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- - Zenith 25MC33 HV Trouble (http://www.videokarma.org/showthread.php?t=270622)

If you disconnect the anode from the CRT and get hi voltage, the CRT is bad.

Quote:

Originally Posted by compucat (Post 3201544)

I was watching my Zenith tonight for about two and a half hours with no problems and I leave the room for a moment and I hear this loud hissing sound. I check on the set to find no picture, and a slight burning smell. I pop the back off the set, open the HV cage and the flyback is hot but I can keep my finger on it. I turn the set back on and find the high voltage rectifier red plating. I have not swapped any tubes as it is late but I hope it is not the flyback. I restored this set eleven years ago, replacing all the sweep and High Voltage tubes in the process. I set the cathode current per the Sams and have never had high voltage problems with this set. Can a shorted HV rectifier cause the red plating or might I be looking at a bad flyback?

I have the same set and was lucky enough to grab an extra FBT. Check the boost rectifier diode. It sounds like time to break out the schematic and do some troubleshooting.

I took the second anode lead off the CRT and still no HV. At least I know the CRT is no loading it down. I do have the schematic and when I get time, I will pull the chassis and really dig into it, hopefully this weekend.

I pulled the chassis today. After partially dismantling the high voltage cage, I found the trouble. A hole had burned into the hv rectifier filament wire. I found another wire in a box of Zenith roundie parts and installed it. I also replaced the 1.5k resistor with a 1k since it is all I had. The high voltage came right back up with no arcing. I put the set back together and checked the high voltage. It was a 30kv presumably because of the resistor replacement. I turned it down to 25kv as recommended in the Sams. Now for a new problem. The horizontal hold will not lock. I suspect the afc diodes because it will operate throughout the range of the horizontal hold control but at center it will not lock in. I have been having trouble up to this point where the horizontal hold would drift especially after being on for a half hour or more and would have to be set further toward the end of the control range to bring it in. I don’t have any diodes to replace them with so I will have to wait until I can order them. I did replace the 6U10 horizontal osc, afc and discharge tube to no affect. I am just glad the flyback did not fail.

There may be more problems lurking around the flyback transformer. Generally a pulse winding on the fly back feeds the phase detector and if it is open, you will lose lock.

Can you post the schematic focussed on the horizontal afc?

I got it fixed. It turned out it was a dirty tube socket causing the horizontal hold not to lock. I touched up the convergence and a few other adjustments and now it is back as great as ever. These old sets will keep you guessing sometimes.

Quote:

Originally Posted by mrjukebox160 (Post 3201573)

I think the CRT lost vacuum.

If the CRT lost vacuum you will see a blue purple glow in the neck of the tube.

That burned resistor on the 3AT2 socket was a common problem, even when those sets were relatively new.