In the first two segments of this series we picked a controller, and discussed the practicalities in building a repeater. It is time to get to the guts of the project. 

MAIN CONNECTOR
This first step is to remove the main connector to modify it. There are two screws on the face of the connector, and another on the top inside the cover. The pins are all numbered so this will be a piece of cake. 

In it's commercial application the Mitrek uses a variety of grounds for various reasons. We don't need but one, so it is time to consolidate things. Install jumper wires between pins 2, 6, 7, and 17 (main ground). 

Normally the PTT circuit is supplied by a secondary +12 VDC circuit. Since we aren't using that, install a jumper from pin 12 to pin 19 (main +12 VDC). Also install a jumper from pin 4 to pin 19 via a 2 amp fuse. This supplies power to the transmitter circuits. 

RECEIVER ANTENNA CONNECTION
Now we need to install a second antenna connection for the receiver. In the section behind the circuit boards are the helical resonators and the receiver input. Disconnect the coax where it enter the resonator. Mount a chassis shielded type BNC connector on the side of the case. Then run a miniature 50 ohm coax from the connector to the input point. 

If when you put your repeater on the air your experience severe spurs in the transmitter output, check the tantalum electrolytic bypass capacitors on the PA 12 volt supply rails. Also pay close attention to the driver stage bypassing. You may have to add a larger value capacitor. 

DUPLEXING THE RADIO
There are several modifications needed to set the radio up for full duplex operation. The first is to ensure the power remains on the receive crystals when the transmitter kicks in. To do this install a jumper from the emitter to the collector of Q1 on the receiver board. 

Next remove CR403 on the receiver board. This will allow the local speaker amplifier to remain in operation during transmission. You will use this on the local speaker. 

CARRIER SQUELCH MOD
We need to get a signal to the controller as to the state of the squelch circuit. Connect a 10k ohm resistor to the collector of Q406 on the receiver board. Connector a wire to the other end of this resistor and run it to the union of the pads at JU3-A and JU3-B on the front interface card. This is on the solder side of the board toward the center of the radio. 

The COS signal will now appear on pin 18 of the main connector. When a signal is present the voltage on pin 18 will be .2 VDC, and no signal equals 4.0 VDC. 

MAIN CONNECTOR PINOUTS
Here are the pinouts for the main connector after modification:
1. Mic HI
3. Speaker +
4. +12 VDC
5. Speaker -
6. Ground
11. Detector audio
13. PTT
14. Squelch (Return from squelch pot)
15. Volume (Return from volume pot)
18. Squelch sense

SQUELCH AND VOLUME CONTROLS
The detector feeds not only the controller for receive audio but also both the squelch and volume controls. Since we have done away with the original controls, we have to recreate these controls. This takes two pots of 25K ohms each, a 3.3K resistor, and a 10K resistor. 

Connect the 3.3K resistor to the detector line, pin 11, and one side of one of the pots. Ground the other side of the pot and connect the wiper to pin 14, the squelch return line. 

Proceed the same way with the other pot, substituting the 10K for the 3.3K and connecting the wiper to the volume return, pin 15. One note here: do NOT ground either speaker lead when hooking up the local speaker, as this could seriously damage the audio output of the radio. 

CONCLUSION
As I started this series I was just finishing my first conversion for the N0ZAK repeater on 145.45 in Centerville. I was so impressed with the Mitrek performance on that project that I have also converted a UHF radio for use on my 444.850 repeater in Ottumwa. And I am still impressed with the performance of both machines. The Mitrek series makes great radios for conversion to repeaters, and should make excellent packet radios also.