Date: 10 Jun 89 19:41:32 GMT

Subject: Kenwood HT-45A mods?    ( and TH-25AT )



Ok, I've promised it for a long time and now it's time to live up to my

promises.  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 on a non-amateur

frequency without FCC type accepted equipment (in the United States).

Lament:when are the Japanese going to produce a 440 FM HT for the US

market that has receive coverage outside the US amateur band?  Lord

knows that there's a market for it - there's MONEY to be made.



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 modifications.



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.



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.  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.



I may have some more information on these radios someplace.  If I can

find it, I will add to this posting, and post it again with the updated

information.  Enjoy.



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

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

Internet:hendrick




[ Editor Note: With my radio, I had to remove the following resistors

to enable full rx/tx: R19, R20, R21, R22, R28   de VE3PZR ]