CTC 7 Question

[droptop]

Hey everyone,
What would cause an arc inside a 21CYP22A? My beloved CTC7 Grenoble developed this out of the blue after near flawless operation over the last 3 years with no warning. After exhaustive testing and troubleshooting I’ve located the arcing sound by removing the metal crt cover and running the set face down with the crt and HV compartment open for visual inspection. It is definitely coming from inside the crt which tests full scale 100% good on all three guns with no internal shorts indicated. The trouble started when I happened to notice an occasional vertical roll as a video disturbance flashed across the center of the screen. Upon investigation I found that turning the brightness up caused an arc that was audible but not visible and the picture would show visible noise that we’ve all seen as an indicator of arcing somewhere. Turning the brightness higher causes it to get worse along with lowering the HV and focus voltages. At a slightly less than normal brightness level it makes a beautiful picture.

Every voltage and current reading is correct throughout the B+, audio, vertical and horizontal circuits. Having said all that, does anyone know the cause? I’ve been told that connecting all elements together at the base and impressing a high voltage (2K) between this and the second anode might burn out whatever is causing the arc. The internal dag coating is in place showing no signs of “letting go” around the ultor connectors. If I did this procedure which anode button would be used?

I’m really hoping someone here knows about this. The CRT in the set is in absolutely pristine condition from a visual as well as emissions standpoint and makes a great picture. I hate the thought of losing a tube this good.

Thanks for any input the community might have.
Rick

[mstaton]

Is it arcing in the neck? Tube may be a little gassy. Look for purple glow. Is the CRT grounded well?

[droptop]

The arcing sound is in the vicinity of the anode connectors, not in the neck and the sound, although hard to describe is distinctly inside the tube. There is no blue glow at all and no signs of gas. Emission of all guns is at 100%. The CRT and metal shield are well grounded throughout. All copper strips are clean and tight. The arc was audible but muffled until the metal CRT cover is removed and then becomes quite audible but not visible.

[mstaton]

Probably best to clean around that area thoroughly with some alcohol.

[Zenith26kc20]

I agree with cleaning. My Zenith NC chassis was mildly arcing on turn on. All looked clean and the rag came away relativly clean but the arcing stopped. Can you run that set with the shield off in a dark room to look for any problems?
What appears on the screen when it arcs?

[marty59]

When it arcs over like that, is it also noticable in the neck? Are the focus and high voltages consistant/regulating okay?

[droptop]

Sorry for the delayed responses. I have run the set in a dark room with the shield removed and there is nothing visible. There is nothing in the neck of the tube when it is arcing. Setting the brightness just below normal displays a good picture. As the brightness is turned up slowly a small tear flashes across the center of the screen and the picture will roll one frame. Turning it further up causes the center of the picture to break up as it upsets the horizontal and vertical oscillators with the usual results. It will ultimately bloom. If I can get a screen shot I’ll post it. The HV regulates fine and current readings are within range even when it starts to bloom. Everything in the horizontal circuit from sync takeoff to output checks out correctly. I plan to pull the CRT completely out to check for cracked aquadag and anything else I can find. This CRT tests as new on all guns. I may ultimately re-assemble it and leave well enough alone since it doesn’t seem to affect current readings and the flyback runs without signs of overheating. Thanks guys and wish me luck.

[Eric H]

There's a resistor between the two anodes, perhaps it's arcing internally? or has changed value so much it's causing the tube to arc internally?

[Penthode]

Sorry for your disappointment. It must be great.

When you turn the brightness up, the beam current goes up. This would tend to diminish the high voltage except that the shunt regulator would relinquish current to keep the load hence high voltage constant. The fact that the arc intensifies with increased beam current is a clue.

What I believe may be the case is the internal conductive aquadag coating is cracked and the increased beam current is causing an arc to the part of the coating which may have broken away from the 2nd Anode (HV connection) The arc is caused by the increased current within the tube.

When the arc begins, does the picture shape distort slightly?

That is my guess. The fact the arc increases intensity with increased beam current is important. I expect you did not see an arc in the neck and the arc seemed confined to the bell?

And are you sure it was in the tube and not in the nearby HV components?

Terry

[miniman82]

It might not be inside the tube at all, nearly any arc can cause the sync circuits to hop like that. My CTC-4 did the same thing, I traced it back to the coating on the plastic CRT insulator being degraded. That caused stray static charges to randomly build up, which would then discharge when it approached a certain potential (much like lightning searching for the ground). Scrubbing off the dag and reapplying is the answer of course, but for now some foil attached with spray adhesive does the trick. Find your bad dag, you'll likely cure the problem. Oh, and make sure that ALL ground straps are in place, even the ones going to the trim around the front of the set.

[Zenith26kc20]

I agree. It sounds like grounds are loose or not contacting. I have had sets with marginal DAGs cause this exact problem. Subbing the tube will stop it but I think grounds are the culprit.

[Einar72]

Mine, now long gone, used to arc intermittently as well.

[old_tv_nut]

Possible the connection to one anode is open (either the lead/resistor, or the connection itself) and arcing across?

[droptop]

Hey guys,
Thanks for all the tips. I’ve changed the resistor between them with no improvement. There are no cracks or arcing visible around the anode connectors in a totally dark room with the boot and metal shield removed. All ground straps are in place and clean. I’ve suspected a dag issue that may be around the rim of the tube where I can’t see it. I plan to pull the CRT soon and give the entire bell a good once over in the hope that it is a dag issue. I have some spray graphite dry lube that dries tack free that should serve the purpose for replacing the dag. I’ll scrub off any loose stuff and re-spray it to fill any missing areas and create a good coating all over. Wish me luck. Thanks again, Rick

[kx250rider]

Anything is possible, but there is no real way that it could be arcing inside the tube, unless there is air or foreign matter in there.

Does the speed of the arcing sound change if you make the picture roll fast with the Vertical Hold pot; and does it stop if you throw the service switch? If so, I bet you're hearing the pulsing of the yoke, which has fooled me a couple times. It is a muffled arcing sound, like what you would perceive an HV arc to sound like through the glass.

Charles