DuMont RA-102 vertical transformer bad?

[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

[Penthode]

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]

Originally Posted by Phil Nelson

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]

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]

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.



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.



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]

Phil,
How did you get that chassis so clean looking, it almost looks new.
Ed

[Phil Nelson]

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