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 Post subject: I like my yolk well done but not my yoke
PostPosted: Sep Mon 30, 2019 9:10 pm 
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Location: tucson az usa 85745
In this RCA KCS120, the phenolic spacer to keep the yoke and centering rings located was completely shot. Taking advice from a group on FB, I used car gasket material to fashion spacers so all would stay in place. Great but I notice that as this TV gets warm, the image becomes distorted vertically. If I leave the back panel off, this does not happen at least for a long time. Closed up, the problem is visible within 15 min. Pushing fan air through helps but not enough. Is the yoke getting too hot or what can be expected of a thing this aged? SAMS shows no thermistor in yoke assembly; I have seen that on other TVs so wondering what if anything can be done. All the really hot big tubes are pointed horizontally directly under the yoke and neck. This seems to invite trouble if in fact heat is the cause of this. Thank you for advice. --Tom in Tucson

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Sep Mon 30, 2019 10:20 pm 
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How is it distorted vertically?
What makes you think the problem is the yoke?

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Sep Mon 30, 2019 10:25 pm 
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I wonder if something in those gaskets is changing with heat, moving the yoke more than the old phenolic spacers did. Or there might be some metallic material in the car gaskets that's changing a magnetic field with temp.
And of course it might be that there's a slightly gassy vertical tube that is slowly being affected by temperature, too, though I think that's less likely.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Sep Mon 30, 2019 10:43 pm 
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I'd like to see pics of the parts concerned and the problem on screen too.

I've never seen phenolic disintegrate, but have seen the primitive plastic in yoke back covers/centering ring mounts disintegrate repeatedly...some of the early plastics naturally rot with age.

Deflection changes after warm up can be caused by many things one of the more common ones is defective carbon comp resistors. They will test fine on a meter but with heat of normal operation will drift in value....the best method of diagnosis is to take freeze spray/canned air and cool parts of the circuit till you find a part that when cooled fixes the problem on screen...then change that component.


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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Sep Mon 30, 2019 11:08 pm 
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Electronic Memory wrote:
Deflection changes after warm up can be caused by many things one of the more common ones is defective carbon comp resistors. They will test fine on a meter but with heat of normal operation will drift in value....the best method of diagnosis is to take freeze spray/canned air and cool parts of the circuit till you find a part that when cooled fixes the problem on screen...then change that component.

If the set still has paper caps, the most likely cause of the problem is the caps. Paper caps become more leaky as they are heated and the vertical circuits are much more sensitive to leakage than any other section of the set. Paper caps are much more likely to cause problems than resistors. But after the caps, the resistors are the next most likely cause of the problem.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Sep Mon 30, 2019 11:18 pm 
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Yes lots of polyester type black plastic coupling capacitors but the board has to be removed to unsolder them. At least in my predicta, I could clip off the leads from above and solder new ones to them. Not possible here due to use of radial leads. I guess I go back to original question about lack of thermistor on the yoke. Lots of TVs have those and I just wonder if leaving it out of this design saved money for RCA. What is the purpose of the thermistor built on to some yokes is my underlying question. The upper part of the image is elongated while the bottom is compressed and comes up above the edge of the phosphor coating. This happens after a half hour or so but is dependent on whether rear panel is in place trapping heat versus rear open to the room letting heat escape.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Tue 01, 2019 12:20 am 
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Dont think its anything to do with the yoke. If that is the chassis I think
it is its a dog to work on. Start EASY.
1) new vert tube /s NOS only. Dont believe a checker.
2) there is an electrolytic at the cathode of the vert out. its usually part
of a can. That causes the bottom to pull up.
3) use freeze mist to find the bad cap. Throw a blanket over the back to
build up heat so it will act up. Usually you find a cap bad in the vert feedback
( parts between the Vert out plate & Vert osc grid).

after that good luck

73 Zeno 8)
LFOD !


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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Tue 01, 2019 1:37 am 
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thomash85715 wrote:
I guess I go back to original question about lack of thermistor on the yoke. Lots of TVs have those and I just wonder if leaving it out of this design saved money for RCA. What is the purpose of the thermistor built on to some yokes is my underlying question.

