66 zenith high voltage dies after 30 minutes
 

I figured id post this looking for ideas before I get any deeper into it. Its a combo and these thing are not easy to work on.

25mc36 chassis 1966 color.

After 30 minutes the picture gets yellowish and in seconds fades away. The high voltage drops to nothing, then slowly comes back up but only to about 15kv. Once the set completely cools off it will work great for another 30 minutes.

The HO tube drive remains the same as does the cathode current. I removed the HV regulator tube, no change.

I have subbed the HO and most of the others in the horizontal HV circuit with no change.

Any thoughts or experience would be appreciated.

http://boxcarcabin.com/25mc.gif

I would check to see if you have over drive on the CRT runing down the high voltage. Your high voltage may be fine and the issue could be a video problem. Try pulling the video when the set has this problem to see what the high voltage does.

Forgot to mention, I unplugged the socket off the back of the crt, no change.

Hows your B+ (340v) feeding into those circuits. Yea I know, other circuits use that voltage too but make sure it's ok where you suspect and trace back if need be. And does the B+ Boost drop too along with the high voltage? Anything getting warmer than normal? It does seem that something is loading down...

And you say the image gets yellowish...something in the blue drive or screen loading down??

It sounds like possibly excessive beam current in the CRT is dragging the HV down. This can happen if the G1s in the CRT go excessively positive. Could you post the part of the schematic showing chroma outputs, vid output and CRT? Does the 3AT2 get real hot when this happens?

You may already be familiar with a problem chronic to RCAs of the CTC-15, 16, 17, and 25 lineage. Two ground lugs on the far right end of the chroma board go open, killing the heaters of the 6GU7s, causing their plate voltage to go sky high, which is direct-coupled to the CRT G1s. Beam current skyrockets, dragging the HV down, causing a symptom like you describe in your Zenith.

Or maybe it's something else altogether.

Bill(oc)

WOOPs cancel that. Didn't see your post about unplugging the CRT.

Ill check the boost. Its like something 'pops' and the picture looses all red and gets yellow and maybe 5-10 seconds later the hv drops. (I dont actually hear a pop)

The hv regulator tube is located in front of the flyback can and being a combo I cant really tip it up on end. Its a tough one to get to.

Its hard to tell if anything is overheating.

Does the picture bloom off the screen or just gets darker and then black?

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If the horizontal is still running fine, and the HV isn't being loaded down, I wonder if that 1.5K @1-watt resistor (R106?; can't quite read the middle digit) is going up in value as it heats up? It's the only thing I see that could do that; assuming that it's NOT a load-down problem and that the horizontal oscillator and output are running, and that you've swapped out the rectifier.

Charles

Yes it blooms out. I have a hv probe so I can watch it. The hv also goes away at the reg tube so I dont think its that resistor.

Im going to go dig in right now for a few hours. Ill get some more info posted soon. Thanks again for the ideas. Ill check them out.

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Yes, ok just checked it out and it appears to be the HV rectifier. The boost voltage went up a bit and I got a good 1" arc off the cap of the rectifier when there was no hv on the cathode.

This time I got a good 30 seconds of color bars and funky colors before it died. Maybe the tube is arcing inside before it dies.

I cant see it glowing even when its working so I cant tell if the filament is going open. I let it run in my tube tester for an extended time and it didnt fail.

What a trip, I never would have expected a tube like this to fail after running for awhile. This is the one tube I dont have a replacement for, just got one coming off ebay.

Well....not so fast. I found another 3at2 new in the box, put it in and the same thing happened. This time I could see the filiment glowing and it went out after about 15 minutes.
The inside of the tube was glowing blue so the plate is hot.

Whats going on here....is there an open in the filament supply turns on the flyback? Thats like 2 turns of pretty heavy high voltage wire how could that ever go open.....

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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.

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......

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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.

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.

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

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.

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.

http://boxcarcabin.com/DSC03182.jpg
http://boxcarcabin.com/DSC03186.jpg

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?

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. :scratch2:

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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.

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Believe it or not...
 

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.

Perhaps you could place a few paper towels around the lower part of the flyback, and simply spray some 409 on the filament winding. The paper towels under there will catch the drippings and keep from making a mess. Might also try using a cotton swab on the wire after spraying the 409.

BTW, that's a very nice looking set! :yes:

Yuck, Imagine what his lungs look like... :no: That crap probably has been absorbed by the old HV wire insulation and cannot be cleaned out. A source of suitable HV wire can be had from junked computer CRT monitors.

On the set I had changing the tube solved the problem; no other trouble from the crt.

This will be a good lesson for all of us...whatever the problem ends up being, we all will know to look for it next time!