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Subject: TH-45 diodes



        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 10   kHz

        in      out      5 and 25   kHz (TH-45AT default)

        out     in       5 and 12.5 kHz (author's preference)

        in      in      10 and 20   kHz



        R23 probably has no effect on the TH-25AT.  I never checked.

        I was quite satisfied with the 5 and 10 kHz 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.



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:     1.6 MHz offset (perhaps for 220, or Europe)

        removed:       5   MHz 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 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.

(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 439.2-468.6 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.



---------------------------------------------------------------------



In the rare case that original ideas   Kenneth J. Hendrickson    N8DGN

are found here, I am responsible.      Owen W328, E. Lansing, MI 48825

Internet:kjh
SOURCE: The QRZ Windows Ham Radio CDROM



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