The schematic is in Post #5 in this thread, based on the principles at the link in Post #2. It's taken a while for me to understand how these flyback circuits work because the principles are pretty different from other amplifier circuits. (I do have a background in analog electronics, but not many people work closely with inductors and transformers anymore, unless building switching power converters.)
A Class-AB amp will work great for the vertical deflection (and I think that's what is there now) but the horizontal is too fast, with too high of a voltage during retrace to generate easily. I'm sure it's possible, but it would run pretty hot.
But studying these old circuits you come to admire the cleverness of what people came up with with very limited means. With just a single switching transistor, the sawtooth generates itself! While the transistor is on, you get +Vcc across the inductor, which creates a linear ramp in current. When the transistor switches off (retrace) that current has to go somewhere so it charges the capacitor (C12) and rings for a half-cycle, during which time the direction of the current reverses.
The DC offset comes, I think, from the losses in the system. If there were no resistance anywhere, the ringing would be perfect and at the end of the retrace period, the current in the yoke would be exactly equal and opposite to where it started. But some of it is lost to resistance (heat) so it ends up with a slightly smaller current in the negative direction compared to the positive. Averaged over time, the net result is a DC offset. It's not huge, maybe 10% of the total, but as you say it's heat that we don't need!
This doesn't happen in the regular SE/30 analog board because of C15 (nonpolar 3.9uF cap), which blocks DC current. But (as I understand it), to use a cap in my simplified circuit, the whole operation needs to be driven from something resembling a current source rather than a voltage source. I think the way it works on the analog board is that the flyback primary is a big inductor, probably bigger than the yoke, so Q2 does effectively see a constant current from it. But the full operation of the transformer is still a bit confusing. And in any case, the whole point is to get rid of the flyback, so I'll need to find a different solution.
Interesting idea on the HV caps! Sounds possibly a bit hazardous though.
A Class-AB amp will work great for the vertical deflection (and I think that's what is there now) but the horizontal is too fast, with too high of a voltage during retrace to generate easily. I'm sure it's possible, but it would run pretty hot.
But studying these old circuits you come to admire the cleverness of what people came up with with very limited means. With just a single switching transistor, the sawtooth generates itself! While the transistor is on, you get +Vcc across the inductor, which creates a linear ramp in current. When the transistor switches off (retrace) that current has to go somewhere so it charges the capacitor (C12) and rings for a half-cycle, during which time the direction of the current reverses.
The DC offset comes, I think, from the losses in the system. If there were no resistance anywhere, the ringing would be perfect and at the end of the retrace period, the current in the yoke would be exactly equal and opposite to where it started. But some of it is lost to resistance (heat) so it ends up with a slightly smaller current in the negative direction compared to the positive. Averaged over time, the net result is a DC offset. It's not huge, maybe 10% of the total, but as you say it's heat that we don't need!
This doesn't happen in the regular SE/30 analog board because of C15 (nonpolar 3.9uF cap), which blocks DC current. But (as I understand it), to use a cap in my simplified circuit, the whole operation needs to be driven from something resembling a current source rather than a voltage source. I think the way it works on the analog board is that the flyback primary is a big inductor, probably bigger than the yoke, so Q2 does effectively see a constant current from it. But the full operation of the transformer is still a bit confusing. And in any case, the whole point is to get rid of the flyback, so I'll need to find a different solution.
Interesting idea on the HV caps! Sounds possibly a bit hazardous though.