Quote:
Originally Posted by Electronic M
It's possible the issue is a H-K short or bad H-K hum coupling in sweep or sync tube(s)...If a cathode voltage is hum modulated it would be harder to change it with B+ filtering than B+ hum.
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Yet another thought are the caps bypassing the filaments to chassis ground. If any are shorted or leaky they could place AC on chassis ground which could in turn back feed into other circuits through their associated bypass caps. In other words the supposedly floating chassis could be at some AC modulating potential.