TH-25/45AT MODIFICATIONS
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Part 3 of 4

On both radios, R27 is located to the right of the four blue buttons on the
back of the radio (M CTCSS SHIFT REV). You should be able to see this on the
back of the now removed top assembly.

On both radios, R36 is for the European tone burst to "whistle up" repeaters.
R36 is located on another portion of the flexible circuit board, closest to
pin 1 of IC2. IC2 is the IC closest to the portion of the flexible circuit
board shown above. Most users will not want to change R36.

Here is the function of the diodes and pull up or pull down resistors (for
both the TH-25AT and the TH-45AT unless otherwise specified):

D4 selects the display type.
   installed: channel display
   removed: normal frequency display
You probably don't want to install this.

R6 is the pullup resistor for D4. Do not remove it, so that if D4 is removed,
pin 44 of IC2 will be properly pulled up.

D3 selects VHF or UHF.
   installed: VHF
   removed: UHF
You definitely don't want to change this!

R7 is the pullup resistor for D3. Do not remove it, so that if D3 is removed,
pin 45 of IC2 will be properly pulled up.

R25 selects automatic offset selection.
    installed: automatic offset selection enabled.
    removed: automatic offset selection disabled.
R25 seems to have no effect on the TH-45AT.

R23,27 frequency selection step size.
Frequency selection step sizes is only known for TH-45AT.
R23,R27 frequency selection step sizes:
out,out 5 and 10KHz
in,out  5 and 25KHz (TH-45AT default)
out,in  5 and 12.5KHz (author's preference)
in,in   10 and 20KHz

R23 probably has no effect on the TH-25AT. I never checked.
I was quite satisfied with the 5 and 10KHz step sizes for the TH-25AT. I
never tried changing R27 to see what would happen.

R24? something with the CTCSS tone ?
     installed: When a TX offset is selected, the CTCSS tone is always
                transmitted. The TONE button does not work correctly. When
                the TONE button is depressed, the display goes to 430.000 c,
                and nothing seems to work. (A "c" is displayed in the channel
                number position.) The action of the TH-25AT is unknown.
     removed:   CTCSS tone selection works correctly. Both the TH-25AT and
                the TH-45AT are delivered this way.
You probably don't want to install this.

R19 function unknown.
R19 is not installed in the European version.

R20 function unknown.
R20 is not installed in the European version.

R21 function unknown.
R21 is not installed in the European version.

R19, R20, and R21 are all in parallel. Thus, if any of them is installed, it
is like all of them are installed.

R26 function unknown.
R26 is installed in the European version.

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