If it was a 1978 Plymouth Valiant, you might hesitate at the cost to rebuild it. A cheap fix-up probably would not make it into a daily driver after 40 years.
This radio may have a lot of miles on it, but there's no odometer to tell that. A high-mileage radio might need a double handful of small electrolytic capacitors replaced, or just one or two to get it running. Replacing every one of them is a "restoration". Replacing only the ones that go bad is more like a "repair".
How many original hoses and belts would you try to keep using in that Valiant?
Years ago we had a stack of this model and the 480, all with a bad VCO module. Couldn't buy that part. Good chance the radios got ditched years ago.
Sams CB volume 227 is the only source of service info I know for that one.
A check to see that the PLL is locked onto the channel you select is a good place to start. The radio will lock out the transmitter if the radio has jumped to a frequency that's not legal. If your radio has the chronic VCO problem, it can cause this symptom.
Have you checked to see that the radio is receiving the channel that it should be? If not, the radio's transmitter may just be locked out as a result.
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