Philco 48-2500 Restoration Blog

[Tom Albrecht]

Originally Posted by John Folsom

I would think it is not the focus coil current that needs to be controlled, rather the HV needs to be regulated. As the As these supplied to be rather anemic, I would think a shunt regulator would not function all that well. But it would be interesting to try it . Or, as the Philco HV supply is independent of the horizontal sweep, it should be possible to build a feedback design which compares the HV to a reference voltage and used the difference to drive an amplifier to modulate the screen grid voltage on the horizontal output drive to the flyback, or modulate the B+ voltage to the flyback. Either approach would be interesting to try.

I agree the origin of the problem is in the HV sag, not the focus coil. However, the HV is derived from the horizontal sweep flyback transformer, so I don't think we can apply the fix you've suggested. Or did I misunderstand your idea?

Interesting to hear that the RCA projection sets are worse. I'm surprised they marketed the sets with these design weaknesses. It can't be that hard to design around the problem.