I'm just going to throw out some thoughts here on this matter.....FWIW....
As far as tubes are concerned never trust a tube checker of any kind for tubes in power and rectifier applications. A checker can only reliably tell you if a tube is bad.
A flyback coil has a large number of windings between the HOT anode tap on the winding to the top end where the HT diode anode connects. Any of a number of things can be going on--several shorted out turns, etc, that can drag down on the efficiency of the generation of HT without necessarily greatly affecting horizontal sweep and such.
Also never assume that a newly-bought tube or component for which you may have paid serious dough is actually of good merit, and not defective. I've witnessed techs wasting many hours because they refused to believe that a new part they installed could be the culprit.
