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Trc 431 issue

Discussion in 'General CB Services Discussion' started by Staybolt, Aug 11, 2019.

  1. Staybolt

    Staybolt Well-Known Member

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    Got one at a yard sale. Hooked it up today and all was well then i heard a pop and the meter light went dim and the modulation light stays on and radio doesn't transmit or receive. I had another one a few years ago do exactly the same thing. So what causes this?


     

  2. Chevboy0167

    Chevboy0167 Well-Known Member

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    I would bet some if not all electrolytic capacitors need replaced. Also possible the voltage regulator in the power supply section went south.
     
    Staybolt likes this.
  3. nomadradio

    nomadradio Analog Retentive

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    A look inside will reveal a cylindrical-shaped capacitor that has swollen and spewed some of its internals. A wet spot is a clue. A puddle of dried, brown or tan powder will be left behind if leaked liquid dries out.

    This kind of part usually fails as a short circuit when they go "POP" like that. The next question is what collateral damage it did. A short circuit in a capacitor can overload other components and damage them. In particular, the components in the power-supply regulator are vulnerable.

    If you can hook it up to a 12-Volt DC power supply, you could find that the radio is okay, and only the built-in power supply is bad.

    Maybe. But a good look inside for a swollen or ruptured part is a better place to start.

    Safer than just hooking it up to a DC supply and crossing your fingers.

    73
     
    Robb likes this.
  4. Staybolt

    Staybolt Well-Known Member

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    Well that tells me nothing. I know caps leak and fail but im wondering why i have no tx rx or audio?
     
  5. Staybolt

    Staybolt Well-Known Member

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    I replaced the audio chip. No change.
     
  6. guitar_199

    guitar_199 Well-Known Member

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    Well.... electrolytic caps that blow up have a tendency to reduce supply power in the device. Depending on how they fail they can take the DC that feeds the audio circuits or anything else in it. Might be worth checking.....

     
  7. Staybolt

    Staybolt Well-Known Member

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    Not everything involves a cap however i did a complete cap change and nothing change...
     
  8. nomadradio

    nomadradio Analog Retentive

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    Did you find the damaged cap that made the noise?

    It could lead you to the damage it caused.

    73
     
  9. nomadradio

    nomadradio Analog Retentive

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    Might check the power supply voltage.

    Put the meter's black lead onto the negative pin of the DC-power jack on the rear panel. The metal chassis is not grounded to the radio's internal power-supply ground.

    Touch the red probe to each of the two lugs on the rear-facing end of the volume/power control.

    Should be 13.8 Volts. If it's a lot less, that causes trouble.

    73
     
  10. guitar_199

    guitar_199 Well-Known Member

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    Understand that I have no knowledge of what you know..... so I am feeding you what I can...
    All things in the radio are impacted by:
    1) The power supply. Make sure all voltages are present and behaving correctly(that is, RCV voltages present when OFF the key, TX voltages present only when ON the key. Check all of them)
    2) PLL/Local Oscillators. In PLL or even in older crystal synthesizer units - at some point - both RX and TX work off of local oscillators. If that fails, nothing works. In PLLs you have both a reference oscillator and a VCO. Older Crystal synth... usually have a couple of oscillators with switched crystals and mixers.

     
    Christopher17 likes this.
  11. Staybolt

    Staybolt Well-Known Member

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    Not the power supply as the radio acts the same on 12 volt as it does on 120. Ive put the radio aside and have moved on.
     
  12. guitar_199

    guitar_199 Well-Known Member

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    Certain parts of the power supply are shared/common. It is entirely possible that it acts the same on both wall and 12v supply because the problem is downstream from the point where the sources switch. The AC power supply generally is configured to create a 13.8V power supply internally. So if the problem is "after" that point then neither the AC in or DC in will work.
     
  13. Staybolt

    Staybolt Well-Known Member

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    Its not the power supply at all, its in the audiio chain. Im surprised Nomad hasnt chimed in as i figured hed know about this radio as its very common.
     
  14. guitar_199

    guitar_199 Well-Known Member

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    from your initial message: "modulation light stays on and radio doesn't transmit or receive."

    Doesn't transmit suggests that it is not putting out a carrier. A problem in the audio chain that might stop everything transmit...might be the AM modulation regulator... but that usually doesn't affect the receiver. Does it put out a carrier or not? Is it just not modulating? If that is the case.... one last.... does the PA work? Can you connect a PA speaker or a headphone... and hear it when you talk in the microphone? That would close in on it pretty quickly.


     
    Shadetree Mechanic likes this.
  15. guitar_199

    guitar_199 Well-Known Member

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    This video presents a "how to troubleshoot a no modulation at all" problem from the git go....

     

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