Yea if you've recapped, and even changed chassis... Its sounding to me like your blue gun inside the tube itself is shot. You didn't mention the neck board however, so if you haven't yet messed with that I'd give it one last try.
This is just stating my opinion here but... The Nanao 293x is a hunk of shit CRT, prone to faults and hard/expensive to repair. Best decision I ever made was to toss the entire thing and replace it with the Wells Gardner. That freaking chassis is built like a tank! Downside is no trisync, but who give a fu*k about anything but 15khz anyways?
Do keep the original Sega Nanao frame, you'll need it to mount another tube per 300wins (thanks man) back mount method.
As you can see I used some washers to help push the tube to the proper depth, depending on the tube you use adjust accordingly.