Improving the MICOR Receiver for 435-450 MHz

This is a detailed explanation of a procedure you can use to modify a UHF MICOR `M` range (450-470 MHz) receiver board so it operates adequately in the 435-450 MHz range. This is an enhancement to Kevin`s text-only article. These receivers are commonly found in a Spectra TAC chassis as well as base stations, mobiles, and satellite receivers. They

will meet specs down to about 445 MHz; then things start deteriorating. While the receiver will, in general, perform adequately down to nearly 440 MHz, the crystal oscillator multiplier stages seem to run out of tuning range around 445 MHz. This condition can be fixed by replacing some components with values appropriate for the "L" range boards. You can do this one of two ways: Kevin`s original article only mentioned replacing C117, C119, and C125. An e-mail received by Repeater-Builder from an ex-Motorola Service Shop technician contained a parts list for an export model MICOR that covered 420-450 MHz. This conversion included replacing several coils (L111 through L116) with parts that are no longer available. Information about C136 came from this source. Sensitivity with C136 padded is marginally better but you`d need sophisticated instruments to see it. Here`s a table showing the capacitor reference number, the original value for the "M" range (450-470 MHz), the value you`d need to replace it with (the "L" range), or the value you`d need to add across the existing component to get it to the "L" range. All values are in picoFarads.

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