RCA T-120 in failure mode! - Videokarma.org TV - Video

Ralph S · #1 09-17-2010, 06:14 PM

Hi guys. Attached you'll see a picture of the onscreen effect of a failure which started out of the blue after 3 weeks of 8 hour days of perfect operation of my T-120. The set has been completely recapped and all resistors in the horizontal and vertical sections checked. The B+ and B- are within 10 volts of being dead on with 115 vac and about 1.7 amps input. The 2nd anode voltage is about half of what it should be (~5kv) and the picture (in spite of what it looks like here) is very dim as one would expect. I've not had a chance to scope out the Hor Osc (6SN7, both halves) or the Hor driver yet, but I suspect things are normal there. No smoke anywhere - all caps and electrolytics are cool. I'm suspecting either the yoke or the flyback may be preparing to sound a death knell. Anybody seen this symptom before?

leadlike · #2 09-17-2010, 06:30 PM

A shorted out yoke is about the only thing that would cause a keystoned picture like that. A shorted yoke would probably also load down the hv as well. Luckily, the yoke you need is the most common yoke ever, being pretty much on every RCA 12" set. I'm sure someone has a spare. I have a T-120 chassis that I've hacked apart. I believe the yoke may still be on it, but I could only ever get audio to work on that set, so I have no idea if it was good or not.

*Bill Cahill* · #3 09-17-2010, 06:37 PM

Definately the yoke. Unusual in these earlier sets, but, not un heard of.
That's a 10" set, I believe.
I think the replacement is DY-1.
I used an RCA yoke on a 10" Bendix when vertical winding shorted out, and, I got a keystone picture. Yoke worked.Was a little short on top, but, I think there were other issues, as well.
Bill Cahill

Ralph S · #4 09-17-2010, 08:44 PM

Thanks, Bill. You're right. The tube is a 12LP4 and Scotty just rebuilt it! Now I'll have to go scouting for a replacement yoke!

Ralph S · #5 09-21-2010, 05:59 PM

Hello again. I'm having trouble figuring out whether or not a yoke now listed on eBay is in fact a reasonable substitute for the RCA 71420 yoke used in my RCA T120 mentioned above. The eBay entry calls out its listing as being a substitute for an Emerson 708130, Stancor DY-1 and/or Merit MD12. However a catalog listing for the Stancor DY-1 shows a resistance of 8.3 ohms for the horizontal windings and the schematic for the T120 calls out a resistance of 13.5 +/- 10% ohms for the 71420 yoke. Seems to me to be too big a difference for the Stancor not to load down the flyback transformer. What's right here? I'd appreciate a quick answer as this item goes out tomorrow at 9:52 am PDT. Many thanks.

John Folsom · #6 09-21-2010, 06:42 PM

The RCA 71420 does sub for the Stancor DY1A. The resistance is not critical, what needs to match is the inductance.

kvflyer · #7 09-24-2010, 10:18 PM

After you get your new yoke, don't throw the old one away. You may be able to find the shorted winding and repair it. Then you will have a spare for testing purposes...

Ralph S · #8 09-25-2010, 10:48 AM

Thanks for everyone's advice. The "new" yoke is on the way. I'll let you know how this comes out!

Phil Nelson · #9 09-25-2010, 11:57 AM

Great keystoning. In an episode of "Mad Men," people watch election returns from the Kennedy-Nixon race on a TV whose screen geometry is distorted. It almost looks like everything was shot through a fisheye lens.

When they switch off the TV, you can see a shrinking keystone shape as the image fades. I wonder if they happened to use a TV that had a keystoning problem, or stuck magnets on the CRT to give the picture a "vintage" look? None of the TVs I watched in the 1960s looked like that, anyhow.

Phil

Ralph S · #10 09-28-2010, 12:36 PM

The set's repaired. After pondering the symptoms of the failure, it became obvious that something had to be dead-shorting half of the horizontal windings. Either it was a wiring or insulation problem or the resonant cap had shorted out. The answer was the latter. The pictures below tell the story. The 56 pf cap is C-169 in the original RCA schematic and unnumbered in the Photofacts schematic. The manufacturer is EL-Menco (made in USA) and is obviously 62 years old. Its resistance is .3 ohms and it looks slightly burned along the bottom row of dots. That's what caused the keystone and crimped the HV down to ~5kv. Luckily I've got a very good supply of new picofarad caps and the results of the replacement show in the off-the-air photo. Thanks to all for their suggestions.

kvflyer · #11 09-28-2010, 02:15 PM

Excellent! Congratulations...

miniman82 · #12 09-28-2010, 07:20 PM

Faults like this are exactly why I tell people never to trust those supposed 'mica' caps. The people on ARF didn't believe me, but half the ones I removed from a Philco I restored were actually flat package PIO's! After finding a whole lot of them bad, I got curious and broke them apart, that's how I know what's inside. So they were no better than a black beauty, but there's no way to tell without destruction.

My take is that if these caps aren't in HV circuits (i.e. under a lot of stress), they will probably be OK. Example, I left the ones in the tuner of my VT-71 in place and they work fine. But none of the ones in the flyback circuit of the Philco were good, and some of the ones in oscillator circuits were causing the wrong frequencies to be generated. if in any doubt=replace.