Kenwood TH-25/45AT MODIFICATIONS
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Part 1 of 4

Here are some mods for the TH-25AT and the TH-45AT. Please note that I do not
encourage transmitting on a frequency for which you do not have a license,
nor do I encourage transmitting in the United States on a non-amateur
frequency with equipment that has not been FCC type accepted.  This is
illegal in the United States, even if you have a license to transmit on the
given non-amateur frequency.
First let me recommend that you buy the service manuals for these radios. The
service manual is not expensive (about $15 I think) and it will greatly help
you in performing these mods.
Before performing any of these modifications, make sure you know what's in
your memories, because they will be lost.


Look at the schematic for your radio. In the lower left hand corner is an IC
labelled IC2. This is an ASIC microprocessor. At the lower right hand corner
of this uP are several diodes and pull-up or pull-down resistors. They are
D4, D3, R19, R18, R28, R20, R21, R22, R25, R26, and a couple of resistors
hat are not even on the schematic that attach to B2 (pin 51 on IC2) and B3
(pin 50 on IC2). The TH-45AT schematic shows R23 on the ASIC uP pin B2.
Another important resistor is R27; on the schematic it is located next to the
frequency selection switch EN1 above IC2. A less important resistor (for
modification purposes) is R36; on the schematic it is located to the right of
IC2 in line with the top edge of IC2.

The schematic for the TH-25AT shows:

                    R18-R21  R25  R26,27  R28  R36
TH-25A  M,M2   -12     O      X      X     X    X
TH-25A  M3,M4,X  -23   O      X      X     O    X
TH-25AT  K     -11     O      O      X     X    X
TH-25AT  M,M2   -12    O      X      X     X    X
TH-25E  T     -52      X      X      O     O    O
TH-25E  W     -62      X      O      O     O    O

And the schematic for the TH-45AT shows:

                    R19-R21  R22  R23  R25  R26,27  R28  R36
TH-45A  M1,M2,X  -21   O      O    O    X      X     O    X
TH-45A  M3,M4   -22    O      X    O    X      X     X    X
TH-45AT  K     -10     O      X    O    X      X     O    X
TH-45AT  M1,M2   -21   O      O    O    X      X     O    X
TH-45AT  M3,M4   -22   O      X    O    X      X     X    X
TH-45E  T     -51      X      O    X    X      O     X    O
TH-45E  W     -61      X      O    X    O      O     O    O

where O means USED, and X means NOT USED.

(Downloaded from GB7SYP)