Tektronix TDS 754D machine gunning

[okw]

Hi! I just acquired a Tektronix TDS 754D with a known faulty display (CRT). They tech salvage company I bought it from said it worked on external VGA. They probably just quickly tested it with a VGA and turned it off again.
I powered it to warm it up at little while I was getting my external screen. Suddenly (3-4 minutes) I heard sparking machine gun sounds. Not quite that loud but still, like some piece of paper was caught in the fan, more more high pitched. I quickly turned it off. By then I felt the smell. Something was definitely happening electrically, like arcing or something. I opened it, found no brown spots, no bursted caps, etc.
It's not too dirty, but some build up here and there with that fine gray dust powder, and some bigger bunnies here and there.

Any ideas?
Done a lot of live stuff, but I've never worked on CRTs, so I'm a bit worried.
I'd be happy to disconnect and use external screen. Or remove the CRT all together, unless they are easy to fix?

[TERRA Operative]

Disconnect the ribbon cable from the top PCB on the CRT side of the unit just in front of the fan above the disk drive.
This will completely isolate power and signals to the CRT board and CRT (but not affect the VGA output).

If you got a bad CRT, it can arc internally. I have a TDS600-something in my collection that was dropped by a previous owner, cracking the neck of the CRT from the shock, and it made similar zappy noises. Luckily I had a spare CRT, so the scope is all working happily again.

[okw]

Thanks. Would it also be possible to disconnect the high voltage circuit too? Or is part of this also powering VGA?

[TERRA Operative]

Disconnecting the connector I described will totally isolate power to the CRT and CRT PCB, including all HV circuitry.

[Jwalling]

My money is on the flyback transformer. It's arcing to the sheet metal that the CPU board mounts on. I've seen this many times.

[okw]

Which ones would that be? I'll probably see it when I plug it in :p

[TERRA Operative]

That's your main PSU board.
The CRT board is to the left, the flyback transformer is the largeish black thing to the rear of the CRT board.