[old_tv_nut]

There are two resonances of interest in a flyback. The first is that of the primary and yoke with a discrete capacitor, which sets the retrace time. Reflectance of the secondary into this circuit probably affects it, but I can't say how much, because when I was designing flybacks we always went from an existing design, never tested a primary only by itself.

The second resonance of interest is the secondary. It is formed by the secondary inductance and distributed capacitance (not a discrete capacitor). While the total turns sets the output voltage, the number of layers/turns per layer plus the spacing between layers plus the characteristics of the potting compound determine the distributed capacitance. This second resonance is tuned by adjusting all these things that affect distributed capacitance. In the usual designs I worked on, the secondary was tuned to the third harmonic of the retrace. Some later ones were tuned to the fifth harmonic. The idea was to have the third harmonic in the secondary out of phase with the retrace pulse at the high voltage winding top, and have just the right amplitude so that the third harmonic tended to flatten the top of the transformed flyback pulse. This meant that the high voltage rectifier had a broader flatter voltage pulse input, making the high voltage output more stable with varying amounts of beam current. In other words, the impedance of the high voltage supply was reduced so that it was "stiffer" and there would be less variation with beam current, meaning less blooming. It is this improvement that you will lose if you wind a new secondary without redoing it by trial and error to get the tuning right.