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Cobra 142 gtl Transmit problem.

Discussion in 'CB and Export Equipment and Accessories' started by gogo_gadgit, Nov 10, 2010.

  1. gogo_gadgit

    gogo_gadgit Member

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    I used the radio before I went to work, came home, and now it dosn't transmit at all. The Meters don't move, the light dosn't turn red, alsmost like it was a microphone problem, but its not, I tried about 5 mics, and nothing,

    there isn't a relay in the rig, so what else can I check or replace,


     
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  2. North Texas Mudduck

    North Texas Mudduck Active Member

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    is it in PA mode?
     
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  3. gogo_gadgit

    gogo_gadgit Member

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    no, but i did flick the switch and put it on pa mode,, and when you key up the mic the green light goes out (rx light)
     
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  4. Robb

    Robb Yup

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    Did anyone else have a chance to mess with it while you were out?
     
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  5. Shockwave

    Shockwave Sr. Member

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    First you need to rule out if it's a PLL issue. Do this by checking to see if the radio is actually able to receive signals on the channel you are tuned to. If it does this, the PLL is likely to be locked. Then we have to suspect it may be a voltage switching issue. If you ruled out mic connections and checked the internal connector also, then look at the 8 pin MB3756 voltage regulator mounted next to the audio chip on the heatsink to the right. It's been a while but at least one pin should go from 0 to 8 volts on TX. I think it's pin 8. If that's not happening, you may have a shorted capacitor or this chip could be bad.

    To determine if the chip is working check pin 5 to see if the control voltage changes from RX to TX. If not you probably have a shorted cap on the control line. If the control line pin 5 changes and pin 8 doesn't have 8 volts on it in TX, lift the pin from the board and check the pin again. If you still have no 8 volts on the pin in TX the chip is probably dead. If there is 8 volts, you may have a short on the load side of the chip. Good chance it's a cap. They are getting old.
     
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  6. Radio Tech

    Radio Tech Radio Operator

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    Ok,
    I take it the receiver is working? Just want transmit....

    Lets take the easy route first. Check pin 8 of IC 5. This is your 8 volt regulator. You should have 8 volts there.

    If the receiver is working then the TX supply should be ok, but we need to see if TR37 is pulling pin 2 of IC 5 low to switch it in TX mode.
    The emitter of TR37 is the tx line to the mic plug.
     
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  7. Shockwave

    Shockwave Sr. Member

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    I'm only looking at the data sheet for the MB3756 and it indicates pin 2 it the main supply voltage to the chip and not the control line. Pin 2 should have constant 12 volts on it and pin 5 should be the transmit and receive control line to the IC.
     
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  8. Radio Tech

    Radio Tech Radio Operator

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    Correct,
    I guess I did not word that good enough. If the circuit of IC 5 is bad, then when TR37 is switched to transmit, pin 2 will go low if there is a shorted circuit. If pin 2 (supply) goes low, probably a bad cap in the T/R switching circuit or an internal short in the IC itself.
     
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  9. gogo_gadgit

    gogo_gadgit Member

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    Okay, the radio Seems to be receiving on the same channel no matter what channel I try to go to. The mb3756 seems to be compleatly dead. So I found a new chip on ebay and ordered it, hopfully that will fix me up, I'll let yas know how I make out
     
    #9
  10. Radio Tech

    Radio Tech Radio Operator

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    Please keep us updated
     
    #10
  11. Cable Guy

    Cable Guy Member

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    Hi, I dont mean to post on someone elses but I am having the same trouble, the 8volt regulator is shot, will replace soon. But it wont receive anything either. Wondering if the PLL is shot too, if I turn it all the way up, I can hear some slight static, but not RX anything. Same radio, same problems, and gogo-gadgit hasnt responded back if it fixed it yet or not, so I will when mine comes in. I just wanted to know if there were a procedure for checking the pll using basic tools. I do have an oscilloscope if it would prove useful.
     
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  12. loosecannon

    loosecannon break on through

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    that voltage regulator chip supplies the PLL with the 8 volts it needs to generate the signal frequencies, so if the voltage reg is shot, then the PLL is not going to work either.

    you can confirm this by measuring the voltage on pin 9 of the PLL. it should read about 8 volts, but yours probably wont.

    best not to operate the radio or leave it on longer than needed to do this quick test, as you could cause further damage to the radio.

    if you measured the voltages on the MB3756 voltage regulator chip and found that they were off, you might want to check the electrolytic capacitors immediately surrounding the chip to make sure none of them are shorted and causing the bad voltage readings.

    another test you can do when you find a pin on the chip with a weird voltage reading is to unsolder just that pin from the board and check the voltage on the pin with it disconnected.

    if the voltage now reads normal, you know that the problem is external to the chip and is along the path connected to that pin.
    do this with each pin with an out of spec voltage reading.

    if the voltage on the pin(s) is still weird with it disconnected, then the chip is probably bad and needs replacing.

    if you have trouble finding an MB3756 chip, NTE makes a sub, the NTE1271.

    good luck,
    LC
     
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  13. Cable Guy

    Cable Guy Member

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    Hi, thanks for the reply. I now see that the chip does in fact supply the pll with 8 volts. I checked the control voltage for rx/tx and when I try to tx, it does change, but only by about half a volt. Im not sure this is enough threshold to switch it, but its not switching. I checked the caps around and they appear fine, I checked the transistor that relays the tx to the voltage regulator, its functional. I still need to check if I have voltage on the pll and I will do that today. All other pins read fine from the voltage regulator with exception to pin 8, which reads about .2 and pin 6 which reads 8 always, even under tx attempts. I will check and get back to you.

    Thanks again!
    Cable Guy
     
    #13
  14. Cable Guy

    Cable Guy Member

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    Okay, pin 9 of the PLL is hot with 8 volts, so its being powered up. SOO, I should be able to rx even though the VR for tx is not working, right? If so, then there is something else wrong with this, maybe even the pll itself. I got this radio from a friend, and after examining it, it appears they have cut a diode, resistor, and attempted a clarifier mod. I repaired all connections back to stock and am starting from there. I imagine they messed up the VR when trying to mod it. People should just leave well enough alone I guess and this would be a fine radio. Never the less, I will keep pushing on toward a repair solution.
     
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  15. loosecannon

    loosecannon break on through

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    good luck with it.
    those can be real headaches.

    best bet is to put things back to stock one by one and check operation at every step.
    LC
     
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