2215 scope - channel B vert position knob isn't working?


Hey there, I bought a Tektronix 2215 recently and it looked like it was in great shape. I mainly have been using it in XY mode to visualize stereo width since I'm a musician.

However, when I turned this scope on last night, it seems like the trace for channel B is stuck at the lowest position, off the screen. The channel B vertical position knob has basically no effect, in fact, when holding down the beam find button, turning the channel B position knob moves the trace ever so slightly in the opposite position that I'm expecting. When I am viewing channel A, holding down beam find and moving the channel A vertical position knob works as expected.

I don't really have any experience repairing electronics but want to try and troubleshoot this. Does anyone have any advice, or at least any tips that may point me in the right direction? I haven't opened the scope up yet to check for anything visually obvious as I don't own a torx screwdriver as large as required.

Thank you for your help :)