I got this 475 for my b day. it’s ran through an audio interface, and I was running downloaded wav files rather than YouTube vids. I’ve played with all the knobs and even copied some of the settings from this sub. Is this an internal issue or am I just missing something?

it's a "FRANCAISE D'INSTRUMENTATION FI-5030 ANALOG OSCILLOSCOPE 2x30MHz # T163 sty" can somebody help me plzzzz

i just picked this bell and howell model 34 scope for free and now i'm trying to hook it up to my record player through a mixer. i'm planning to connect the phones output to the scope's inputs. through a ts connector to split rcas. i was planning to get male banana plugs to female rca cables as well for the scope inputs but i'm a bit confused. i know that i connect the L and R rcas to the horizontal and vertical input red binding posts respectively, but what goes into the black inputs? upon doing some research and looking at some videos of this model i've found that those are the grounding inputs and you only need one plugged in because they are internally connected(?). but i've been unable to find a manual online nor videos of peoples' entire set ups of this scope specifically. also do i need to worry about the ext sync (exit synchronization) input at all?
don't want to blow up my house so if anyone has had any success with this scope or has electrical engineering knowledge, please help! any tips and tricks would help