66 zenith high voltage dies after 30 minutes

[ctc17]

Originally Posted by andy

I would check the rectifier socket, soldering, and wiring. Sometimes there's a resistor in series with the heater that can fail.

I took the socket all apart thinking there may be a resistor and theres not. The heater circuit is just 2 turns of wire around the core. The socket looks new.

I even ran a 2 amp load on the wire using a 12 volt battery and auto bulb for 30 minutes and it never opened up.

I put it back together, enough radiation for today. I have an old flyback I may pull that winding off and try it in the next day or two.

[Zenith26kc20]

I would isolate the anode cap (discharge it first, then carefully remove it from the CRT) in a plastic container above and away from anything (to prevent arcing). Leave the HV probe connected and the regulator tube connected. Watch the HV probe to see if you still have high voltage after the time it would take to fail. I had a 26KC20 a long time ago that developed CRT leakage and would do this trick after it warmed up and had a bright scene. I replaced the CRT after doing this test and it remedied the problem. It's the only time I ever saw that happen. I always wondered what happened in the CRT to cause that.
Hope my experience was very isolated but you are beginning to describe it......

[andy]

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[drh4683]

Zenith reported a strange loss of high voltage symptom on the 26KC20 chassis to be caused by a gassy 6HL8, 1st color amp tube. I think the 25MC36 uses a 6KT8 for that function, but its worth a try anyway. I thought there was another case history where HV was lost caused by some other fault in the chorma circuit and it was a tube also if I remember correctly such as a demodulator. This is possible since your set has a color problem just moments before the HV is lost.
I'd swap out those tubes immediately after the bloom out and see if operation is restored. These Zenith tough dogs are the best ones to figure out.

[ctc17]

Thats interesting! How could something in the color circuit basically cause the heater in the hv rectifier to cool off?
I'm going to change them. I wouldn't be surprised if it fixes it, although it hasn't died sense I put it back together.

My work light is one of those compact florescents, and with the door open on the hv cage that thing lights up 6 inches away from the radiation. That cant be healthy!

Thanks again for all the input. I love this combo unit. Its so well built its just nice to look at.

[Charlie]

If you get a chance, post a photo of the console. I'd like to see what this thing looks like.

[bgadow]

The work light is probably just lighting up due to HV AC, same reason you can test for HV with a neon bulb.

I had a CTC-31 that would go bezerk, it would lose HV with a pronounced "thump", the neck of the crt would glow purple, ouch! Turned out to be a bad video amp tube. I don't know why, I just know that was it.

[ctc17]

It has one other sightly annoying problem. A tube or something has become vibration sensitive because whenever there is any kind of bass in the sound it can be seen in the video. I know its related to vibration because I can bypass the power switch and use the stereo part and I get the same thing. Tap on it or pound on the floor same thing.

[old_coot88]

Originally Posted by andy

... The HV is connected to the guns, so a piece of metal, or flake of dag could short out the HV.

Just curious, but could you elaborate a bit more on the HV being "connected to" the guns? And are you saying the aquadag coating is "inside" the CRT?

[Jeffhs]

Originally Posted by bgadow

I had a CTC-31 that would go bezerk, it would lose HV with a pronounced "thump", the neck of the crt would glow purple, ouch! Turned out to be a bad video amp tube. I don't know why, I just know that was it.

What happened to the CRT when the video amplifier tube went bad? If, as you say, the neck of the CRT glowed purple, I would almost suspect the defective video tube had somehow caused the CRT to become gassy. Did the set work after replacing the tube or did you have to replace the CRT as well? Hmmm.

[andy]

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[wa2ise]

Originally Posted by bgadow

I had a CTC-31 that would go bezerk, it would lose HV with a pronounced "thump", the neck of the crt would glow purple, ouch! Turned out to be a bad video amp tube. I don't know why, I just know that was it.

The circuit was probably designed to be DC coupled from that bad tube to the CRT. This is designed to maintain a dark scene without the blacks going gray. Many B&W TVs did go gray with scenes like that because the CRT was AC coupled to the video amp tube. So the bad tube would mis-bias the CRT, and possibly cause that purple glow.

[andy]

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[TV Engineer]

The "2 turns of heavy wire" on the flyback for the rectifier filament has been know to leak HV to ground. The result is exactly what you describe, and I have seen a few times before. You would think you'd hear it arcing, but usually not.

Replace the HV rectifier filament winding. I'd bet that cure the trouble.

[ctc17]

Originally Posted by TV Engineer

The "2 turns of heavy wire" on the flyback for the rectifier filament has been know to leak HV to ground. The result is exactly what you describe, and I have seen a few times before. You would think you'd hear it arcing, but usually not.

Replace the HV rectifier filament winding. I'd bet that cure the trouble.

Thats interesting. I think the previous owner was a smoker and there is a lot of cigarette juice on anything hv. The hv rectifier was totally dripping wet with the stuff.
I cleaned most of it up because it was causing everything to leak and hiss, thats the one wire I cant get to without removing the flyback.