I'm uncomfortable with the term 'ballast'...
To my mind the better term is CWA -- constant wattage autoformers.
Ballast -- as a term -- reminds me of old, old classic fluorescent fixtures in which the ballast performed more like a 'choke.'
BTW, these lamps need healthy field wiring, too.
While the damage certainly shows up in the fixture -- it's often the result of failing connections elsewhere up the line.
Rather like an induction motor -- CWA's draw heavy until the lamp fires off. If there is too much impedance ( particularly on the neutral ) one or more lamps won't 'strike.' Then, after powering down, a re-strike attempt will cause the pattern to entirely reverse: the dead lamps fire up -- and the previously lit fixtures won't re-strike!
This situation is not that rare -- because by the nature of the devices they are used across long distances from the panel. Even bumping up to #10 is not enough, sometimes, to get these babies to fire.
Sometimes you'll have to split the run in half ( double the #10's ) joining them together in a gutter near the panel. This up-sizing is purely for impedance reduction.
So your job is not done until you've inspected the condition of the field wiring.
