Kenwood HT-45A/25AT

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/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.

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. Some unsoldering of
obvious grounding wires may be necessary. You will see where the
flexible circuit board plugs into a socket on the main circuit board.
Before unplugging it, make sure you know what's in the memories, because
they will be lost. Unplug 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:

+----------------+
| R R R D3 R R |
| 2 2 2 7 2 |
| 5 3 2 1 |
| |
| R O O R20| The O's are solder pads.
| 2 O O R19|
| 6 O O R18|
| O O R28|
| |
| R |
| D4 6 |
| +-------+
| |
| |

The fold out board is actually square, but with only characters for
graphics, I couldn't draw it that way.

On both radios, R36 is for the European tone burst to "whistle up"
repeaters.

On both radios, D4 is for selecting the type of display. With D4 in,
the display is normal. With D4 removed, the display is a channel
display.

D3 is for selecting VHF or UHF. With D4 in, the radio thinks its a VHF
radio. With D4 removed, the radio thinks its a UHF radio. Don't change
this on your radio.

On the TH-25AT:
(All frequencies given in MHz.)

R22 in
R28 out
This is how the radio is delivered in the USA. TX 144-148, RX 141-163
(I think).

R22 out
R28 in
The radio tunes from 142-151. This may be the modification given
to US MARS members. I don't remember where the unit will transmit. It
may or may not transmit outside the range from 144-148.

R22 in
R28 in
The radio tunes only from 144-148.

R22 out
R28 out
Frequencies may be selected from 100-200 MHz (on the display only - your
PLL will not lock up in this entire range). In addition, TX is possible
where your PLL locks up.

R25 out
Removing R25 disables automatic offset selection.

R23 and R24 are used for selecting the step size for tuning. I can't
remember which positions are for which step sizes, and alas I didn't
write down what I found. If you want to play with this, go ahead.

On the TH-45AT:
(All frequencies are given in MHz.)

R18 in
R28 in
This is how the radio is delivered in the USA. The radio covers
438-450 MHz.

R18 in
R20 out
The radio is prohibited from tuning outside 440-450 MHz.

R18 out
R28 in
The radio will only tune from 215-230 MHz. Note that the PLL would not
lock up! (What did you expect?) Could it be possible that Kenwood
originally planned a 220 version of this radio, but then scrapped their
plans?

R18 out
R28 out
The radio will tune from 200-500 MHz (on the display only - your PLL
will not lock up over this entire range). Transmitting is possible
anywhere your PLL will lock up.

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.