First thing I would want to know is if the radio really is receiving the channel that's selected. If the radio's PLL loses its grip on the channel frequency, FCC rules require a circuit that locks out the transmitter. And if you're certain the radio is receiving on the right channel, this is where a five-dollar yard sale CB with a coax jumper on it can pay for itself. Plug the coax jumper into the second radio and set it to the same channel as the broken one. Unscrew the shell of the coax plug at the other end and back it down the cable. Now you have a half-inch long "sniffing" antenna on the end of that plug. Remove the broken radio's case and key the mike on the broken radio. See if you can hear anything at all in the monitor radio, or see the S-meter flicker. Sniffing around the transmit section can reveal a weak signal it won't hear a foot or more away.
If you can see a weak signal, even if it's only a carrier with no audio this tells you your transmit signal is being produced, but not amplified.
And if you don't hear anything at all, this shifts focus AWAY from power stages like the driver and final. A total lack of any response in the sniffing radio points to a fault in the lowest-level transmit circuits.
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