Zenith pic problem

[Charles Vesser]

On those early Zeniths the yokes are are known to slide back on the tube neck, also the spacers up aganist the back of the tube (rubber feet) are know to get loose and falloff. Thus if the set is moved or viberated during cleaning the yoke can tilt or move. Some Zeniths also have a tendency to rub a bare spot in the anode lead and cause arcing to the metal frame.
Of course the yoke could just be bad. My guess is its in or around the yoke. Not the tube.