If it won't key, I would first look at the electrolytic filter capacitor for the relay circuit.
What baffles me now is that nobody has asked how big is the radio you're driving into it.
The JB150 was built to work with a typical tube-type CB base radio of the late 60s. Radios that would bust a gut to show a 3.5 Watt carrier, and 14 Watts PEP.
A fairly-stock 40-channel legal CB radio with only one final transistor is bigger, maybe twice the "swing" of the 23-channel radio from 50 years ago.
And a "10-meter" radio with two final transistors will hit this amplifier at least three times as hard as it's built to allow.
So, what kind of radio are you using to drive it?
73