Modifications for the Kenwood TM-2550

TM-2530/2550/2570 MARS/CAP

The TM-2500 series radios may be easily modified for use on MARS/CAP frequencies from 141-150.995 MHz.

Locate the control unit (X54-1860-XX) on the bottom of the radio. This is the same board that the TU-7, and or MU-1 are mounted on. Just to the left of IC3 you will find a row of diodes. Cut diodes D8, and D11. After the diodes have been cut you will have to "reset" the microprocessor.
Microprocessor Reset.
  1. Press and hold the [PS] key.
  2. Turn ON the power switch.
  3. Release the [PS] key.
All of your programmed data such as memory channel data, DCL data, and Phone number data will be erased and you will have to reprogram them. If the radio is not reset, it will not recognize that you have cut the two diodes.
TM-2550 DCL Call Sign Data

Some owners of the TM-2530/50/70 have reported a loss of the DCL system call sign data when the power switch is turned off. The call sign data actually remains in memory, but timing prevents its utilization in some radios. Adding a resistor will correct this problem.

If you receive a radio that exhibits this symptom add a 47K ohm resistor to IC3 to the Slave CPU on the control unit (X53-1440-XX) between pins 4 and 20. Cover the resistor leads with "spaghetti" type insulation. Adding the resistor to radios that do not have this symptom will not cause any problems, so you may wish to add the resistor if you receive a radio for service for some other problem.

Before soldering unplug the DC power cable and antenna from the radio.
When soldering to this IC you must use a soldering iron that provides voltage isolation on the tip since IC3 is a CMOS device and can be easily damaged by static electricity or voltage on your iron. Use normal CMOS precautions. One final precaution: Don't bridge any of the pins on the IC with excess solder!

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