GE Adventurer II / 15SF not locking sync

[Electronic M]

It's most likely in the resistors and caps between the plate of the sync separator and the vertical tube grids or possibly in the parts between the vertical output plate and the vertical osc grid. Resistors of this era tend to drift far higher than tolerance and caps can short, leak DC current or go open. If the tube was bad you'd probably have no deflection or little deflection... Though odd ball failures like a heater cathode short introducing AC could cause it. (Granted if it was a heater cathode short the set would loose sync strongly for an instant every ~11 seconds, assuming NTSC color vertical rate ,then relock on its own and continuously repeat the cycle).

Resistors you can check. Caps you can't properly check for leakage without an eye type capacitor tester that can perform leakage testing at full rated voltage...So substitution with new caps is usually the best test.

If you can make it roll up or down it's almost certainly a sync issue. Horizontal locking range will also tell you something about where the issue is...If horizontal hold is stable for almost a whole turn then it's the components between the separator plate and the vertical, but if hold just barely stably locks the whole sync separator stage, its tube, the components that feed it video and the level of that video become suspects.