Any idea how many attempts have been made to fix it since it first broke down? A radio that still won't work after somebody has "worked on" it tends to have added faults in it besides whatever it was that failed to begin with. I would guess that the average is about 1.5 new faults for every failed attempt at repair.
The built-in power supply may be the whole problem. What's inside is a 12-Volt (13.8 Volt) DC mobile radio. There is a DC power jack on the rear. Do you have a power cord that will fit and a 3-Amp or larger DC power supply to try it that way? Just make sure there is a 3-Amp fuse in line with the DC cord before you key the mike. This model has a failure-prone capacitor that can pop the fuse, or damage the relay inside the radio when you key the mike. That relay is no longer made and finding one is neither easy nor cheap.
This is a 1977 or 1978 radio, so who knows what has been done to it over the last 40 years?
The mike socket is NOT wired like all the other modern-day radios with this socket. Pins 1 and 2 are the reverse of a "normal" Cobra/Galaxy mike. Pin 3 is the same, and pin 4 is not used.
Any of those that get serviced here will also get the mike socket changed to match the Cobra/Galaxy pin arrangement that has become more or less universal over the last 30-plus years. This is just easier than having to have a separate "Robyn-only" mike for the radio.
It's not just the years, it's the miles. If it has a lot of wear and tear that could make the cost of "restoring" the radio out of reason. After 40 years the line between "repair" and "restore" can get mighty thin.
73