DuMont RA-102 vertical transformer bad?

[Phil Nelson]

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

[Kevin Kuehn]

Can you give us the DuMont part number? From there I can check in my Thordarson substitution book.

[Phil Nelson]

The Dumont part number is 20D-4901.

Thanks!

Phil Nelson

[Kevin Kuehn]

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

[Kevin Kuehn]

Thordarson specs.

[Phil Nelson]

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

[kramden66]

Isn't that the same transformer used in admiral tv sets ?

[Kevin Kuehn]

Originally Posted by kramden66

Isn't that the same transformer used in admiral tv sets ?

I'm not aware that they used a dual secondary.

[Kevin Kuehn]

Originally Posted by Phil Nelson

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

My main concern would be mounting centers, which I can't verify for either of those.

[Phil Nelson]

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

[M3-SRT8]

Ummm...if it was me, I'd buy both of them.

[Kevin Kuehn]

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/

[Phil Nelson]

OK, I contacted Tracktronics. Haven't heard of them before. I'll need to add 'em to my list of suppliers.

Phil Nelson

[Kevin Kuehn]

I think I've seen an ad over on the ARF, but I've never dealt with them either.

[Phil Nelson]

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