THE UHF MITREK AND THE KPC-9612
This is a "This is how I did it" piece and not necessarily representative of the best way to do it. I use a KPC-9612 to run 1200 and 9600 baud AT THE SAME TIME on one radio. The changes I made might not be adequate for a different type of TNC.
The UHF Mitrek is a fair performer on a 9600 baud packet circuit. I'm not as enamored of the Mitrek as some others are. It has been my experience that its receiver characteristics are substantially inferior to the Micor. It is also more difficult to add a worthwhile, low noise, preamp to the receiver input. Most available units do NOT have a preamp in them and that might be somewhat of a blessing as the stock Motorola preamp leaves some things to be desired. The use of a quadrature detector indicates that cost was a factor when it was designed. Nevertheless - it works.
TRANSMIT: One end of R514 (10 ohms), in the transmit audio amplifier, should be disconnected from the circuit board as this amplifier isn't required. The Kantronics KPC-9612 two output ports are 600 ohm ports and can put out a maximum of 4 volts p-p. This is ample for 3 to 3.5 KHz deviation. Some brands of TNCs may not have enough output for the required drive. Examine the PC track that connects to pin #4 of the transmit crystal sockets. This track will also connect to R513 and R515. Cut the PC trace at a point where the pin #4 crystal pins are disconnected from both R513 and R515. (Early release boards will require one trace cut. Late release boards will require two trace cuts.) Pin #4 of any of the transmit channel elements will be the new audio input point to feed audio FROM the TNC to the transmitter. Pin #4 is connected to the minus side of a 10 MFd tantalum cap within the channel element. The positive side of the cap is connected to the high end of the 50K ohm IDC control in the channel element. This IDC pot is "hot" with approximately +9 VDC. Set port #2 (9600) of the 9612 to DC coupled by placing jumper J6 in the 9612 on both pins. Connect a 470 ohm resistor and a .15 Mfd cap in parallel. Place this combination within the DB-9 connector that mates with port 1 of the KPC-9612. Connect it in series with the lead that would normally connect to pin #1- this is for 1200 baud pre-emphasis.
RECEIVE: The detected audio output line to the volume and squelch controls has a level of approximately 640 millivolts RMS. This level can be increased by changing R231 to 2,200 ohms. (I get approximately 850 mv RMS with a 1200 baud, 3.5KHz deviation signal and approximately 800 mv RMS from a 9600 baud, 3.0KHz deviation signal.) High frequency roll-off occurs too low in frequency for 9600 baud operation so remove C240 and C451. A shield plate, soldered to the bottom of the board, interferes with easy access to some of these components. I unsoldered R231 and C451 from the top of the chassis. I grasped C240 with a pair of pliers and gently rocked it from side to side until its leads broke. The new R231 (2.2K) was soldered in place from the top of the board. I took detected audio from the receiver from pin #9 on the interconnect board. This point is positive with respect to ground. Both ports of the KPC-9612 have a DC path to ground so a *D.C. blocking capacitor must be inserted between pin # 9 and the input to the TNC. 9600 baud requires a frequency response flat down to 3Hz. The input impedance of the 1200 baud port is 600 ohms. A very high value of capacity is required for a blocking cap if ports 1 and 2 are simply placed in parallel. An 85MFd capacitor has an impedance of approximately 600 ohms at 3Hz. This would cause an approximate 3dB loss at 3Hz. A 350MFd cap would have an impedance of approximately 150 ohms at 3Hz. I do not simply place the 2 ports in parallel. I use a scheme that provides me with de-emphasis on the 1200 baud port and raises the total load impedance of the 2 "paralleled" ports to approximately 5,000 ohms. This brings the required capacity of the blocking capacitor down to 47MFd. Place a 6,800 ohm resistor in series with the audio that goes to pin #5 in the DB-9 connector that mates with port #1 (1200 baud) on the KPC- 9612. This resistor, plus the input capacity of the port, will approximate proper de-emphasis. This resistor also prevents the port #1 input impedance from severely loading the relatively high impedance of U403A in the Mitrek! Place a 10,000 ohm resistor in series with the line that would go to pin #2 in the DB-15 connector that mates with port 2. The input capacity of port #2 is low and the input impedance is 100,000 ohms. The resistor merely provides some isolation from the 1200 baud port. *NOTE: The 9600 baud modem that is used in Pac-Comm and MFJ TNCs may not work very well with this D.C. blocking capacitor installed and they do not require it as the input of these modems is already A.C. coupled.
Radiation from the transmit and receive crystal oscillators can be reduced (somewhat) by soldering a .1 Mfd ceramic cap between pins 1 and 3 of the transmit channel element socket and the receive channel element socket. The receiver crystal oscillator of a Mitrek operating on 445.975 MHz will completely obliterate a nearby VHF packet receiver operating on 145.09 MHz!
I drilled a hole in the right side of the Mitrek, very close to the front. I mounted a 5 pin, female DIN jack on a small bracket outside the cabinet over the drilled hole. Shielded audio leads and a push-to- talk lead were run from appropriate points within the Mitrek to the DIN jack. The cable from the TNC plugs into the DIN jack.
Installation of a low noise preamp presents some problems. The coax from the antenna relay to the receiver input is hard soldered at the receiver end on early release units while late release units have jacks. What has worked for me is to cut the receiver coax at a point about half its length where it runs along the top, left side of the case. RCA type phono plugs are then installed on each of the cut ends. The WELL SHIELDED preamp is then installed by use of brackets to some of
the holes which the PL board was originally mounted to. Mount the preamp so that the RCA phono plugs can be plugged into the input and output of the preamp. If your unit has a jack at the input of the receiver you may not want to cut your coax. B+ can be obtained at pin #8 on J3 on the interconnect board.
Verne, 26 December, 1995. W9ZGS @ KB9KKN.#NEIL.IL.USA.NOAM