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  #1  
Old 01-18-2010, 11:16 AM
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Zenith26kc20 Zenith26kc20 is offline
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Vertical Output failure on Zenith Roundie

Replaced the vertical output on my 24MC33 this weekend. It appeared not to be the transformer (badly stretched vertical, rolling, popping in sound in sync with dots on the screen) but it was... Transformer 95-1239. The new one was used but was perfect on turn on. Checked voltages and waveforms, all fine. No overheating, set was great. Did notice convergence was out so reconverged. Watched it for a few hours, no problem.
Saturday nite, watched the game, spectacular picture! 5 minutes after the game, same problem!
Winding "JK" shows 500 megohm leakage on my megger to the primary (the one with b+ on it). Removing the two wires to the winding makes the vertical normal again but convergenge is poor. Checking the voltage on the "JK" winding shows B+ and pulse present so it is leaking (shorted) to the primary.
I am sending the original transformer to a transformer manufacturer I have used in the past with excellent results. My question is, do the roundie Zeniths share a transformer chassis to chassis (the replacement had a note showing how to use it in another application)? If so, I could get more than a couple made (if they can reproduce it) with improved insulation to prevent this problem.
Maybe this problem can be cured!
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Old 01-18-2010, 12:56 PM
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Aww, that sucks. Is that the set from me? I don't know what would cause that, maybe the used replacement was about to go too. Something in that part of the circuit might be loading it down maybe? Hopefully it's only the transformer, I'd hate to see it cook a rebuilt one. Good luck with it. I would send my spare but already gave it away a while back.
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Old 01-18-2010, 01:08 PM
DaveWM DaveWM is offline
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Does the vertical output tube have a cathode bypass? the reason I ask is I had a RCA roundie that had the same short, Primary to secondary on a convergence winding ground. The tranny looked like it had been slow cooked.

The bypass cap was leaking badly, my guess is the leaking cap upset the bias, too much current flow thru the tranny, heat builds up and breaks down the insulation. I don't know if there is a spec on the current flow of a VOT like there is on a HOT but if so I would check it out.
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Old 01-18-2010, 02:02 PM
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Just got a call from Moyers. Another one is on the way.
What makes me think the transformer is the cause is the way it failed. It seems the primary punctured the insulation from the primary to one of the secondaries. The transformer runs cool so current is doubtful. The 6HE5 appears fine also, no red plate or sign of distress.
Linearity is good also. I think Sams photofacts have the Vertical output current listed. I'll look and check it. Once the transformer builder gets into it we'll know what actually happened.
Don't worry Mark, I'll bring the critter back to life. At over 40 years old, anything can happen!
I've grown quite fond of the three color oldies I have. I like the Roundie best!!! Well worth keeping them going!
Has anyone used the Thordarson replacements? That's what is on the way.
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Old 01-18-2010, 02:12 PM
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Zenith26kc20 Zenith26kc20 is offline
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Just looked at a Sam's Photofact. It was for a 25NC38 (closet Sam's I have). The circuit appears nearly identical. It lists vertical current at 38 milliamperes. I will have to wait to check it as the Zenith is in my Mississippi weekender. I would have brought it home but my better half (who helped me get it up the 14 steps (over ten feet up in the air) said after she helped me "never again".
She does like the picture on that roundie though!
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Old 02-01-2010, 12:15 PM
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Put the new transformer in. The parameters are nearly textbook values. I did check the electrolytics and the ESR fine. Cathode current measures around 36milliamperes.
Ran the set eight hours and grabbed the transformer. It was just warm to the touch. I'm waiting to see what the transformer rebuilder says happened to the old one.
Now to get Maggie up and running....
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Old 02-01-2010, 01:57 PM
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Great! I hope it works fine this time. I'm interested in hearing about the old transformer myself. Good luck on the Maggie!
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Old 02-02-2010, 10:27 PM
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Question

Any news on what killed the old tranny? It's probably too soon to ask huh?
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  #9  
Old 02-03-2010, 08:40 AM
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No word yet. They do very small batches if requested. The problem is, the smaller orders take a while. Their engineer is a friend of mine and he's going to dissect the old one to try and find out.
As soon as I get word, we'll all know. Hopefully he can duplicate the transformer with better insulation to prevent this from occuring again.
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Old 02-03-2010, 10:51 AM
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Is this a company that you can tell us about in case we need to have something similar done?

John
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Old 02-03-2010, 11:28 AM
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The company is Transformer Manufacturers Inc. They are in Norridge, Illinois. I have had a lot of work done by them. Small batches are a bit expensive but if the transformer is unavailable they are the way to go.
Awhile back, a big (750 watt, no kidding!) tube amp killed it's power transformer and the company that made it wouldn't sell me one. TMI made me a replacement that worked and actually ran cooler than the original.
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Old 02-03-2010, 12:04 PM
ctc17 ctc17 is offline
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They can take one apart and rewind it? That would be a good thing as transformers for these old sets are becoming impossible to find.

I dissembled a zenith vert out transformer with a primary to secondary short and the insulation between the two winding was very poor. It was like one turn of clear tape and it had a little hole burned through it. Looked to me like they were relying to much on the wires varnish, theres a fairly high potential between those windings and there should have been more tape.
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Old 02-03-2010, 01:22 PM
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They have reverse engineered a number of transformers for me. One of the strangest was the start transformer for one of my Infinity SWAMPS. They have been in business for a long time and apparently have made almost every transformer imaginable over the years.
It seems to me that Zenith put the plate connection on the inside part of the winding. This has 1200 volts signal (vertical) and 390 volts DC. When mine failed, it appears that the puncture in insultion would cause the arcing that I noticed, but no sign of distress with the 6HE5 pulled. It also makes sense with JK unhooked. I noticed a set on EBAY that the owner says gives a picture with the convergence panel unplugged. Unplugging the panel disconnects JK so that set may need a transformer too.
It the winding had the plate end outside, the only thing between windings would be 390 volts.
The engineer at TMI said the insulation may only be 3 mils thick (the nearly transparent tape). No wonder it may get into trouble....
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Old 02-03-2010, 06:42 PM
ctc17 ctc17 is offline
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On mine, the center winding was plate with the inner most being B+ the the outside of that winding being plate, tape, convergence windings the on the outside the yoke drive winding.
I would be curious to know about what it cost to get one rewound? I wonder they could do flybacks...
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Old 02-03-2010, 09:50 PM
3Guncolor 3Guncolor is offline
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I would be interested in getting an upgraded new unit if the cost was not too high. I feel on Zeniths the vert output TX is a weak spot. I've had several fail just like listed here. Even had one go bad on a 71 25" Zenith "B" line chassis.
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