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Thread: Kenwood TM-271A Receive Issues

  1. #1

    Default Kenwood TM-271A Receive Issues


    I have a TM-271A. The Wideband (normal) receive sensitivity has dropped off extremely. I can't hear anything unless I am close to the transmitting station. The only way I can hear now is on Narrowband mode.

    Anyone else or ideas?

  2. #2

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    I would guess that someone keyed up with lots of power way too close to your radio and it gave your crystal filter a major headache. Try replacing the wideband crystal filter. It is labeled CF301 (part number L72-0993-05).

  3. #3

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    Hi Brandon,
    No, no one was ever close to the radio with too much power. Radio was never near anything high power.

    Quote Originally Posted by brandon lind View Post
    I would guess that someone keyed up with lots of power way too close to your radio and it gave your crystal filter a major headache. Try replacing the wideband crystal filter. It is labeled CF301 (part number L72-0993-05).

  4. #4

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    If narrowband works, the wideband problem is in either the wideband crystal filter itself, the switching diodes (D302 and D303) that switch the RF between the two filters, or something on a digital level controlling the low voltage signal that operates those switching diodes. Everything else is common to both narrow and wide suggesting everything else is functioning. I would check the diode bias voltages on D302 and D303 in both narrow and wide settings and see if they are getting the voltage necessary to bias them (which allows RF to pass through). If the voltage is there (and consistent with the diodes expected forward bias voltage), it really cant be much aside from the filters.

    EDIT: Wideband should have 0v on pin 64 and 5v on pin 65 of the processor. Vice versa for narrowband (5v on pin 64 and 0v on pin 65). Thats the very first thing I would check. If thats not there, Id try a reset on the radio. If the diode or filter was bad, I wouldnt expect it to have much effect on the processor signal (5v) as the diodes are fed using 10k resistors. That said, the presence or lack of those voltages at the processor should differentiate between a diode or filter issue vs a digital processor(ing) issue.
    Last edited by brandon lind; Sat 4th Jan 2020 at 04:29. Reason: simplifying procedure

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