N6OCS
Receiver
The receiver started life as a 800 MHz Motorola Maxtrac D35 or D45 with the 16 pin accessory connector. The 800 version was chosen because they are simple to modify, less expensive than the 900 version, and why waste a good 900 radio. Modifying this radio is pretty straight forward.
First, the radio is checked out on the bench to make sure that it will perform to spec BEFORE being modified. One of the things to check is the small fuse for the Ignition sense line on pin 10. Check for continuity between pin 10 and the main power supply connector on the back of the PA. This will be the new power supply for the receiver.
The PA is removed and discarded. This makes the radio smaller but also gets rid of the mini-UHF connector and any power problems that might be associated with the PA. Next the receive connector and first filter are removed and discarded. Since a large amount of external receive filtering will be used the first filter is unnecessary. I opted to replace the second filter with a 915 MHz filter. A small pigtail is made up with a female SMA bulkhead connector and soldered onto the board where the first filter’s output was formerly connected. I cut a small flat washer in half to help with chassis irregularities and installed the SMA through an existing hole in the rear of the chassis. This makes a much better connection for the receiver than the original mini-uhf connector.
The other modification to the RF board requires disabling the VCO range-switching transistor, Q205. This will keep the radio in “talk-around” mode even while in receive. In “talk-around” the receiver will receive 902 MHz quite well. A simple solder bridge across the base and emitter does the trick and is reversible.
The last task was to make a clean mounting bracket to meld the Maxtrac into the Nucleus chassis. Since the Maxtrac does not use much power, it is powered from the Nuc's 12v supply.
Below is the PHEMT LNA used on the receive side after the filter. Thank You! Chip Angle
11-25-07
I am currently working on a Spectra Tac (aka Micor) receiver to replace the Maxtrac in hopes of better selectivity. The older Spectra Tac receiver has huge, tunable, helica resonators that I'm hoping will improve receive performance at the site.
