I am currently working on a cobra 29 ltd. Radio keys up and has power and receive but will not modulate carrier. My question is how can I trace the audio through the board to find out where the loss of the modulation is happening. Is there an audio tracer available for this task? Thanks in advance for the help.
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Question. Tracing Modulation audio
- Thread starter harborpc
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Is the receive audio ok? Do you have an oscilloscope?
techman210
Member
Most CB radios use the same audio amplifier stage for both modulation and the public address function.
See if the PA works.
See if the PA works.
tvengr
Well Known Member
No modulation is usually caused by a broken wire in the microphone connector or cable.
Some radios have a mechanical T/R relay that routes different signals between transmit and receive. Could be a bad relay contact. Some radios use other methods to route the audio to the final. I have also seen a power transistor in line with the 12V line to the final transistor where the emitter and collector are in the 12v power line and the audio amp feeds the base, thus applying modulation to the final transistor. Since you have receive audio and especially PA audio the problem will probably be in the T/R switch area. A schematic and an oscilloscope should find it easy.
tvengr
Well Known Member
If PA is working, microphone is OK. Is the microphone audio going to the PA speaker output or just the receive audio? I agree to check if there is a T/R relay which can route audio between the transmitter and receiver. Could just be some dirty contacts.
The same audio power amp that feeds the internal speaker and PA output is the same that modulates the radio in AM mode. I looked at a schematic for the radio and it looks like in one version there is an audio output transformer and they feed 12V through that then through a diode to the driver and final which provides power and modulation. If that diode was open there would be no transmit at all. There could be several different schematics and circuits depending on the vintage of the radio.
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