The thermister makes small adjustments to the overall height to compensate for the increased resistance of the windings in the yoke as it warms up. But it is small changes and only for the overall height, not the linearity.
The symptoms you are seeing are the classic symptoms of leaky paper caps in the vertical circuit. We saw that a lot at the TV shop I worked at.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Tue 01, 2019 3:44 am 
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This TV has only one can with four sections. Two of them are for audio; the others are on either side of choke. I had never seen a TV with no electrolytic dedicated to the vertical. If there is one in here somewhere, I cannot find it. The tiny pentode used for V out had a bad cathode short in my eico yet the thing was sort of working. With a NOS 6aq5 in there now, the behavior is a bit better but not much. This client will play old time DVD through it for background when he has people in the office for a reception or something. At some point, this is a cost benefit analysis. Does he want to pay me by the hour to tear this thing down completely and replace deeper and deeper layers of passive parts? If it were my own TV and I had nothing else to do, yeah I might go ahead and make it real pretty with new coupling capacitors. That always makes the difference; I was just hoping against hope there was a work around or palliative remedy. Thank you.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Tue 01, 2019 3:27 pm 
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Those caps are junk.......back when those sets were only a few years old they were failing. By the time they were 10 years old, we were replacing all of the black stand up caps at one time in the shop on every RCA that came in which used that type.

There's no doubt that if you checked them today with an appropriate tester like one of the Sprague TO- series, all of them would fail the leakage test quite badly.

Bottom line the set will never work properly with the originals still in place after more than 60 years. There is no work around or other remedy........ if your customer is not willing to pay for a proper repair, the appropriate thing to do is give it back unrepaired and collect a diagnostic/estimate fee to cover the time you have spent looking at it. You'll likely never hear the end of it if you don't.

EVERY vintage TV or radio of that age is full of leaky caps unless they have already been replaced with improved quality parts during the time they were still in daily use.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Tue 01, 2019 3:41 pm 
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I assumed the OP knew that it is pointless trying to troubleshoot a pre-1962 set before all the electrolytic and paper dielectric caps have been changed, and seemed to recall from their other thread that a recap was mentioned...

Tom Schulz wrote:
Electronic Memory wrote:
Deflection changes after warm up can be caused by many things one of the more common ones is defective carbon comp resistors. They will test fine on a meter but with heat of normal operation will drift in value....the best method of diagnosis is to take freeze spray/canned air and cool parts of the circuit till you find a part that when cooled fixes the problem on screen...then change that component.

If the set still has paper caps, the most likely cause of the problem is the caps. Paper caps become more leaky as they are heated and the vertical circuits are much more sensitive to leakage than any other section of the set. Paper caps are much more likely to cause problems than resistors. But after the caps, the resistors are the next most likely cause of the problem.



Thermistors came in with color sets and IIRC were meant to keep deflection stable so pincushion and convergence on color sets would be more stable.


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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Tue 01, 2019 3:45 pm 
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Location: Dallas, TX
I agree with Mr. Detrola.
The time it would take to troubleshoot every problem far outweighs the cost of new capacitors.
The common wisdom one something this old is to replace the electrolytic and tubular caps ( and perhaps resistor far out of spec.) then troubleshoot. It is much more time effective this way.
You certainly should fine an economical source for capacitors if you want to keep expenditures down.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Fri 04, 2019 3:31 am 
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Yes as others have said do recap the TV before doing anything else.

Also my suggestion is while you have the board out go ahead and replace all the carbon comp resistors.

Metal film should work just fine.

Now is the TV going to be used with the tuner or is a composite video input being added?

The best video quality I have gotten on a B/W TV is to add a composite video input, put a 75 ohm resistor between the input and ground then use an S video to dual RCA plug cable only plugging in the luma cable which is a B/W signal.

The device will then need to have an s video output or a box that can convert whatever input to s video will be needed.


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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Wed 23, 2019 5:37 am 
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Sorry for long delay in reply to questions... the thing will be hooked to a DVD player only. No attempt to show live reception will happen. The IF strip and tuner are in deep failure mode but baseband sound n pix is fine so that is how it was left.

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 Post subject: Re: I like my yolk well done but not my yoke
PostPosted: Oct Wed 23, 2019 5:38 pm 
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Did you get the vertical problem fixed?

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