Tektronix 7904, no horizontal trace on input B

Hermansson Amplification
 

I just bought a Tektronix 7904 scope and realized that something is wrong with horizontal input B.
Horizontal input A works fine but I got no trace on input B. If I press
“Beam finder” slightly I see dots for each inputs that are activated in the vertical slots and if I press the “Beam finder” harder I see bigger dots. I only see the dots when I press “Beam finder” otherwise the screen is blank when horizontal input B is activated.

Any suggestions where to start? I’ve measured the voltages on the Z axis-PCB but nothing alarming there. Since the horizontal Input A works something might be bad on the “Trigger B”-PCB? Or where should I start?

Here’s a video showing the problem:
https://youtu.be/KuPenOSkyJg

I’ve been exercising the switches and cleaned the input slots for the plugin.

Thank you in advance!

Best regards,
Henric

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