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THX5AT.MOD
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1989-12-04
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315 lines
---------------------------------------------------------------------
Copyright (C) Notice: Some people have been taking
information posted to this network, publishing it, and selling it.
Not only is this immoral, it is also illegal. You may not do this.
I give everybody the right to copy this material, and give it to
others. You must not sell it. You may NOT include this information
in a newsletter or compendium that you charge for, but you may incude
it in a free newsletter or compendium. You may NOT distribute this
information on a computer bulletin board system that users are
charged any fees for, but you may distribute this information on a
computer bulletin board system if you provide the service free of
charge. You must not alter the information provided here in any way,
including removing this copyright notice and/or removing the authors'
name and/or e-mail address at the end of this message.
---------------------------------------------------------------------
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 in the
United States on a non-amateur frequency with equipment that has not
been FCC type accepted. This is illegal in the United States, even if
you have a license to transmit on the given non-amateur frequency.
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 mods.
Before performing any of these modifications, make sure you
know what's in your memories, because they will be lost.
---------------------------------------------------------------------
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 or
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. Another
important resistor is R27; on the schematic it is located next to the
frequency selection switch EN1 above IC2. A less important resistor
(for modification purposes) is R36; on the schematic it is located to
the right of IC2 in line with the top edge of IC2.
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.
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.
---------------------------------------------------------------------
(1) Only 8 screws must be removed from the case to take the radio
apart; these are the 4 screws that hold the battery clip on, the 2
screws on the back half of the case near the PTT switch, the screw on
the back of the case under (and in line with) the BNC connector, and
the screw on the back half of the case adjacent to the external
speaker jack.
(2) After removing these 8 screws, remove the front half of the
case from the back half. You may unconnect the blue plug from the
touch tone circuitry on the front half of the case to make further
work on the radio more convenient, but it is not necessary.
(3) Place the radio back side down with the antenna away from
you. Now, unsolder the two grounding wires from the plate which
covers much of the radio's circuitry. These wires are near the top
of the plate and will be quite obvious.
(4) Next, remove the 7 screws that hold this plate on. 5 of the
screws for this plate are up on top and are quite obvious; the other
2 are at the bottom of the plate, and may be hidden by your CTCSS
unit. Remove the plate.
(5) Remove the red plug with about 7 wires that connects just
under the volume and squelch switch.
(6) Remove the screw that holds the PTT/LAMP/MONI switch
assembly; this screw is on the left side of the radio just under the
PTT switch.
(7) Remove the two screws that hold the top assembly to the back
case of the radio; these two screws are on either side of the orange
flexible circuit board coming from the top assembly and plugging into
the back case of the radio. You may need to use a screwdriver or
stick to move the orange flexible circuit board out of the way in
order to see these final two screws.
(8) Unplug the orange flexible circuit board from the back case.
At this point, the whole top assembly should come free quite easily.
The BNC connector will stay attached to the back case, and the whole
top assembly will come up over it.
(9) Unfold 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: (the component side is shown up)
+----------------+
| R R R R D3 R R |
| 2 2 2 2 7 2 | The O's are very large solder pads.
| 5 4 3 2 1 |
| |
| R O O R20| Not all components will be installed on your
| 2 O O R19| circuit board.
| 6 O O R18|
| O O R28|
| | D3 & D4 are two diodes in a three terminal
| R | package. The anodes of the diodes are in
| D4 6 | common.
| +-------+
| |
| |
The fold out board is actually square, but with only characters for
graphics, I couldn't draw it that way. It comes out reasonably
square on a printer with 10 characters/inch and 6 lines/inch.
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 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.
---------------------------------------------------------------------
Kenneth J. Hendrickson N8DGN