IC-746 Receive Drops on Transmit

Discussion in 'Radio Circuits, Repair & Performance' started by W4ZZK, Apr 2, 2012.

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  1. W4ZZK

    W4ZZK Ham Member

    Wondering what is going on with my Icom IC-746 (Non-Pro)
    After turning on radio, it receives fine, but after tuning up or even keying the mic, the receive is gone.
    Any suggestions?

    Josh W4ZZK
     
  2. W4ZZK

    W4ZZK Ham Member

    Now no TX on this rig!!
     
  3. KO6WB

    KO6WB Premium Subscriber

    Josh, did the receive and transmit disappear at the same time?
    Could be any number of things that have gone wrong. Have you checked the circuitry fuse inside the IC-746? It's on page 81 of the manual.
    Do you have any other devices hooked up to the IC-746? If so, disconnect them and check their operation. I believe the IC-746 has an automatic shutdown if the input voltage goes high.
    Does anything work on the rig, frequency display, S-meter, band select, volume control, etc?
    Get back to us with these.
    Hope this helps
    73
    Gary
     
  4. KO6WB

    KO6WB Premium Subscriber

  5. W4ZZK

    W4ZZK Ham Member

    Gary, the receive went first... and it was intermittent to begin with. That scenario was where I could turn the radio on, and receive would do fine.
    Then, I'd key up or use the tuner, the receive would be gone after un-keying. At that point it was still passing RF.
    I have another rig (Ten-tec Omni V) in the shack and had been using it on the air. We'd start smelling that 'uh oh' smell when we'd tune up the other rig! After sniffing around, the smell was coming from the IC746! Soon after, it would not pass RF on any band, even on the 2m side!

    Yes, fuse is good. No devices connected.
    Display, S-Meter, Band Select, Volume Control.... everything such as that works fine!
     
  6. KE3WD

    KE3WD Ham Member

    Sounds like it may be due to dirty T/R relay contacts. or filter relay contacts...
     
  7. W4ZZK

    W4ZZK Ham Member

    How would this possibility yield the electronic smell?
     
  8. KO6WB

    KO6WB Premium Subscriber

    The total loss of transmit has usually been attributed to the IC151 problem. This has happened a lot. I am still kind of at a loss about the receive since the display seems to be normal and at least that part of your rig thinks it is working okay. You may have lost an internal voltage regulator or some reference voltage some where. A distinct smell emitted by the rig is a valuable clue that something didn't like having electrons pass through it. Start taking measurements of your voltages as a starting point. Hopefully you didn't lose something that is unobtainium.
    73
    Gary
     
  9. KO6WB

    KO6WB Premium Subscriber

    Josh, what kinds of test equipment do you have or can borrow?
    For troubleshooting the receive problem a signal generator would be very handy. You would do the half split method. That would be to inject a signal at the 9.0115Mhz crystal filter and see if the signal is detected. If not then you go down the receive chain until you do get a signal. If you do get a signal then you go up the receive chain until the signal disappears.
    For the transmit side an oscilloscope is a very good item. Doing the same half split method you would check to see if you have a signal being generated at the output of the 9.0115Mhz crystal filter (use CW for this test). If you have a signal there then go up the transmit chain until it goes away. If you don't have a signal then go back down the transmit chain until it shows up.
    Hope this helps
    73
    Gary
     
  10. K0RGR

    K0RGR Premium Subscriber

    You were transmitting on one radio, and it caused the other radio to smoke?

    I'd check immediately to see if I somehow cabled the RF out from the Ten Tec into the 746! Next, I'd be sure that the antenna outside wasn't shorted to whatever is on the 746.

    It could just be a coincidence and one or more components in the 746 decided to release their smoke. If there is a common power supply, I'd check that, too, and make sure I wasn't somehow inducing voltage spikes when transmitting on the Ten Tec - maybe RF back into the power supply?
     
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