Kenwood TH-25/45AT MODIFICATIONS
           ================================

Part 4 of 4

On the TH-25AT:

R22,28 select RX and TX frequency range.

R22,R28  RX range TX range
~~~~~~~  ~~~~~~~~ ~~~~~~~~
in out   144-148  141-163(USA default)
in in    144-148  144-148
out out  100-200  100-200(only where your PLL locks up.)
out in   142-151  ???-???

R18 exact function unknown.
    installed: as delivered in USA.
    removed:   TX disabled from 146-148.

The RX and TX frequency range based on R22 and R28 are unknown if R18 is
removed.

On the TH-45AT:

R18,28 select RX and TX frequency range.

R18,R28  RX range TX range
~~~~~~~  ~~~~~~~~ ~~~~~~~~
in out   440-450  440-450
in in    438-450  438-450(USA default)
out out  200-500  200-500(only where your PLL locks up.)
out in   215-230  215-230?(PLL won't lock up.)

R22 TX offset selection
    installed: 1.6MHz offset (perhaps for 220, or Europe)
    removed:   5MHz offset (USA default)

PLL adjustment
~~~~~~~~~~~~~~
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.

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 1MHz 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. (Be very careful when you adjust TC1.  If you are not careful, you
could break TC1 loose. This will cause your radio to have microphonics.
Mechanical vibrations will change the value of this capacitor, and thus
change the tuning on the PLL.  If you encounter this problem, you can repair
it by dripping wax through the small hole onto TC1, or by replacing TC1. It
is of course better not to break it in the first place.)
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
439.2-468.6MHz 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.

In the rare case that original ideas are found here, I am responsible.

Kenneth J. Hendrickson N8DGN, Owen W328, E. Lansing, MI 48825

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