Brightness fluctuating

[vol.2]

My Toshiba CX1467 is working correctly except that it is varying in brightness when it gets rapped at all.

I noticed this when I moved it recently; I turned it on and then was adjusting it's position and I jostled it a little bit in the process while it was on.

When that happened, the raster all of a sudden got darker. Hitting it again seems to make it brighter again, but it's as if something is loose. When I whack it repeatedly on the side of the case, it can get kind of darker, or very dark or light, but if I keep on rapping on it, it eventually returns to the original brightness.

I'm worried this could be a problem with the tube itself, although I do have a replacement tube in storage I could try, I would rather not use it if I don't have to of course. It's also very non-trivial to retrieve said replacement tube, so I'd like to exhaust some tests on the chassis parts before I commit to hauling it out.

I opened it up and knocked on the back of the neck board (right in the center of the socket) while it was on and I was able to replicated the issue.

I reflowed the solder on the neck board, checked all the resistors and Deoxited the service switch. I put it all back together and turned it back on and the problem seemed to go away at first, but it returned after the set had been running for just a little bit.

I don't think it could be one of the transistors or the bias/gain controlls because the whole image gets darker when I rap on it, it doesn't seem to effect the color balance.

I do have another chassis here I could swap in the neck PCB for, but the main PCB of that chassis needs a huge recap to get working, so I don't really want to do that.

I though perhaps it could be the CRT socket, but it looks alright and I did clean it out with contact cleaner. I suppose if it's super loose maybe, but it seems okay. It's hard to know if it's a connectivity issue in the socket if it only happens when it's inserted.

Any ideas?