Flyback Failure

Yes,you do need vert. sweep to see the keystone effect.You may
be able to hotwire the vert.sweep section to the low voltage power
supply to get at least some vert. height so you can see a raster
and get a better idea of what's going on.It sounds odd that there
would be a .25 uf cap between the b+ line and the damper cathode.
There is usually a few thousand volts of rf on the damper cathode.
Usually the boost cap is between the damper plate and a tap on the flyback.
It's basically stacked on top of the b+ and the damper tube charges it up.
I would definitely change those caps.If you could post a partial schematic
of the horiz. sweep section that would be helpful.This sounds like a
very old flyback setup that uses a real transformer instead of the
more modern autoformer type flyback.The damper tube may not
normally have that much rf on it at all.The boost voltage may not
even be that much higher than the b+ normally either.Also check
to see if you have a doorknob type capacitor under the 1B3 socket
or somewhere else hanging off the hv line.These caps are known to get leaky or short out.
Regards,
Swanson