Time to back up a little...........
Remove the plate cap from the 1G3 & let it hang away from metal.
Take a well insulated screw driver & get it near the cap ( wire end).
You should get about a 1/2 inch arc, NOT just a spark if you touch
it or get very close.
Boost comes from the flyback. Its stacked on top of regular B+ voltage
( think of 2 batteries in series ) & filtered by C50 & C51. R79 must also
be good.
On your set if pin 3 of CRT goes to low it will kill the raster BUT not effect the HV.
Next is to get good HV ( that nice arc ) you need the following stages running.
1) horz osc
2) horz output
3) damper
Any defect will lower & or kill the HV & boost.
Bottom line it nice big arc = good till that point
Small arc = problem is before HV rect.
73 Zeno
LFOD !