The commonest 'real' problem with a Tx/Rx that was 'unlocking' was the Magnetron was shot.
Trying to remember was it Green Satin where you had to use a long copper bladed screwdriver to tweak the Klystron. If you used one with a normal steel blade the moment you got close to the Klyston the magnet on the maggie would pull it over on to it's EHT, and zap you found yourself on the other side of the room. Happy times.