N6EX Receiver Mods

This page identifies all the hardware modifications necessary to adapt a Kenwood TK-931 transceiver for 902 MHz repeater receive operation. Not shown here is the effort required to program the radio for the desired operating frequency. This is accomplished using the Kenwood KPG-5D programming software and KPG-4 programming cable, or equivalent. It
N6EX Receiver Mods - schematic

also assumes that the receiver portion of the radio was properly operating in its original frequency range prior to modification. Check-out and alignment of the receiver after modification is recommended. The overall receiver modification photo shows the audio/buffer interface board installed in the front, right section of the top cavity of the receiver. This area is located near the microprocessor of the radio. The board is drilled on the two front corners to coincide with two unused holes in the radio frame. Not only is this area of the radio available for convenient mounting of the interface board, but it also provides the necessary interface points for receive audio and signal present. For the receive audio signal, an unsquelched line is preferred. The audio switching (squelch) of the TK-931 is done prior to the de-emphasis circuitry, therefore any pick-off point will have flat (relatively) audio. Two easily accessible pick-off points are available in the vicinity of the microprocessor. One point is an unfiltered buffered version of discriminator audio. There is another point that has a sharp filter to remove the PL and DPL signals below 300 Hz. The plot below shows the frequency response of the two points. While some applications may prefer having the low PL frequencies filtered-out prior to injection into the controller and transmitter, I chose to use the unfiltered point. The unfiltered discriminator point offers a...

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