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Cobra 142 gtl Transmit problem.

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  #9  
Old 11-11-2010, 10:47 AM
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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

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  #10  
Old 11-11-2010, 11:48 AM
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Please keep us updated

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  #11  
Old 01-04-2011, 06:27 PM
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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  
Old 01-04-2011, 07:48 PM
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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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Old 01-05-2011, 11:18 AM
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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

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  #14  
Old 01-05-2011, 11:28 AM
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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  
Old 01-05-2011, 09:57 PM
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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.
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  #16  
Old 01-12-2011, 07:03 PM
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Alright, I replaced the voltage regulator, and after a little tuning, she receives great, but when I tx, my meter shows some swing in modulation, but my watt meter dosen't move, so I assume I have modulation without power? My neighbor can receive me even though I put out no watts. Maybe since we are so close he is picking me up for some reason. I tried ssb and lost modulation swing and still no power, but am has modulation swing. huh, weird. Could this be my finals? In PA mode it works fine.

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