

Kenwood TH79 Dual Bander Modification and Feature Guide  r1.5  9/13/94
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Contents of This Document
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I.    Disclaimer and other foolery
II.   Brief Description of the Kenwood TH79A Dual Band Handheld Radio
III.  MARS/CAP Modification
IV.   Extended TX/RX Modification ('Beyond MARS')
V.    Remote Base Operation
VI.   Post Modification Specification Chart
VII.  Cross Band Repeat
VIII. Other Stuff

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III. MARS/CAP Modification provided by Mike Musick, N0QBF
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Here is the MARS/CAP modification for the new Kenwood TH-79A
dual-band HT. Usual disclaimers about doing mods apply.

1. Remove battery.

2. Open case by first removing 3 screws (#00 Phillips) on back
and 1 screw on side hidden underneath the rubber flap covering
the DC power jack. Gently pry apart starting at top; BNC
connector will move with back, knobs with front.

3. Spread unit out. Wires will "hinge" on the side with the PTT
button.

4. Looking on the back of the keypad (front) half, locate green
wire on upper left, just below the CTCSS decoder module.

5. Neatly cut this green wire, and tuck back in so ends won't
touch each other or other components.

6. Reassemble.

Programmed memory will be lost since the CPU will notice the
strap change and reset itself.

End result -

   Before: VHF RX - 118.000-173.995  TX - 144.000-147.995
           UHF RX - 438.000-449.995  TX - 438.000-449.995

   After:  VHF RX - 118.000-173.995  TX - 142.000-151.995
           UHF RX - 420.000-449.995  TX - 420.000-449.995

Automatic repeater offset and shift selections remain normal.

And this tip comes from Duane Voth, KC5BGV...

"The MARS/CAP mod can be made without opening the entire case up.  You 
can fairly easily cut the green wire by simply (and carefully) removing 
the CTCSS decoder cover and using a small pair of diagonal wire cutters."
  
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IV. "Wide Band" Modification, provided by Cole Cunningham, AA7RD
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Following is a modification for the Kenwood TH-79A Dual Band HT which
opens it up to tuning from 67-174 MHz and 400-511 MHz.  Actual receive was
85-135.995 AM and 136-179.995 FM with reasonable (0.5 microvolt
or better) sensitivity at all the points I measured.  Transmit was 
136-179.995 and 400-511 with power dropping off at the ends but still
usable (about 0.5w on the high scale at the band edges).

As usual, I take no responsibility for the mods, or your use of the unit
after mod.  Please exercise good judgement and don't endanger our ham bands
with your irresponsible actions.

The observations noted are all I tried and I have no further data.
This is such a delicate operation that one shot was all I wanted to do,
and when it worked I quit!!!

WARNING-CAUTION-LOOKOUT-ETC!!!
The diodes to be moved are microscopic!!  About 1mm long by 0.5mm wide.
This mod should be done by professionals on a closed track only.
Children, do not attempt this at home.  Requires extremely small soldering
tools and MUCH skill.  I had a professional surface mount facility do mine...


1. Open the unit by removing three screws from the back and one under the 
   DC power input cover.  Carefully separate the front and back pieces.
   The two case halves may be disconnected by pulling the wired plug and by
   prying the brown latch on the ribbon cable connector up enough to
   release it.

2. On the front PC board in the lower right corner as viewed from the rear
   with the knobs pointing up there are seven spots for diodes in a row,
   with one diode missing.

                                    |
                     o o o o x o o  |
                    ________________|
                     2 3 4 5 6 7 8

   The numbers are D302-D308, left to right, with D306 missing.

3. Remove D304, and D307. Put them away in a safe place (Just in case...)_

4. You should now have:
                                    |
                     o o x o x x o  |
                    ________________|
                     2 3 4 5 6 7 8

5. Reassemble and good Luck.

Enjoy, but don't create problems with the new capability...Wouldn't it be
nice if Kenwood would make a RX only mod with these capabilities and we 
wouldn't have to worry about accidental transmissions where we shouldn't
be transmitting.   I, for one, don't want or need to TX, just RX...

73, Cole, AA7RD
August 11, 1994

(Note - This mod also adds 2 more menu settings. One of which is an
        AM/FM demod selector for the 300-400 Mhz band. To access this
        band, select the VHF band, press F, then press the Low/Hi key.
        Repeat to return to 144 Mhz. The AM/FM selector will allow you
        to listen to Military transmissions in the 300Mhz band in 
        either AM or FM.
        - Clayton)

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VI. Post Modification Spec Sheet, by Clayton Wagar, KD4IDN
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Here is a small chart showing the measured specifictions of TH79
S/N 60300272 after modification. The measurements were made with an
IFR 1200S Service Monitor, on the bench, with a 6V power supply.
Your milage may vary - these are just what happened with mine, and
I would suspect that most units are pretty similar.

73 de Clayton
KD4IDN


Band - VHF -----------------------------------------------------
Frequency       Sens (uV)       Power           Mode
---------       ---------       -----           ----
108             4               -               A3
110             2               -               "
115             .75             -               "
120             .40             -               "
125             .30             -               "
130             .30             -               "
135             .30             -               "
140             .12             2.9             F3
145             .12             2.9             "
150             .13             2.8             "
155             .15             2.4             "
160             .18             1.9             "
165             .20             1.45            "
170             .21             1.10            "
175             .30             0.85            "
179.995         .35             0.70            "

 
Band - VHF (2) (set to 300Mhz) --------------------------------
Frequency       Sens (uV)       Power           Mode
---------       ---------       -----           ----
300             50/75           -               A3/F3
320             4/20            -                 "
340             .3/.6           -                 "
360             .2/.5           -                 "
380             .2/.6           -                 "
400             .2/.75          -                 "


Band - UHF -----------------------------------------------------
Frequency       Sens (uV)       Power           Mode
---------       ---------       -----           ----
400             11              1.0             F3
410             4               1.5             "
420             1               1.85            "
430             .2              2.2             "
435             .12             2.3             "
440             .10             2.5             "
445             .10             2.5             "
450             .10             2.5             "
455             .15             2.3             "
460             .16             2.3             "
465             .20             2.0             "
470             .45             1.8             "
480             1               1.6             "
490             2               1.3             "
500             6               1.1             "
510             no RX over 509  0.8             "


  BTW, close inspection of the RF deck in my radio shows no obvious
missing parts.  I didn't remove the board to check the other side, but
my suspicion is that the 800 MHz parts are installed.  I'd be interested
in input from others on that.  So far I've not discovered how to make it
display any 800 MHz frequencies though."