CRT filament lights up, but no picture?
 

I have a RCA 21t9275 black and white tv set. I've replaced ALL the capacitors(including the filter capacitors) all ive gotten the CRT filament to light but theres no picture(although there is a slight hum coming from the flyback) i think the issue might be the tube that runs the flyback but i'm not sure. but the sounds works(i tested it with a VCR)

Sounds like no HV to me. You need to find out if there IS HV--and enough of it. Unless the tube is TOTALLY DEAD emissions-wise (NOT likely.. but possible it is just "asleep")..there should be SOME raster...

If there is HV present then the CRT may have an open cathode connection. To fix this you need to re-weld the cathode. To do this you need a CRT tester with a remove G1 shorts function. What you do is press remove G1 shorts on the tester and as you press the button immediately hit the neck of the CRT with a screwdriver handle (not too hard though!). If you see a bright flash when you hit the neck then the cathode should be re-welded. Sometimes this only works temporarily before the cathode opens up again and sometimes it just doesn't work at all. Or your CRT could just be absolutely dead.

Can you pull an arc from the top of the HV rectifier tube to an ungrounded screwdriver?

The most common cause of no high voltage is a non functioning Horizontal oscillator, can you hear the high pitched whine of the oscillator, and if so, does it change pitch with the Horiz Hold control?

If you have High voltage check for an ion trap of the neck of the CRT. Rotate it until you see the picture on the screen. You may have to move it forward or backwards on the tube neck.

The 21CEP4 doesn't use a trap.