Modifications for the Kenwood TH-45

KENWOOD TH-25/45 out of band modifications>

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 on a non-amateur frequency without FCC type accepted equipment (in the United States).

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 modifications.

Look at the schematic for your radio. In the lower left hand corner is an IC labled IC2. This is an ASIC microprocessor. At the lower right hand corner of this uP are several diodes and pull-up/pull-down resistors. They are D4, D3, R19, R18, R28, R20, R21, R22, R25, R26, and a couple of resistors that 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.

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.
On the TH-25AT:
(All frequencies given in MHz.)

R22 in
R28 out
This is how the radio is delivered in the USA. TX 144-1, 141-163
(I think).

R22 out
R28 in
The radio tunes from 142-151. This may be the modification given to US MARS members. I don't remember where the unit will transmit. It may or may not transmit outside the range from 144-148.

R22 in
R28 in
The radio tunes only from 144-148.

R22 out
R28 out
Frequencies may be selected from 100-200 MHz (on the display only - your PLL will not lock up in this entire range). In addition, TX is possible where your PLL locks up.

R25 out
Removing R25 disables automatic offset selection.

R23 and R24 are used for selecting the step size for tuning. I can't remember which positions are for which step sizes, and alas I didn't write down what I found. If you want to play with this, go ahead.

On the TH-45AT:
(All frequencies are given in MHz.)
R18 in
R28 in
This is how the radio is delivered in the USA. The radio covers 438-450 MHz.

R18 in
R20 out
The radio is prohibited from tuning outside 440-450 MHz.

R18 out
R28 in
The radio will only tune from 215-230 MHz. Note that the PLL would not lock up! (What did you expect?) Could it be possible that Kenwood originally planned a 220 version of this radio, but then scrapped their plans?

R18 out
R28 out
The radio will tune from 200-500 MHz (on the display only - your PLL will not lock up over this entire range). Transmitting is possible anywhere your PLL will lock up.

Some of the above codes are:
K     USA
T     England
X     Australia
M     Other Areas
These components are found on the flexible circuit board under the display. To get to them, take the radio apart. Some unsoldering of obvious grounding wires may be necessary. You will see where the flexible circuit board plugs into a socket on the main circuit board.
Before unplugging it, make sure you know what's in the memories, because they will be lost. Unplug the flexible circuit board and unfold it so that the components are accesable. One of the fold-out parts of the flexible board will look something like this:

! R   R R D3 R R !
! 2   2 2    7 2 !
! 5   3 2      1 !
!                !
! R  O      O R20!            The O's are solder pads.
! 2  O      O R19!
! 6  O      O R18!
!    O      O R28!
!                !
!              R !
!           D4 6 !
!        +-------+
!        !
!        !
The fold out board is actually square, but with only characters for graphics, I couldn't draw it that way.
On both radios, R36 is for the European tone burst to "whistle up" repeaters.
On both radios, D4 is for selecting the type of display. With D4 in, the display is normal. With D4 removed, the display is a channel display.
D3 is for selecting VHF or UHF. With D4 in, the radio thinks its a VHF radio. With D4 removed, the radio thinks its a UHF radio. Don't change this on your radio.

I have found a quick and easy way to retune your PLL (in the TH45-AT) with a minimum of test equipment. All you need is a scope and a small tuning tool. First, take off the battery pack holder plate. Then, remove the silvery sticker covering the tuning pot access holes. If the radio is positioned on its back, with the top folded over so that the touch tone pad is also facing down, the test point you want (TP1) is on the bottom half of the radio, near the center (left to right), and close to the battery; the tuning pot you want (TC1) is on the bottom, and closest to the PTT switch. Under no circumstances change the tuning of TC51. This is used to calibrate the output of the radio with the display the radio is giving; you don't want to mess with it. Once again, the Service Manual makes it very clear where these points are, if you are having trouble with my descriptions. On with retuning the PLL.
With the radio on, and receiving, monitor the voltage and the waveform on test point TC1.

Tune the radio DOWNWARDS in frequency until the PLL unlocks. Note that the radio will beep when this happens, and the waveform on TP1 will change. Tune the radio about 1 MHz higher so that the PLL locks up again, and note the voltage on the testpoint, TP1.
Now, tune the radio to the LOWEST frequency that you want to be able to receive. Adjust TC1 until the voltage on the test point TP1 is the same as what was noted earlier. Button the radio back up, and you're done. You will not be able to tune the PLL to any range you want.
There are limits. On my radio, I have been able to retune the radio so that I can recieve from 439.2-468.6 MHz with a set of batteries fresh out of the charger. The tuning range will probably diminish as the battery voltage decreases. I have not retuned the PLL on my 2m HT, but I'd imagine the same technique will prove fruitful.

I may have some more information on these radios someplace. If I can find it, I will add to this posting, and post it again with the updated information. Enjoy. Kenneth J. Hendrickson N8DGN
TH-25/45AT Automatic Power OFF Function

Some users of the TH-25AT/45AT have expressed a desire to defeat the automatic power off function. The following modification will explain how to do this. It should be noted that this modification does not effect the battery saver function.

Required Part:

Diode Kenwood Part #1SS133
  1. Disconnect the battery pack and antenna.
  2. Remove the Volume, Squelch, and Tuning Control knobs by pulling them straight up from the top panel.
  3. Using a 7mm spanner wrench, remove the nut from the volume control and the nut from the tuning control.
  4. Remove one screw located by the PTT switch.
  5. Reve one screw located by the speaker jack.
  6. Remove two screws from the battery terminal plate.
  7. Carefully pull the front panel up from the transceiver (do not break the wires connected between the front panel and the body of the transceiver.) Lay the front panel to the side of the transceiver (Keep track of the F.LOCK cover if it comes off.)
  8. Remove the PTT cover.
  9. Remove one screw from the PTT switch unit.
  10. Gently lift the top panel from the transceiver by pulling it forward and then up (the "O" ring on the BNC connector will produce some tension.)
  11. Carefully unfold the flex Printed Circuit Board (PCB) to expose IC2.
  12. Using a 45 watt (or less) soldering iron that has an isolated or grounded tip, add a diode between pins 23 and 58 of IC2.
  13. Carefully assemble the transceiver by reversing step 1-11. Pull the BNC connector "O" ring up a little before installing the top panel.
  14. Reset the microprocessor by following the procedure in the instruction manual (page 16) called CLEARING ALL MEMORY.
This is an optional change that is not covered under warranty. Time required for this modification is 1 hour or less.

Later versions of the TH-25/45AT came with a programmable defeat for the Auto power off function. Do not install this modification if your manual provides a procedure for turning the power off function off! Step 11 tells you to carefully unfold the flexible pc board. It is extremely important that you do not try and bend the board in a direction that is opposite from its current bend. To do so will break the board or the foil traces inside the board!

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