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  #1  
Old 08-21-2014, 01:31 PM
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Zenith26kc20 Zenith26kc20 is offline
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Another vertical output transformer????

Everyone keep you fingers crossed! My 25MC33 is going to get it's FOURTH vertical output transformer tonight! The original shorted around two years ago. Got another Zenith replacement (really a beat up one!) that failed shortly (I did expect that failure from it's looks). A Thordarson from Moyers that went a little over a year. Now another Thordarson is going in. This one is really interesting! It is the only cloth wire of the four. It is also a Zenith! Thordarson covered the Zenith number with black paint!
I've checked all the voltages when the set is running and replaced the 6HE5 just to be safe. From looking at other postings, it looks like Zenith had serious problems with these transformers.
The set is used only on Friday and Saturday night. The second Zenith transformer "exploded" internally according to the transformer rewinder I sent it to. He said he couldn't rewind it and then said "no more vertical output transformers".
Wish me luck!
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Old 08-21-2014, 01:40 PM
DaveWM DaveWM is offline
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any caps in the primary of that circuit?

hmmm never mind I was thinking of RCA, had a .001 2kv across the primary, I see zenith does not.

I smoked a vert out not to long ago on a 23XC36, also had the replacement with the black painted out part number.

Last edited by DaveWM; 08-21-2014 at 01:50 PM.
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  #3  
Old 08-21-2014, 01:53 PM
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There is a screen grid electrolytic (40mfd@475V), a cathode electrolytic (100mfd@50 volts). The rest are the feedback/shaping caps. I've ESR'ed the electrolytics and replaced the integrator. The feedback caps have been replaced a while back. I've checked the old transformers after a couple of hours running and it's not hot.
The rewinder also noted the wire used in the transformer is not exactly a real high voltage type wire. I guess when these were made, cost was a bit of an object.
Also, when running properly, vertical is very stable and has a full screen. I thought if the boost voltage was high it could cause a problem but the transformer/output tube is fed from 390 volts.
Funny thing is my other Zenith has a 94-2345 and it failed exactly the same way. They short to the convergence winding, the one to ground. I plugged the old thordarson primary into 120 volts AC to see if the transformer would overheat but it say all day hooked to my scope. I did find out it is a 10 to one ratio for just the vertical output part.
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Old 08-21-2014, 01:56 PM
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Boy talk about coincidence, I need one for my 25MC30. Where'd you get this latest one, Moyers also?
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Old 08-21-2014, 02:17 PM
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yep same here short to convergence circuit from primary, bet they are RIGHT next to each other.
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Old 08-21-2014, 02:56 PM
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ESR is a poor safety test for original capacitors in a tube set because it does not check for leakage at full working voltage. To properly confirm the safety of those caps you need a Heathkit C-3 or one of the other eye type capacitor checkers of the time period.

I recently unwound the primary of a late 60's Zenith vert. Xfomer out of a monochrome set, and was surprised by the lousy (thinner than paper and brittle like onion skin) insulation between layers of the primary, and more surprised that they used the exact same crap between the primary (connected to B+) and the secondary (connected to ground)....I'll give you one guess what the failure mode was...
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Old 08-21-2014, 03:19 PM
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Agree that testing equivalent series resistance(ESR) has no relevance to high voltage leakage(insulation break down).
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Old 08-21-2014, 05:10 PM
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Quote:
Originally Posted by Electronic M View Post
...was surprised by the lousy (thinner than paper and brittle like onion skin) insulation between layers of the primary, and more surprised that they used the exact same crap between the primary (connected to B+) and the secondary (connected to ground)....I'll give you one guess what the failure mode was...
You should see the vertical dynamic convergence x-former out of a CT-100, thousands of volts on those it's no wonder they all fail eventually. Jeff the gentleman I bought my set from had a whole series of pictures burned onto CD-ROM of him taking the thing apart, and one where it clearly arced through from primary to secondary. I really should upload them.
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Old 08-21-2014, 07:35 PM
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You'd think that a special transformer for a flagship product that already was VERY expensive would be built better....
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Old 08-21-2014, 08:48 PM
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It's been my experience that frequent failures like this are more likely due to external factors like capacitors or power supply problems
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Old 08-21-2014, 09:38 PM
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My Zenith 25MC33 roundie is on its third vertical output transformer. The previous two shorted to the convergence winding. This is definitely a weak point in these components. The vertical on my Zenith has never been quite right. It frequently looks like it is going to collapse and sometimes does for a second or two and then comes back. I have not figured out what the cause is yet. I wonder if there is a way to have a more reliable replacement made for these vertical output transformers.
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  #12  
Old 08-22-2014, 04:11 AM
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Hello, my 2 eurocents worth about vertical output transformers, if I can...

I have no experience with color TVs, but I have a 1954 27" TV that burned it's vertical output transformer more than once. When I unwound all the primary winding (in reality, it's an autotransformer coupling a 6SN7 tube to the yoke), composed of 6000 turns of 0,10mm enamelled wire, I found many spots of verdigris on the interlayer paper, not on the copper wire, and some arcing that burned the winding open. So, last time it happened, I replaced the paper insulation with mylar tape, the Yellow tape used in SMPS power supplies. This tape is quite thin, 0,05mm, but it should offer more dielectric strenght (it should withstand about 1Kv between layers no problem) and it's not water-absorbent, unlike paper. Ok, it does not look anything like the original paper insulation, but until now, it works like a champ.


Cheers

Giulio

Last edited by Giulio Maiocco; 08-22-2014 at 04:16 AM.
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  #13  
Old 08-22-2014, 08:50 AM
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Quote:
Originally Posted by compucat View Post
I wonder if there is a way to have a more reliable replacement made for these vertical output transformers.
This is what I'm thinking as well. Or, if there's some other type of replacement that's more robust that can be subbed in.
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Old 08-22-2014, 09:18 AM
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Quote:
Originally Posted by Giulio Maiocco View Post
Hello, my 2 eurocents worth about vertical output transformers, if I can...

I have no experience with color TVs, but I have a 1954 27" TV that burned it's vertical output transformer more than once. When I unwound all the primary winding (in reality, it's an autotransformer coupling a 6SN7 tube to the yoke), composed of 6000 turns of 0,10mm enamelled wire, I found many spots of verdigris on the interlayer paper, not on the copper wire, and some arcing that burned the winding open. So, last time it happened, I replaced the paper insulation with mylar tape, the Yellow tape used in SMPS power supplies. This tape is quite thin, 0,05mm, but it should offer more dielectric strenght (it should withstand about 1Kv between layers no problem) and it's not water-absorbent, unlike paper. Ok, it does not look anything like the original paper insulation, but until now, it works like a champ.


Cheers

Giulio
That's about .028 USD and worth much more. Thanks Giulio
The 1200v P-P pulse on that winding, and no option to lift/isolate the ground makes me very nervous.
But many of these transformers stood a test of time, when they were young.
So its likely to be a deteriorating insulation condition and we should investigate a rebuild like you say.
Dave G
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Old 08-22-2014, 12:18 PM
philcophan philcophan is offline
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I'm not sure if this applies, but... older transformers had sulfur impregnated paper which had (I'm told) the end effect of eating into the copper... again, I may be all wet with this comment and YMMV for what it may be worth.

Jim
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