View Full Version : DuMont RA-102 vertical transformer bad?


Phil Nelson
11-28-2016, 07:13 PM
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-102CliftonVerticalProblem.mp4

You can download the Riders service manual here:

http://www.earlytelevision.org/pdf/dumont_ra-102_manual.pdf

The transformer in question is T2, the vertical blocking tube oscillator xfmr:

http://antiqueradio.org/art/DuMontRA-102CliftonV16Schematic.jpg

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
11-28-2016, 07:23 PM
Can you give us the DuMont part number? From there I can check in my Thordarson substitution book.

Phil Nelson
11-28-2016, 07:58 PM
The Dumont part number is 20D-4901.

Thanks!

Phil Nelson

Kevin Kuehn
11-28-2016, 08:07 PM
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
11-28-2016, 08:18 PM
Thordarson specs.

http://i452.photobucket.com/albums/qq245/Kuehn/VK%20Pictures/Thordarson2026A012001_zpsazyq5bvc.jpg?t=1480295812

Phil Nelson
11-28-2016, 08:28 PM
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
11-28-2016, 08:42 PM
Isn't that the same transformer used in admiral tv sets ?

Kevin Kuehn
11-28-2016, 09:04 PM
Isn't that the same transformer used in admiral tv sets ?

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

Kevin Kuehn
11-28-2016, 09:07 PM
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
11-28-2016, 09:26 PM
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
11-28-2016, 09:36 PM
Ummm...if it was me, I'd buy both of them.:smoke:

Kevin Kuehn
11-28-2016, 09:56 PM
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
11-28-2016, 10:10 PM
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
11-28-2016, 10:14 PM
I think I've seen an ad over on the ARF, but I've never dealt with them either.

Phil Nelson
12-12-2016, 05:52 PM
I ended up getting a Stancor A-8124 transformer from Moyer Electronics.

http://antiqueradio.org/art/DuMontRA-102CliftonVerticalTransformerReplacement.jpg

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:

http://antiqueradio.org/art/DuMontRA-102VerticalModification01.jpg

http://antiqueradio.org/art/DuMontRA-102VerticalModification02.jpg

(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

Electronic M
12-12-2016, 06:26 PM
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
12-23-2016, 03:14 PM
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-102CliftonJitteringScreen01.mp4

http://antiqueradio.org/art/DuMontRA-102CliftonJitteringScreen.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/dumont-ra-102-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
12-23-2016, 03:31 PM
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-102CliftonJitteringScreen01.mp4

http://antiqueradio.org/art/DuMontRA-102CliftonJitteringScreen.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
12-23-2016, 04:58 PM
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-102CliftonJitteringScreen02.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
12-23-2016, 08:25 PM
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?)

Penthode
12-24-2016, 11:14 AM
There is probably a one megohm resistor in series with the HV lead. It often breaks down and offers the symptoms described.

Phil Nelson
12-24-2016, 04:00 PM
Thanks. There is a 47K resistor (R12) in series with the HV lead. It has been replaced with a new 1-watt 47K resistor. Those solder joints have been coated with corona dope, but they are candidates for corona, along with several other spots in the HV cage, and I'll probably go over them again when I get around to it.

It's hard to observe whether the image jitters coincide with ticking sounds, since the screen is at the other end of my workbench from the power chassis, where I need to hold my ear close to the HV section to hear any ticking. Maybe I can rig up something with a mirror and a long listening tube. That might be in a day or two.

Regards,

Phil Nelson

Penthode
12-24-2016, 11:03 PM
The intermittent picture expansion would result from the HV dropping. It appears that the high voltage regulation is bad due to the insulation breaking down.

I would also suggest paying close scrutiny to the HV capacitors. I had the same intermittent problem because the 500mmfd HV cap was breaking down.

Username1
12-25-2016, 09:54 AM
Hi - I believe your problem is some kind of spike in the Horiz. Osc. Signal
that is then sent to the Horiz. Output. While doing a freeze-frame on yer
video, I can see what looks like a a spike or reduction in amplitude of
the horiz. Scan line. Making a few scan lines run long or short. You can
also see the Damper circuit faithfully reproduces the effect over on the left
side of the screen. That makes me think it's source is Before the Horiz Output
tube, and Damper circuit. I would look over in the Horiz. Osc. area as being the
cause. Effects of the spike, well, larger, or shorter scan for that instant,
possibly raise or lowering of B+ Boost? Raise or lowering of HV? Anyway, to me
it looks like a Horiz Osc. Origin - I did not look at the schematic, but you know
it's so short in duration, and intermittent, you may just need to change
parts to get it..... Of course it's possible for some type of discharge somewhere
else to be causing a spike to give you this display, you will have to figure out
if you have a HV arc somewhere.... It would be really neat to get this problem
figured out down to a single part, and post it's location on a schematic because
the effect is just so different.....

