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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