Sounds like a blanking problem. Try the tube first ( 6KT8 ? ).
For grey scale on almost any tube chassis set
After warm up service switch to service & off channel.
Bring up one at a time then back off til the line just disappears.
Go back to normal & tweak if needed.
Be sure you can turn down the bright, contrast & color to get
little or no raster. remember these sets were NOT brite. To enjoy the
best pix use in dim lighting.
73 Zeno
LFOD !