Good Luck.

.

Ampico-kid
01-08-2017, 11:40 AM
Hi Phil. In answer to your question #1....My Clifton had rock solid vertical locking and no vertical jitter at all after I recapped it. The vertical jitter modification had not been done on my RA-102 chassis, it was still the original configuration.



I ended up getting a Stancor A-8124 transformer from Moyer Electronics.

http://antiqueradio.org/art/DuMontRA-102CliftonVerticalTransformerReplacement.jpg

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:

http://antiqueradio.org/art/DuMontRA-102VerticalModification01.jpg

http://antiqueradio.org/art/DuMontRA-102VerticalModification02.jpg

(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

M3-SRT8
01-08-2017, 03:16 PM
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-102CliftonJitteringScreen01.mp4

http://antiqueradio.org/art/DuMontRA-102CliftonJitteringScreen.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.:smoke:

Phil Nelson
01-09-2017, 09:28 PM
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

Penthode
01-12-2017, 12:41 AM
I know I am repeating myself on this. But the similarity of what you depict to the problem I had on my 1948 RCA striking. I was in a similar dilemma trying to determine the twitching in the video when I found the 500pf hi-voltage filter with a crack. The were tiny sparks in the crack which I could not see but heard and located them by using a listening tube around the hi-voltage cage. The ticks coincided with the twitching in the video.

The hi-voltage capacitors generally have a clip which you can remove. (The aquadag provides ample filtering anyway). You may wish to disconnect the capacitor just to see if the twitching disappears.

M3-SRT8
01-12-2017, 12:17 PM
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.

I ended up replacing the DoorKnob Cap as well. It was fried. Put in a 20kV one. It screws in place IIRC.

I also replaced the four micas under the high voltage chassis around the 807. They all failed in quick succession.

Phil Nelson
01-12-2017, 02:00 PM
Thanks. I did replace the micas under the power chassis, along with resistors in that area.

On this set, the previous owner eliminated the doorknob HV filter cap, possibly because he was using a substitute CRT that had an aquadag coating (the 12JP4 that I'm using has no aquadag). I got a new doorknob from Surplus Sales of Nebraska and installed it as shown in the schematic. I suppose it's possible that the new doorknob is bad; I can pull one from another restored TV and try substituting that, to rule it out.

Meanwhile, I removed the HV transformer and coated the whole thing with thin corona dope, along with the likely connections on the HV line and 1B3GT HV rectifier socket. This seems to have quieted any corona noise.

If subbing the doorknob has no effect, I'll look into the horizontal circuits as suggested by username1.

Regards,

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

Phil Nelson
01-16-2017, 12:44 AM
Well, the doorknob cap was bad, after all. When I played the set to check another issue, a faint tick-tick returned in the HV section and then the cap failed, causing hash in the video and loud static in the audio. When I swapped in a doorknob borrowed from my National TV-7W (which has four to choose from), the picture stabilized.

http://antiqueradio.org/art/DuMontRA-102CliftonJitterFixed.jpg

I don't have a cap checker that can test at such high voltage, but out of curiosity I temporarily clipped the old doorknob cap in parallel with the good one. It immediately started hissing, so I guess it really is a goner.

http://antiqueradio.org/art/DuMontRA-102CliftonJitterFixed01.jpg

There's more work to do on this set, but I'm counting this problem resolved. Thanks again for the good advice.

Regards,

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

EdKozk2
01-18-2017, 12:05 AM
Phil,
How did you get that chassis so clean looking, it almost looks new.
Ed

Phil Nelson
01-18-2017, 12:54 AM
Both chassis on this set had bad rust. After using Naval Jelly & the usual tomfoolery, this chassis still looked rotten, so I painted it (not my usual practice).

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

EdKozk2
01-18-2017, 01:17 AM
Better than having rust. :thmbsp:

Penthode
01-18-2017, 11:56 PM
Nice job :thmbsp: