Modifications for the Kenwood TH-75

Modifications for the Kenwood TH-75
TX across entire tuning range TH-75A

I have a couple of mods that you may or may not have. For the TH-75A MARS/CAP mod, removing D4 will allow TX across entire tuning range, except that actual TX range will be limited by tuning of the PLL.

For the TM-731A, there is a "Beyond MARS/CAP transmit Modification". It provides TX from 136 to 174 and 400 to 500 MHz. Again, actual TX range limited by PLL tuning. If you don't already have this and would like it, I'll send a copy if you furnish me your address. It involves cutting R25 on the control unit, and installing a jumper (diagram required to locate position), then two resets in a row.

I have tested these mods on my own rigs.
73 - Kevin, KD2SL @ KC3BQ
9600 baud with the TH 75

I tested the follwing modification only for the 430 MHz part. Perhaps it works also on 2 meters if you try the modification on the 144 MHz IF-Unit.

For opening the TH 75 follow the instructions of the manual page 31 ( installation of the TSU-Unit ).
After opening the handy, it is not necessary to disconnect any connectors between the front and rear part. All connections have to done at the rear part.

-TX-

On the right side of the open handy is a connector (No. 201). The wires of this connector are leading to the lower board of the rear Part.
Disconnect this connector. Next remove the five screws holding the upper board. Now it is possible to lift this board for about 5 mm.
On the lower side of the board are the solder pins of the connector which was opened before. Solder a thin wire to pin No 5. (Start counting at the antenna side of the TH 75).
This wire can be connected direct to the TX-Exit of your TNC, if this exit has A/C level only. If the TNC has also D/C level add a capacitor of about 4.5 to 6.8 uF.

-RX-

A little bit higher to the connector were the TX line was connected is the UHF - IF unit (X58-3610-00). It is a small additional board over the main board. You will find the IC TK10485M on this small board. Solder a wire to pin No 11, the first one the left side (Start counting again at the antenna Side). This pin is connected to a capacitor. In order to prevent the IC from being demaged, I recomment to do the soldering at this capacitor.
This wire can be connected direct to the TNC.
If you need for some reason the squelch of the TH 75 during 9K6 operation, add a resistor of 10 KOhms in the RX line.

These are all modifications necessary for 9K6, and now you can start closing the TH 75. If you do not instaubber cap of the DC IN Terminal, you have a fine little gap for the wires. But it is also possible to install an additional jack if the TSU-6 unit is not installed. The ground connection can be made via the speaker jack. The PTT line of the TNC should be connected to the MIC jack.
The TH 75 is able to work with a TX-Delay of 150 - 170 ms (> T15) which is a good value for a modified PLL TRX.
TH-75A MARS/CAP modification

This modification will allow the TH-75A to transmit from 142 to 152 MHz and 420 to 450 MHz. Specifications are guaranteed for the Amateur Bands only. Through the transceiver will display 136 to 174 MHz and 335 to 512 MHz, the PLL circuit may not lock through the entire range.

Modification procedure

Disconnect the battery and antenna.
Remove the three case screws and two battery plate screws.
Lift the front panel from the body of the transceiver, but do not disconnect the two flex cables.
Cut the green jumper wire (W1) that is located to the left side of the CPU in the front panel assembly.
Assemble the transceiver by reversing steps 1-3.
Reset the CPU by holding the M key as the power is turned on.

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