[Penthode]

Did you move any leads or change any components in the IF amplifier? Any lead dress changes?

Any change to the connection between the tunerout and the first IF stage? Any changes to the shielding?

If all looks good or you insist there has been no or minimal wiring changes, have you tried substituting the IF amplifier tubes? I recall this set has 3 IF stages. My preference is to first determine which stage may be or if is mistuned. Assuming it uses exposed bi filar wound coils, try maneuvering a metal screwdriver tip near each coil while watching if the ringing changes.

The video carrier is typically at the 50% point on the response slop at 45.75MHz. the tuner mixer out to first IF and the 3rd IF Amplifier to detector stage both are typically tuned to 44MHz or thereabouts. The stagger tuned stages from around 43 to 42.5MHz will lie in between. If one of these stages becomes mistuned towards 44MHz, regeneration may occur. So try detuning some of the stages slightly only with the steel blade of a screwdriver gently near the coils and watch the result.

I don't think the tuner can cause this as the RF bandwidth is broad. But lead dress eg even location of the antenna lead to the RF input near an unshielded IF stage may cause regeneration.