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It looks like the ray of ch2 sometimes go out of the screen.
I would measure the Voltage on R 278 and R 279 in good and bad condition.
When there are the same values in both cases the fault is behind this stage.
By rotate the position pot the values must change.
With my older Grundig Osc i had a similar Problem.
At the end the cause was a faulty pot ( Makes sometimes no contact ).
Take an isolated skrew driver an push carefully a little bit to the pot's of ch2 with the handle.
When the Osci reacts You've found the damage.