142 GTL No Transmit on am
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jgordon12364
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- Real Name: jim gordon
142 GTL No Transmit on am
I picked up a 142 GTL on Craigslist and it has transmit on SSB but no carrier on am. frequency counter showing slightly off frequency on key up and when I talk into mike, I am using a D104M6B, It swings from nothing to about 6 watts. Unit has two toggles on back obvious channel conversion done. Receive is good. I replaced driver and final did no good. The voltage on the driver and final flange is 13.05 on am and 13,78 on ssb when on receive, driver drops to negative reading on transmit and final goes down to .20 what would cause the over voltage and drop? any help would be appreciated. Thank you in advance. JIM
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redlght
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I would look at TR34 2SC1419 they are a weak link in the 142 gtl then look at the am voltage regulation IC5 MB3756 and any caps in that circuit. You should get 4 to 6 volts at the test points on AM and 13 volts on SSB. The reason for the voltage drop would be a short on tx. The VR in the power supply can set the power supply output voltage it might be turned up to high.
Sunday, 29 September 2013, 8:07 AM
Forgot to mention if someone volted the finals then you would see full 13 volts on am there are two wires running from the drive and final that go to test points they plug on to the posts sticking up out of the board if the final one is not like that and soldered direct to 12 volt (most of the time they use the audio transformer for the 12 volt tap point) then the final is volted, it is very hard on the regulator when you volt the finals.
Sunday, 29 September 2013, 8:07 AM
Forgot to mention if someone volted the finals then you would see full 13 volts on am there are two wires running from the drive and final that go to test points they plug on to the posts sticking up out of the board if the final one is not like that and soldered direct to 12 volt (most of the time they use the audio transformer for the 12 volt tap point) then the final is volted, it is very hard on the regulator when you volt the finals.
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TheCBDoctor
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Hi jgordon12364,
As Redlight mentioned you should make sure the wires running from the test points are in place. The voltage should be between 4 to 6 volts on AM and 13 volts on SSB. There is no benefit to have 13 volts on AM at the Final and Driver. The transistor mounted to the left side of the rear of the radio near the Driver modulates the carrier on AM. If that transistor is bad you may not have no voltage or modulation on AM depending if it is shorted or open.
Since, SSB is created at the balance modulator, and the AM carrier envelope is created at the Driver and Final, it is possible for SSB to work and AM not to work. If the radio works on SSB, then the output transistors are good. I would suspect a shorted 10 volt electrolytic decoupling capacitor at the 7.8 MHz carrier oscillator. If that cap is shorted you will be missing the 7.8 MHz that feeds the balance modulator that generates and creates the carrier. This circuit is disabled in the SSB mode.
Having a negative voltage on transmit points more to the mode selector switch. Unlike exports, the Cobra Base stations manually switch the SSB and AM voltage to the Final. I have replaced bad mode switches in the past. It is not common for them to fail, but in nearly 30 years I have replaced 6 of them.
Respectfully,
Rick
As Redlight mentioned you should make sure the wires running from the test points are in place. The voltage should be between 4 to 6 volts on AM and 13 volts on SSB. There is no benefit to have 13 volts on AM at the Final and Driver. The transistor mounted to the left side of the rear of the radio near the Driver modulates the carrier on AM. If that transistor is bad you may not have no voltage or modulation on AM depending if it is shorted or open.
Since, SSB is created at the balance modulator, and the AM carrier envelope is created at the Driver and Final, it is possible for SSB to work and AM not to work. If the radio works on SSB, then the output transistors are good. I would suspect a shorted 10 volt electrolytic decoupling capacitor at the 7.8 MHz carrier oscillator. If that cap is shorted you will be missing the 7.8 MHz that feeds the balance modulator that generates and creates the carrier. This circuit is disabled in the SSB mode.
Having a negative voltage on transmit points more to the mode selector switch. Unlike exports, the Cobra Base stations manually switch the SSB and AM voltage to the Final. I have replaced bad mode switches in the past. It is not common for them to fail, but in nearly 30 years I have replaced 6 of them.
Respectfully,
Rick
Respectfully as always,
Rick
Rick
