[Heathkit] Heath Apache BFO questiom

[John King]

I have a Heath Apache with a VERY NOISY BFO control. The BFO control has a shaft that goes into a box on top of the chassis. Out of the bottom of the box housing the control , two wires through the chassis to the a tube socket
When I turn the BFO knob a loud scratchy noise is made in the audio. My question is: What is inside the box that is turned or switched by the shaft going to the box. Is it a variable capacitor or rotary switch???  I don't want to go into the VFO box until I know. The schematic is not clear about what is inside the cox.
Your input will be greatly appreciated.
73, John, K5PGW