Many ways to go depending on equipment & knowledge.
Its not a vert problem. That shrinks the pix top to bottom
sometimes so there is only a line going L-R.
Most video problems on old B&W sets leave you with a lit
screen so probably not that.
No HV will kill the pix. If audio good most likely its the HV
rectifier tube. IIRC its a 1X2 the tube the anode wire goes to.
On GE's the sockets used to corrode, check for green crap
on the tube pins & socket. You can usually detect HV by
putting the back of your hand near the SCREEN, you will feel
the static pulling on your hairs. BTW the socket can give you a shock
when off. The cap on the other end is safe when off.
A bad CRT is also possable. Run the set for a while in a room
with NO light. If you see a very dark image its probably a bad CRT.
Beyond that post the model # ( stamped on back).
Chassis # ( look on tube chart) and maybe a screen shot of
the chassis & a lot of us will try to help.
73 Zeno
73 Zeno