The service manuals are excellent in diagnosing problems like this.
I would highly recommend the RF section manual - RF tune section block diagram, the theory of operation, then focus on the PLL lock circuits, then the troubleshooting for those circuits. Below 5MHz span the sweep and tune are continuously locked, at and above 5.01 MHz span it is "lock and roll".
There are 3 smaller brds. in the RF section that are the PLL2 circuitry - marked on the metal covers. The PLL2 troubleshooting will point you at the problem right down to the individual components.?
You will probably find the PLL2 frequencies are off. Probably causes may be the potentiometers on the brds. or the phase lock detector.
