove the top and bottom covers (2 screws on each cover).

  3. Pull the 3 knobs off the front panel.

  4. Remove the nut from the microphone jack.

  5. Remove the nut from the rotary encoder.

  6. Remove the 4 screws from the front panel assembly (1 on each side, 1 on top, and 1 on the bottom).

  7. Pull the front panel off the body of the transceiver.

  8. Remove the 3 screws from the TX-RX board (X57-3340-11 B/2).

  9. Pull the board off the body of the transceiver, but do not disconnect the wire harness.

  10. Locate D208 on the foil side of the TX-RX board (figure 1).

  11. Desolder and remove the diode.

  12. Assemble the transceiver be reversing steps 1 - 9.

This is an optional change that is not covered under warranty.
Time required to perform this modification is ½ hour.