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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@pollux.usc.edu UUCP: ...!uunet!pollux!kjh
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