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Cobra 29 LX - Squelch Not Functioning

Discussion in 'General CB Services Discussion' started by wcsd106, Jul 1, 2019.

  1. wcsd106

    wcsd106 Active Member

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    Hi all,

    I have a Cobra 29 LX that was working fine until a couple of weeks ago when the squelch stopped working. I can turn the squelch knob all the way to the right, and the squelch never closes. This occurs whether an antenna is connected or not, so definitely not a noise level issue.

    I thought possibly the squelch pot had gone screwy, but I checked it on my bench today and it's tracking nicely.

    Gotta be something else in the squelch circuit itself, but I haven't found it so far.

    Checked TR11 and TR12 against the voltage chart I found in a Cobra 29 LTD Service manual. The Collector of TR11 doesn't have the 4.2v collector voltage that it's supposed to when the squelch is at maximum.

    Any ideas?


     

  2. nomadradio

    nomadradio Analog Retentive

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    Moving parts tend to fail first.

    I'd be checking the voltage at the wiper lug of the squelch control, and follow the circuit downstream from there.

    73
     
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  3. wcsd106

    wcsd106 Active Member

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    Thanks, Nomad. I'll check on it later this morning when I get a chance. Not sure that's the issue though, as I tried bypassing the squelch pot with a fixed resistor during testing and still wasn't able to squelch the radio.

    I'll let you know what I come up with.
     
  4. Robb

    Robb Yup

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    VR3?
    Not like there are a lot of components left in that circuit that would fail.
    Solder joint, bad trace, resistor changed value, ??? . . .
     
    #4 Robb, Jul 3, 2019
    Last edited: Jul 3, 2019
  5. wcsd106

    wcsd106 Active Member

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    Measured voltage at the wiper of the squelch control: Tracks cleanly 0v (Squelch Fully Open) to 1.99v (Fully Closed)

    VR3 can adjust that voltage a little more, but again everything tracks fairly smoothly.
     
  6. nomadradio

    nomadradio Analog Retentive

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    TR12 is the next stop "downstream" from the control.

    Should show zero when the squelch control is full left. Should come up to about 6/10 of a Volt and stay there as the squelch control is turned clockwise.

    If TR12 is working, the colllector voltage should fall from 3 to 4 Volts with the knob at the left, and drop to under 1 Volt when the base terminal reaches the 6/10-Volt threshold.

    TR11 and TR13 are next, if TR12 passes this test.

    73
     
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  7. wcsd106

    wcsd106 Active Member

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    Here's the results of TR12:

    The Base pin shows ~0v with squelch at full left. Raises to 0.66v and stabilizes as you begin turning the squelch control to the right.

    However... The Collector shows 0.7v with squelch fully left and drops to near 0 as the base pin reaches 0.66v
     
  8. nomadradio

    nomadradio Analog Retentive

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    Next link in the chain is the collector (center pin) of TR11. The voltages you got so far appear correct.


    TR11 collector should show close to zero Volts with the squelch to the left, and pop up to about 5 Volts as you turn the squelch control to the right.

    73
     
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  9. wcsd106

    wcsd106 Active Member

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    TR11 Results: Collector is at Zero volts regardless of squelch position.

    Replaced TR11 with a new 2sc945. Did not correct issue.
     
  10. nomadradio

    nomadradio Analog Retentive

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    You'll need to find the foil trace where the center pin of TR11 is soldered. Follow it towards the outside to a hole marked "CS" on the component side. Unsolder and pull out the wire from this hole and see if the squelch works now

    If so, there is a short to ground in the "Channel Scan" circuit at the other end of this wire.

    73
     
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  11. wcsd106

    wcsd106 Active Member

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    Nomad, you were spot on! Removing the green wire at point CS restored squelch function. Now to determine whether it's the scan switch or something else causing that short.

    At least, though, this radio is usable again. I wouldn't consider the scan function to be a necessity.
     
  12. wcsd106

    wcsd106 Active Member

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    Looks like the channel scan wire doesn't have a direct short to ground, but it is only showing around 32 ohms between the wire and pc board ground.
     
  13. nomadradio

    nomadradio Analog Retentive

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    Checked the diagram. That wire leads directly to pin 30 of IC301, the brain of this radio.

    This is not an unusual way for a chip to fail. The real surprise is that the rest of the chip is working. A resistance of 32 ohms isn't a "short" like a solder glob across two adjacent foil pads, but it's failed chemistry somewhere inside that chip. It operates at very low currents, so 32 ohms might as well be a short to a low-current circuit.

    Not a part you wan't to try changing, even if you could obtain it.

    Much better to get the radio back by pulling a wire.

    Good work and 73
     
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  14. Robb

    Robb Yup

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    NMR: Is that chip unobtanium?
     
  15. nomadradio

    nomadradio Analog Retentive

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    I just assumed that it is. The labor to change it would probably not be worthwhile even if the chip is cheap. And since there won't be more than one source (even if there is one) the price of the part is likely to be high.

    Even if you get your hands on it, what next? Plaster kapton tape all around it and change it with hot air?

    Maybe for a radio that's worth many hundreds of bucks.

    But for a 29?

    73
     
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