Generally, the frequency range consists of two circuits, one is input protection+shaping (a group of operational amplifiers+peripheral circuits), the other is waveform trigger (usually 7555 circuit), and now there are circuits directly composed of a 4528.

As you said, I think there are two possibilities:

1, the input front-end protection circuit is damaged;

2. When you open the lamp tube, you remove the components of the frequency range circuit.

You can dial the frequency range, and then use another multimeter to check the direction of the red stylus input to the internal circuit of the instrument to check the circuit of the input part.

Of course, the quickest way is to call the manufacturer's service phone to make an inquiry.