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DuMont RA-102 vertical transformer bad?
I haven't had to replace a vertical transformer before, but maybe this will be a first. I'm slogging toward the finish line with a Dumont RA-102 that has an odd (to me) vertical problem which I haven't solved by replacing caps & resistors or swapping tubes.
The TV refuses to lock vertically -- the best I can get is a slow hover. It also shows slanting lines similar to the common retrace lines, but these lines sort of dance along, changing speed & direction as the image hovers upward or downward. Here's a short video: http://antiqueradio.org/art/DuMontRA...calProblem.mp4 You can download the Riders service manual here: http://www.earlytelevision.org/pdf/d...102_manual.pdf The transformer in question is T2, the vertical blocking tube oscillator xfmr: Checking with an ohmmeter, T2 looks open (infinite resistance) between the Red and Blue primary leads. And the voltage at pin 2 of V16 is zero rather than +180VDC as given in the manual. Which leads to my real question. Say that this transformer is shot -- what replacement could I use? The Rider manual lists nothing but a Dumont part number for this item. I see a bunch of blocking osc transformers for sale at Playthings of Past, but they have various ratios and only one of them (STANCOR A-8124) has six leads like this one. Regards, Phil Nelson Phil's Old Radios http://antiqueradio.org/index.html |
#2
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Can you give us the DuMont part number? From there I can check in my Thordarson substitution book.
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#3
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The Dumont part number is 20D-4901.
Thanks! Phil Nelson |
#4
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Yep. I found that the RA-103 in Sams appears to use that same circuit. So the Stancor A-8124 should work.
DuMont 20D-4901-2 crosses to: Thordarson 26A01 Triad A96-X Stancor A-8124 Merit A-3001 |
#5
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Thordarson specs.
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Audiokarma |
#6
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Good detective work! I should have thought to compare the RA-103.
It looks like Playthings of Past has both the Stancor A-8124 and the Merit A-3001, both new and about the same price. Any reason to prefer one over the other? Phil Nelson Last edited by Phil Nelson; 11-28-2016 at 07:37 PM. |
#7
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Isn't that the same transformer used in admiral tv sets ?
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#8
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I'm not aware that they used a dual secondary.
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#9
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My main concern would be mounting centers, which I can't verify for either of those.
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#10
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I measured the mounting centers on my original transformer and they look close to 2 3/8" as given in the Thordarson chart. One of those mounting holes is actually a slot with maybe 3/16" of wiggle room either way, so if either the Stancor or Merit is close in that dimension, it should fit.
Phil Nelson |
Audiokarma |
#11
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Ummm...if it was me, I'd buy both of them.
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#12
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Tracktronics claims to have a 26A01 in stock. Specific part links don't seem to work, so you may have to search for it. http://www.tracktronicstechnologies.com/inventory/
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#13
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OK, I contacted Tracktronics. Haven't heard of them before. I'll need to add 'em to my list of suppliers.
Phil Nelson |
#14
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I think I've seen an ad over on the ARF, but I've never dealt with them either.
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#15
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I ended up getting a Stancor A-8124 transformer from Moyer Electronics.
Replacing the bad old transformer fixed the vertical problem with the peculiar slanting lines and inability to lock. However, there is a faint vertical jitter that won't go away with any adjustment. In the "Rider Receiver Trouble Cures" book at the ETF website, I see the following modification for curing vertical jitter in the RA-102: (The tube shown in that diagram is V16, the same tube shown in the schematic posted earlier.) A couple of quick questions before I start reworking things on the transformer that I just installed: 1. Has any other RA-102 owner done this, and would you recommend it? 2. What's going on with this mod? Regards, Phil Nelson Phil's Old Radios http://antiqueradio.org/index.html |
Audiokarma |
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