Still trying to get a bit further without having to take measurements may yield more info.
If both channels show the same behavior: Since bad contacts are rather frequent with old equipment like this, wiggle the various transistors in the vertical amplifier; they¡¯re all socketed. Also slightly move back and forth vertical position adjustment pot. Don¡¯t know by heart if and where in this model. Preferably do this while power is off.
Should you do measurements at/near the vertical amplifier, be very careful not to short the outputs (or plates) to ground, which very likely would kill the output transistors.
Raymond
Raymond Domp Frank