Working on a Zenith round tube TV. Fired it up for the second time. After it ran for about 30 minutes we heard a noise. I saw a blue arc inside the high voltage cage. After opening the cage it appears the arc was jumping from the plate cap lead of the 1B3 tube to the inside of the cage where the flyback live. Why is this happening and what can we do to eliminate this problem? Thank you.

Well, I'm no expert, but I'd imagine whatever the plate cap is connected to is probably open. So that should be the flyback, right? Bad flyback?

If you have light on the screen (heck even arcing off the 1B3 top cap) the flyback is fine...

Run the set with the cage off to determine exactly which to points the arc is jumping between and report back.

If it will not arc with the cage off because it is arcing to the cage then you need to clean the surfaces and area around the arcing points...Eliminate any sharp/pointy edges at the arcing points and possibly add insulation.

Pictures of the places it is arcing from and the condition of the HV cage and it's contents of will help in providing less general advice.

Maybe lots of dust on top of the 1b3 or the cage .

The arc I saw and heard while looking through the holes in the top of the cage were going from the top of the tube or the lead to the nearest metal which was the inside of the cage. A distance of not more than one inch. We've since opened the cage, removed the tube for testing and will replace it with a known good tube. The cage, chassis, everything is very clean. This was a very clean chassis from the beginning. I cleaned it thoroughly and made it shine. Dust, well there might be a very thin film of dust on everything since we hadn't touched this TV since June. Can't imagine that would create the intense arc I saw and heard twice. The fuse at the bottom of the cage you see is still intact. Other than replacing the 1B3 with a good tested one, what else should be check before we attempt the next fire-up of the TV? Thank you for your previous reply.

There are tall 1B3’s and short ones, you may need a short one.

If the arc was going to the side of the cage (instead of the top), it wouldn't matter whether the tube is tall or short.

The arc is not going to hurt the tube, as the tube is 'downstream' from the arc's current path, "out of the loop" so to speak. But the flyback sees the full overload current of the arc.

If an arc were 'downstream' from the tube, then the tube would carry the current. It would be a DC arc producing a very loud 'SNAP!' or series of snaps.

From the looks of the insulated wire, I'd say a new wire is needed. Do not use standard wire. This wire has to be insulated for nothing less than 15,000 volts.
Another temporary repair would be corona dope.
Bill Cahill

I agree with oldcoot and Bill.
If the tube was good before the arcing,then it should still be fine. The arcing from that point should not hurt it.
I would probably replace that wire with a 15-40KV rated wire....In lieu of that corona dope is one option, but I'd first try to put as many layers of heat-shrink tubing over the wire as I could. The tubing I get at Ace Hardware is 600V rated and some brands may be rated higher. Two layers of 600V insulation is 1.2KV extra insulation 5 layers is 3KV and so on.

Three other observations: Dress the lead as far from the cage as you can. It looks like the grab fingers/terminals on the 1B3 top cap/spring have been pointed at the side of the cage....You probably want them pointed away as they could form an arc point. Have you measured the HV, and compared it to spec?...If the HV is substantially higher than designed insulation may break down, and arcing may occur.

It looks like the grab fingers/terminals on the 1B3 top cap/spring have been pointed at the side of the cage....You probably want them pointed away as they could form an arc point.Yes, there doesn't appear to be much clearance there.

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How close is that end of that screw that protrudes into the cage above the 1B3 to the 1B3 plate cap/clip fingers? It looks like it could possibly be a point for corona/arc generation? :scratch2:

jr