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.

[Electronic M]

2.) what is going on: Removing DC (tube plate voltage) from the transformer to prevent DC core saturation in the transformer, then giving the tube a new plate B+ DC source (the two resistors and lytic) to replace the now cap blocked path through the transformer.

[Phil Nelson]

Well, the screen image is definitely unstable, although I'm not sure if I would ascribe it only to the vertical. The whole image seems to expand and contract in both dimensions -- horizontal and vertical -- slightly and rapidly. I can also see faint "retrace"-like horizontal lines, both light and dark. It's hard to capture with cellphone video, but these brief clips give an idea what's happening:

http://antiqueradio.org/art/DuMontRA...ngScreen01.mp4

http://antiqueradio.org/art/DuMontRA...ringScreen.mp4

Edit: I just noticed that the "retrace"-like lines are light in the top half of the screen and dark in the lower half. ????

I wonder if instability in the high voltage supply could cause the fluttering in screen size? Adjusting the Regulator Control and Voltage Control on the power supply chassis doesn't affect this problem. My high voltage meter shows a steady 10KV output, although that meter may not be sensitive enough to show such small, rapid fluctuations.

Ideas? The RA-102 service manual with schematic is at:

http://www.earlytelevision.org/pdf/d...-rider-tv1.pdf

The HV section in this set needed repair beyond the usual caps & resistors. The previous owner tore up the HV output transformer and did other odd kludges, but Andy Cuffe was able to repair the transformer and it seems to function normally. I have done extensive work on the rest of the TV. In the vertical & horizontal sweep sections, I have replaced nearly every cap and resistor.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[old_coot88]

Originally Posted by Phil Nelson

Well, the screen image is definitely unstable, although I'm not sure if I would ascribe it only to the vertical. The entire image seems to expand and contract in both dimensions -- horizontal and vertical -- slightly and rapidly. I can also see faint "retrace"-like horizontal lines, both light and dark. It's hard to capture with cellphone video, but these brief clips give an idea what's happening:

http://antiqueradio.org/art/DuMontRA...ngScreen01.mp4

http://antiqueradio.org/art/DuMontRA...ringScreen.mp4

I wonder if instability in the high voltage supply could cause the fluttering in screen size? I tried adjusting the Regulator Control on the power supply chassis, with no improvement. My high voltage meter shows a steady 10KV output, although that meter may not be sensitive enough to show such small, rapid fluctuations.

Is there by chance a faint 'ticking' coming from the CRT socket/base, in step with the fluctuating raster size? Sometimes a pin-to-lug connection is open, creating a tiny spark gap that the CRT beam current is jumping across, resulting in the bloomy raster.

[Phil Nelson]

No ticking at the CRT socket, and those connections are clean & bright.

Where I do hear a wee ticking, however, is near the base of the 1B3GT HV rectifier tube. Corona inside the HV cage was a common problem with these sets, the subject of a Dumont service note. I thought I had solved that with multiple applications of corona dope, but perhaps more is needed.

I took another video with a test pattern instead of broadcast video from my agile modulator:

http://antiqueradio.org/art/DuMontRA...ngScreen02.mp4

I don't know if that tells us anything new, but at least I know I'm not watching interference from the home broadcasting setup.

Phil Nelson

[old_coot88]

Phil,
None of those linkys will play on my (Linux) rig for some reason. But a coupla questions.. Is the tick you're hearing in sync with the raster-size fluctuation? And does the period of the fluctuation vary with the brightness setting? (e.g., speed up with hi brigntness, slow down with low brigntness?)

[M3-SRT8]

Originally Posted by Phil Nelson

Well, the screen image is definitely unstable, although I'm not sure if I would ascribe it only to the vertical. The whole image seems to expand and contract in both dimensions -- horizontal and vertical -- slightly and rapidly. I can also see faint "retrace"-like horizontal lines, both light and dark. It's hard to capture with cellphone video, but these brief clips give an idea what's happening:

http://antiqueradio.org/art/DuMontRA...ngScreen01.mp4

http://antiqueradio.org/art/DuMontRA...ringScreen.mp4

Edit: I just noticed that the "retrace"-like lines are light in the top half of the screen and dark in the lower half. ????

I wonder if instability in the high voltage supply could cause the fluttering in screen size? Adjusting the Regulator Control and Voltage Control on the power supply chassis doesn't affect this problem. My high voltage meter shows a steady 10KV output, although that meter may not be sensitive enough to show such small, rapid fluctuations.

My RA-102-B3 Club 15 does that, on occasion. I also could not measure any fluctuations in power from the 2nd Anode.

[Phil Nelson]

Thanks to Ampico-kid and M3-SRT8 for that useful info. I have been laid low with the flu, but in a day or two I should be able to quit whining and get back to work.

My RA-103 has rock steady vertical & horizontal sweep, so given the similarity, that's what I was expecting from this RA-102, as well. I'll start by applying more corona dope, since that issue must be addressed in any case. If I eliminate the corona but the jitter remains, then I guess the doorknob cap would be an easy and logical thing to swap next.

Happy 2017 to all !

Phil Nelson
Phil's Old Radios