This is a view of the main board on a MCX100. The modification is the same for either UHF or VHF. This modification works on "dash" mounts as well as "remote" control head versions of the MCX100 radio. The rear of the radio is towards the bottom of the page. The connection to the TNC is made using a 7 position accessory plug, 15-84193M01 and some 09-84151B01 pins to go into this plug. The seven position accessory housing costs $6.00 each and the pins come in a pack of 20 pins for $4.20. The tool that allows you to remove the pins from System 90 plugs, such as this seven pin housing is a 66-84690C02, at $1.90 each. I recommend you get one of these.
The receiver audio is obtained from the TOP OF VOLUME CONTROL, or TVC test point. The older MCX boards didnt have a TVC silkscreened onto the board, so you have solder your wire here. This closeup shows just where a piece of small wire ( red #30 wire wrap wire in this view) is soldered into the plate thru hole on the main board next to the triangle with a 10 next to it. The receive audio at this point is constant, and is squelched audio. This wire runs down towards the rear of the radio, to a cut jumper that leads to the 7 position options plug.
This view shows the transmit audio being connected to a series resistor and cap, then soldering to a pad on the main PCB. The actual connection point is pin 2 of the eight pin IC chip (pin 1 is the square pad shown in this picture). The Cap is shown laying flat, and the chip resistor is on its edge, connected to the end of the chip cap. The resistor value ranges from 100 ohms to 1K, mainly depending on the amount of drive from the TNC, and the cap is typically a .1uf chip cap. I have used 1/8 watt resistors in place of the chip resistor. One note here. The value of the cap will effect the responce curve of the transmmit audio - the lower in value the cap goes, the more it "favors" the high freq (2200hz) tone.
This view shows the relative position of where the cap and resistor combo is located.
This view shows two jumpers that are cut (JU351 and JU352), and the wires that solder into the holes that go directly to the external pins. The receive audio is present on pin 1, the transmit audio is on pin 2. The pins count in a staggered arrangement from left to right. See my page on trackers using KPC3's for information on setting the jumpers to power the TNC from the DB9 port.
This view (from left to right) shows the 7 position options connector, the two pin speaker plug, the 5 pin microphone plug and the power plug. Of course, the round "white" plug is the UHF antenna jack.