Congrats, I think you got yourself a very nice scope! You can read a bit about it here:
https://www.hpmemoryproject.org/wb_pages/wall_b_page_02.htm Reading the service manual will also be a great learning exercise. Read it thoroughly! Those engineers knew quite well what they were doing back in the days.
Disclaimer: I don't own this scope, so I'm just giving some generic advice / guesses about what I would do.
The scope is an analog storage scope so you have potentially 20+ signals and voltages coming into the CRT which is a challenge by itself.
Also the usual stuff: This appliance has potentially lethal voltages in it. Before touching metal parts in it which are not connected to ground already, you should always discharge the capacitors and the CRT. Document yourself about how to safely do these things without damaging the instrument.
I think you should focus first on getting channel A to work properly end-to-end so that you have something working to compare to when fixing channel B.
The signal showing up multiple times could be as simple as an issue in the time-base or in the blanking circuit or could be as complicated as an issue with the storage mesh.
I'd suggest to leave all the storage features turned off for the initial troubleshooting.
The vertical stage in this scope is also somewhat complicated because the CRT features a transmission-line deflection plate.
So in short: after ruling out all possible power supply issues I would look in this order at:
- Try triggering from an external source, follow trigger signals inside the scope and try to compare with expected signals in the service manual
- Look at the vertical output signal on the back on the instrument with another scope
- Look at the signals before the final vertical and horizontal final stages and try to follow the signal back to its source
- Look at the vertical delay line which drives the transmission-line vertical plate
- CRT blanking signals (Page 8-16 and 8-17 on the service manual)
- CRT storage meshes signals (even if turned off there may be faulty components / switches / whatnot which play a role in that area) after you ruled out other problems