QRZ! Ham Radio 5/QRZ Ham Radio Callsign Database - Volume 5.iso/files/tcpip/amiga/akit29m.lha/AmigaNOS/TCPIP/Digger/KENWOOD.LHA/TH25-45.AT

Text File | 1992-10-11 | 10.6 KB | 247 lines

              Mods for the TH-25AT and TH-45AT
              ================================

Part 1.

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
labelled 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 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. The whole top
   assembly should now come free quite easily. The BNC connector will stay
   attached to the back case, and the top assembly will come up over it.
9. Unfold the flexible circuit board so that the components are accessible.
   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
| 2  O      O R19|on your circuit board.
| 6  O      O R18|
|    O      O R28|
|                |D3 & D4 are two diodes in a three terminal package.
|              R |The anodes of the diodes are in common.
|           D4 6 |
|        +-------+
|        |
|        |

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 10kHz
in  out 5 and 25kHz (TH-45AT default)
out in  5 and 12.5kHz (author's preference)
in  in  10 and 20kHz

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: 1.6MHz 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 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 439.2-468.6MHz 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