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#1
06-17-2020, 09:34 AM
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1966 Zenith 20X1C36 failed vertical output transformer
I've been working on a restoration (with a my friend Roger) of a Zenith 20X1C36 chassis. This set was made in 1966 with a very unique and beautiful "mid-century modern" cabinet. The chassis had come a long way which included a NOS 25XP22 CRT. Just when things looked like it was up and running - the vertical sweep failed dramatically. It was clear that there was arching somewhere - both the picture and sound made that obvious.
Long story made short- the vertical output transformer shorted directly from the primary to one of the secondary windings that feeds the dynamic convergence circuit. The good news is that it is only one winding in the secondary. It took me a week to figure out all the details because the physical damage took place in a distant part of the set. That's where electrically it was shorted to ground. Once I lifted that winding out of the circuit, the set started up perfectly - minus the dynamic convergence circuit. I have looked all over the usual places for a replacement vertical output transformer but have had no success. I was wondering if there would be a way to "hot-rod" an audio transformer to one of the other secondary windings that are working, and feed that output to where the failed windings go? The impedance in these secondary windings are small 1.3 - 6 ohms in places so I though of an audio output transformer wired backwards. Any ideas?? This is the Zenith part number and the after-market replacements in case someone might have one! I'll buy it! vertical output transformer: Zenith 95-2470 Thordarson 26s60 Stancor VO-714C Triad A308X Thanks for your help! Dave P.S. The picture is before the transformer failure... 
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#2
06-17-2020, 08:51 PM
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Quote:
I wired in a small audio output transformer from a junked console stereo and it worked well with some minor value changes, but it was a single primary and single secondary arrangement. If you run into a dead end, check Hammond's catalogue and see how close you can come to something you can make work. John
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#3
06-18-2020, 07:39 AM
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That stinks... I had that exact failure happen to me about 10 years ago in a similar Zenith. My guess is the wire and insulating materials have degraded with time. That old enameled wire really isn't very good these days, you can flake it off with your fingernail.
I am getting ready to rewind a vertical output transformer for my CTC-20. The primary went open for no apparent reason, and I found it open about 30 turns from the finish end of the winding. The set is operating with a replacement CTC-10 part, but the yoke winding is not isolated from the primary, so I had to disconnect the vertical blanking circuit from the yoke secondary for now. To give you an idea of turns ratios, the primary has 3000 turns, the yoke winding 300 turns, and the two convergence windings are 16 turns each with a center tap; 8 turns either side of the tap. The Zenith part is probably in the same range as this one.
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Erich Loepke
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#4
06-18-2020, 12:51 PM
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Thank you both for that information...
The schematic says that that winding has a resistance of 1.3 ohms and reading it with an ohm meter, it is not open, it reads correctly. So it's arching through the enamel from the primary. Right now anyway, all the other windings are isolated and working. Has anyone ever been able to "re-varnish" a transformer using some vacuum method in order to stop the arching inside? Your thoughts are appreciated! Dave
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#5
06-18-2020, 01:58 PM
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These old transformers used some kind of thin paper for insulation of the layers and the windings. Most likely both the paper and the wire enamel has broken down and it's not going to be possible to get insulating varnish down in the interior. Rewinding or replacement is the only option at this point. I personally would rewind it with modern wire and Kapton insulation and have something that will hold up better, but I know this is beyond the capabilities of most of us.
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Erich Loepke
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#6
06-18-2020, 03:45 PM
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Thanks Erich-
What are the possibilities of getting a transformer like this re-wound? I've made a few calls and have not had any responses including Heyboer Transformers. Do you know of anyone who will take on a custom project like one of these vertical output transformers? The Zenith project has come too far to quit now!! Does anyone know where I could find another Zenith set with a 20X1C36 chassis? Thanks, Dave
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#7
06-18-2020, 04:21 PM
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I'm working on doing this stuff now, I will see how the RCA transforner I am doing works out. The first one I did for one of the early color sets; it was the vertical convergence transformer and is pretty much an unobtainable part. They tend to short from the secondary to the frame and take out other unusual parts when it happens. That one uses 41AWG wire, about 20,000 turns total. Unfortunately the CRT lost vacuum a short time ago, but it held up the whole time. These vertical output transformers have a lot less wire, and the wire is larger and easier to handle. The new thing I have to practice is bifilar winding, which is two wires wound side-by-side. In this case it's used on center tapped windings so the wire length (and resistance) will be the same on both sides of the tap.
