Duplexing a VHF Mitrek

Conversion of Motorola VHF Mitrek Mobile to Repeater Service
This page was developed for Peter Harrison for internet publishing at repeater-builder.com

Modification by: Peter Harrison, AA1PL
January 31 1999

The Mitrek mobile is a good repeater candidate for a few reasons:
1. It has a reputation for being very reliable with excellent receiver filtering and a rugged transmitter.
2. After studying the schematics, I determined that it would be easy to make full duplex.
3. The most common VHF version is designed to go down to 146 MHz.
4. All controller connections can be made through the main control plug/cable.

I am a ham who also works full time as a two-way radio systems technician. I have been a big fan of the Mitrek for all of my 13 years in the business.

First determine that the Mitrek is fully operational prior to the conversion. This is just plain common sense. You will need a Mitrek service manual (Motorola part # 68P1045E70 for VHF) which is readily available at many 2-way radio shops. I first put the 2 meter channel elements in the radio prior to the conversion and tuned the receiver and transmitter according to the book. Do not touch the IF section unless you suspect it needs tuning.

The steps for making it full duplex are as follows:

1. The receiver supply voltages must be wired on all the time.
a. Remove transistor Q1 and resistor R3.
b. Install a small wire jumper across the solder pads where Q1's emitter and collector leads were.

2. Receiver audio mute during transmit must be disabled. Do this by removing diode CR2 on the interconnect board.

3. Feed the receiver's CSQ/PL detect point (COR) to the control plug J1.
a. Install a jumper from receiver test point E to J1 pin 18 (on the interconnect board.)
b. Remove jumper JU15 on the interconnect board.

4. Install a second coaxial jack for the receiver. Do this by carefully mounting a chassis mount S0-239 coaxial jack on the side wall of the radio chassis near the receiver section. I only found one place it would fit near L107 but be extremely careful drilling the holes and make sure no metal filings get into the circuits by covering them up as best as possible during the drilling. Then cut the white coaxial cable which goes from the antenna relay jack to the receiver front end and wire the receiver to the new jack.

6. Configure the channel element jumpers to fixed F1 select. Install a jumper JU611 near the RX channel element.

7. Re test the transmitter and receiver to make sure that nothing has gone wrong. Terminate the original antenna jack (now TX out) to a dummy load. You should be able to turn the squelch during transmit and now hear the white noise. The receiver should receive normally during transmit.

REPEATER NOTES: Physical construction may vary. You could mount the control head in your repeater cabinet and splice in your controller connections to the wires behind the head. I chose to cut the control cable to the needed length and tied into a long terminal strip. The control head can then be tied into the terminal strip along with the repeater controller, 12 volt power supply, etc. This is neater and easier to service. Cooling of the RF power amp is critical. Remove the silver shield covering the PA and cut the top cover of the radio so that a 3 to 4 inch cooling fan can be mounted to continuously blow onto the PA output transistors. Mount another fan to blow on the back side of the PA heatsink. I recommend setting the power to no more than 75% of its rating. A good quality band pass, band reject duplexer with at least 90 dB isolation should be used along with quality shielded cables. The Mitrek's PL board can left in or removed depending upon if PL is needed or not. If the Controller is to generate the PL tone, then the PL feed point (J3 pin 3 on the interconnect board) needs to be routed to an unused line on the control cable such as a frequency select line F2, 3 or 4.

Read the particulars on 146.76 in Scituate, Rhode Island.
See photos of the 146.76 repeater in Scituate, RI.

Peter Harrison, AA1PL
24 Forest Glen Drive
Hope Valley, RI 02832

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