TH79

Original from N0NEJ to W0XK

#: 288217 S8/Equipment/Antennas
    23-Jul-94  23:43:01
Sb: #Kwd TH-79A MARS/CAP Mod
Fm: Mike Musick/N0QBF 71301,3106
 
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.
 
If I read the schematic right, the green wire corresponds to "W301", which
also serves the same purpose as removing "D308". More research needs to be
done on other mods; I'm especially interested in extending UHF receive beyond
450.
 
There is a row of diodes right below the uC which appear to be D302 through
D308, the configuration straps. HOWEVER - I need to caution anybody thinking
about playing with the diode straps that it will not be a casual operation.
For one thing, the diodes are SMD "603" style - extra, extra tiny - and the
plastic board hold-downs molded into the front case are right on the landings
for most of the diodes. This means that the board will have to be removed from
the case (a very involved procedure), and working with the tiny diodes *will*
require magnification tools as well as the correct micro-tip iron. It is not a
job for the inexperienced or all-thumbs.
 
Also, as noted previously, the non-US versions of the TH-79 have an 800 MHz
receive section. All RF components of this section are not present in the US
version. Frankly, it would easier to import a gray-market version of the radio
than it would be to attempt restoring this coverage in the US with parts.