Those Zenith transformers don't have a good track record and I wouldn't trust a used one anymore.
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Erich Loepke
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#8
06-18-2020, 04:41 PM
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In Zeniths VOT's were a problem but not big. It probably is the
same or nearly so one in a lot of Zeniths of the era. Your set is an X year chassis so look for other X, Y, Z & A years for a used one. The set ( & chassis) was common, I had a maple one & quite a few others here have one. 73 Zeno  LFOD !
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#9
06-19-2020, 06:46 AM
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Very common problem. I had mine rebuilt, but the core was cracked, and would hum like a sun of a gun.
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#10
06-19-2020, 08:32 PM
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Hi Everyone-
I follow up to the shorted vertical output transformer... Today may be my lucky day! I found a web site of a wholesaler who deals with bulk electronics... Oxygen Electronics. I've been searching everywhere on the internet hoping that I might "get lucky" if I stay at it. I found this web site last night. So I typed in the Thordarson substitution part number: 26S60. BING! There was one that showed up on their list. I just stared at the page in disbelief. I called first thing this morning and the guy who answered the phone was helpful and looked up the part number and told me that the part is located in California at a company that also sells bulk electronics. So I called them and talked to one of the sales reps and he said "sure - I have one in stock". He quoted me a price of $50 and I said "sure!". I'll believe it when I can hold this 26S60 in my hand, but I feel like I have won the lottery! Such a lucky find. Neither of these companies sells vintage TV parts, they just deal in bulk electronics and I'm sure that a part like this came from the purchase of another company selling the remaining inventory while going out of business, something like that. To them, this is just another transformer... to us, it's gold! Dave
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#12
07-08-2020, 06:42 PM
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Hello all!
I'm happy to say that I was able to successfully find a Thordarson replacement vertical output transformer for the Zenith 20X1C36. It was for sure a lucky, random find from an electronic surplus company on-line. I installed it yesterday and was happy that the set started up and the convergence circuit worked as it was supposed to. HOWEVER... the vertical linearity was way off. The top of the raster is stretched and the bottom is compressed. I can adjust the vertical linearity control but it has no effect adjusting the linearity. It has a great effect that is identical to the vertical height control. I am concerned that when the transformer failed, something else was damaged. I tried changing the tubes but that's not it. I've been looking for a bad cap or resistor, but no luck. Does anyone have any ideas? Thank, Dave
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#13
07-09-2020, 05:37 PM
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Quote:
Is that Thordarson definitely the correct sub? John
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#14
07-09-2020, 06:31 PM
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Thanks very much for the thoughts! I do have the schematic so I'll go through all of them and check. I see that there is a Zenith V.D.R. off of the plate of the vertical output tube. I have a newer style MOV I can replace with... Not sure how to read a VDR otherwise.
The replacement transformer is for sure the correct part number... Zenith 95-2470 is directly replaced by the Thordarson 26S60. All of the wire colors appear to be the same except one of them. It was pretty easy to double check the windings of old old one (ohm meter) with the new one in order to confirm the hook-up. I checked it a number of time as I proceeded. Having said that, it's possible I made a mistake but I don't think it would work this way if I did... Thanks John- I'll report back! Dave
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#15
07-10-2020, 03:52 PM
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All are IIRC.........
On some chassii there are 3 electrolytics on the vert out. One in a can & 2 strap mounted. Sub those by cutting one end & using a close value. Also when a VOT went a 5W green resistor cooked, check that. I was the jig man at a major Zenith dealer but only saw abt 6 or 8 VOT's bad from 1970 on. More than most brands but from what I see they are dying from old age now & its almost an expected fail. The VOT's I saw bad were all on early rectangle sets & 1 or 2 in 4 tube hybrids. 73 Zeno LFOD !
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