Rack mounted Motorola Mitrek for link service.
INTRODUCTION ------------ The Motorola Mitrek is a great mobile radio that was never designed to be a full-duplex radio. You can, however, make a few simple modifications to get the radio to operate full duplex. My club, The Sierra Radio Association , has several 30 watt 406/420 MHz government Mitrek radios converted to the 420 MHz ham band used as full-duplex link radios. We typically run the radios at 1/2 to 2/3 rated power. So, a 30 watt transmitter is set to put out 15 watts. The Rx is quite good and the radios have given us years of reliable service. After working on many radios, we have found that at least 3 out of 4 radios will duplex just fine. But, in a bout 1 in 4 radios, the transmitter will desense the receiver. We run a very tight split of less than 5 MHz between the Rx and Tx frequency. Keep in mind the following thoughts: 1) The transmitter was not designed to be a 100% duty cycle transmitter. Make sure you keep the output power at about 1/2 rated output.
2) Keep the heatsink cool. A fan and good air flow is a good idea. 3) If you are at a commercial repeater site, make sure you check the radio with a spectrum analyzer so the transmitter is clean and use a good quality commercial pass cavity like a Motorola 1500 series duplexer. THE BASIC MODS -------------- ____ On the transmitter side of the board: Remove diode CR1.
____ On the receiver side of the board: Remove diode CR403.
You may want to snip one side of the diode and bend it up just in case you ever want to return the radio to mobile service. ____ Flip the radio upside down. Add a wire jumper between the solder pads of pins 1 and 25 of the interconnect board connector P10. This sets the PTT to be active when grounded. These pads are easy to find. They are a row of 25 evenly spaced solder pads along the edge of the circuit board. Pins 1 and 25 are the two end pads.
____ Activate Rx channel element 1. Jumper JU 611. ____ Activate Tx channel element 1. Ground pin 5 of the first Tx channel element.
RF INPUT FOR THE TRANSMITTER ---------------------------- ____ Add an external Tx RF output connector. I drill a hole on the side of the radio and mount a BNC connector just above the RX board.
Lazy man's method: Locate the coax going from the Tx out to the RF connector / TR relay already installed in the radio. Snip the coax at the back of the coax relay. You want as long a length of Tx coax as possible from the Tx output to your new Tx output connector on the side of the radio. ____ Add an external Rx RF input connector. Lazy man's method: Use the RF connector / TR relay assembly already mounted in the radio. Make sure you snip the black and red wires going from the main radio PCB to the coil of the TR relay so it will not be energized by the PTT.
Professional method: Remove the RF connector / TR relay assembly and throw it away. Mount a new connector in place of the old one. Of course, it must be round and long enough to fit the existing hole.
Make sure you use either BNC or N connectors at UHF. SMA connectors are nice but fragile and UHF (SO-239) are not really good at UHF despite their name. Coax from the external RF connectors to the Rx input and Tx output should be either small diameter rigid cable or double shielded coax. I prefer the coax because it is easier to route some people prefer the rigid because it has better isolation in high RF / desense prone radios. For lower powered radios, like the 30w model, use small diameter cable for both the Tx and Rx. This makes the installation much easier. If you install your own Rx coax... Installing your own Rx cable to the preselector (Rx input): ____ Remove bottom cover ____ Locate where Rx coax connects to the preselector on the right side next to the A+ connection of the PA.
____ Pry up the small metal cover. ____ Unsolder the connection on the left. ____ Remove the two philips screws holding the bracket. ____ Unsolder and remove the eyelet and save it for your new coax. ____ Install you new coax. EXTERNAL INTERFACE ------------------ The control head cable connector on the front provides all necessary signals to in and out of the radio. I get a fresh new cable connector and wire all signals through the new connector to my control system. You can order part #9-8016C01 (or possibly new part #0981069C01) from Motorola. Connector pin configuration --------------------------------------------- | | | 8 7 6 5 4 3 2 1 | | | | 19 18 (hole) 17 | | | | 16 15 14 13 12 11 10 9 | | | --------------------------------------------- 1 Tx Audio In 2 Tx Audio Gnd 3 Speaker high out 4 Jumper to pin 19 5 Speaker low out 6 Jumper to pin 17 7 No connection 8 No connection 9 No connection 10 No connection 11 Rx discriminator audio out 12 No connection 13 PTT in - Ground to Tx 14 Squelch pot wiper 15 Volume pot wiper 16 No connection 17 Power ground 18 No connection 19 +12v DC power Local control options - if you want to have your repeater / link radio operate like a duplex mobile... ____ Add a volume and squelch pot per the diagram. ____ Add a speaker across pins 3 & 5. Be very careful not to ground the speaker lead. The speaker leads float above ground and if you ground the lo side you will blow out the audio amplifier. ____ Add a 47 Ohm 2 watt resistor across the speaker to provide a load to the audio output circuit. I/O connector Pin 11 ----------------\ | | | | R1 R2 | | Volume (wiper) | | (wiper) Squelch Pin 15 -------->Pot1 Pot2<---------- Pin 14 | | |-------/ | GND R1 3.3k R2 10k Pot1 25k Pot2 25k THAT'S IT! Now you have a nice full-duplex Motorola Mitrek radio. Good luck and enjoy the radio. '73 de George Zafiropoulos - KJ6VU
