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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Fri 14, 2019 4:41 am 
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Okay, so some more voltage checking in this set revealed that the AGC control is properly set, but moving it to its extremes does not result in a voltage on the grid of the 6CF6 that is as positive as is called for on the schematic. Even at the maximum i am seeing -7V on the grid of the 6CF6 where it should be slightly positive when operating normally.

I checked some voltages around the 12AU7 AGC tube and found that plate voltage was low, actually all tubes that are fed the low B+ voltage are low. The cathode of the 6AS5 audio output tube from which the low B+ voltages are derived is only at 85 volts, where there should be about 120 volts. The high voltage on the plate of the 6AS5 correct, if a tad high. Since only the IF and AGC circuits are fed from the lower B+ so this may have gone unnoticed previously since feeding a signal into the set does not require the use of these circuits. Although the low B+ also feeds the 1st sync and sync output tubes which may explain the touchy vertical hold.

I tried another 6AS5 and both test decently for emissions and shorts, however that doesn't rule out a bad tube, and the issue persists. I am leaning towards a partial short or leaky capacitor on the low B+ line and will continue to investigate further unless anyone thinks this may not be the issue.

Edit: Grid voltage on the 6AS5 is also low, 52V vs the 135V called for. Also, added schematic segment:

Attachment:
Audio (800x389).jpg
Audio (800x389).jpg [ 214.55 KiB | Viewed 948 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Fri 14, 2019 5:01 am 
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The voltage on the grid of the 6AS5 is set by the ratio of the values of R111 and R112. If C113 is not leaky than it is likely that one or both of those resistors is bad.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Fri 14, 2019 5:43 am 
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Tom, you were 100% correct, by process of elimination R111 was open. I disconnected one lead to get a more accurate measurement and found that the other side of the resistor facing the chassis was blackened and the resistor measured open circuit. Checking the two points it was connected to I realized that the chassis presents about 840K ohms impedance at those nodes with the resistor out of circuit. Hence why I did not catch it in my previous checks.

That is my fault though, I should have realized based on the schematic that there were alternate resistance paths present at the nodes to which that resistor connects and disconnected one end to measure it initially.

I replaced R111 and was greeted with more correct voltages, and did indeed get a signal though the tuner and IF stages, inclusion some vestige of a picture. Its not perfect, and there is a large amount of wavy interference in the signal, but that can be resolved on another day.

Attachment:
SAM_5627 (640x498).jpg
SAM_5627 (640x498).jpg [ 171.05 KiB | Viewed 943 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Fri 14, 2019 12:55 pm 
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You are making good progress. That resistor feeding the audio output tube grid was a common failure in all brands and models of sets which used that circuit, they went high in value causing the low B+ voltage to drop affecting performance.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Fri 14, 2019 3:57 pm 
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Since the RF/IF is passing signal and you have played with the AGC you probably need to reset the AGC. First make sure the fine tuning is set correctly then with the contrast maxed turn the AGC down till there is barely any contrast then up till the Image has excessive contrast and starts to loose sync, then back the AGC down just a bit past the point where sync returns. If image is too contrasty turn contrast control down to a pleasing setting.


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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sat 15, 2019 6:03 am 
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I solved the issue with the weird lines, some kind of ground loop caused by the fact that I was feeding the set using two signals from the same DVD player into the set in two different places. Through direct injection to the audio and video circuits and through a coax and matching transformer to the tuner, even though I had disconnected the active lines from the audio and video tubes, the ground connections were still intact and causing an issue. Removing the RCA cables from the back of the DVD player removed those lines.

I believe the majority of the problems in this set have been resolved, thus my plan going forward is as follows:

I want to take a few voltage checks going forward to make sure everything is operating correctly. I also need to double check the AGC potentiometer as it is a bit hard to get the picture and sound set ideally, I think it just needs to be disassembled and cleaned. The notes above were most helpful though.

I have noticed that adjusting both the height and vert. linearity controls more than a few degrees will upset the vertical hold and cause the image to roll. It is possible to readjust the vertical hold control while adjusting either of the other two controls and thus keep the image synced while performing the function of the other control (they do affect height and vertical linearity when turned). I have no idea if this operation is normal, my other 21" Admiral set does not require the vertical hold control to be adjusted when adjusting the height or vertical linearity but it is my only other set of this era that I have worked with. Thus, I am calling on those with more experience to say whether this is normal operation or not.

Schematic section for the vertical circuits:
Attachment:
Vertical Circuits (800x355).jpg
Vertical Circuits (800x355).jpg [ 173.37 KiB | Viewed 904 times ]


I also have the parts on hand to, and plan to build a proper switched box for direct audio and video input similar to that described on Phil Nelson's site: https://antiqueradio.org/A-V_AdapterForVintageTVs.html

Then, I need to do some image adjustments. The centering is slightly off and I need to move the image to the right a bit, also the linearity is not great near the edges of the image. I have the instructions on hand on how to adjust everything, it will take a few hours time to set everything up as best as possible.

Finally, I need to clean the cabinet, as it has been sitting around for a few years and has collected some dust inside and out. I would love to have a NOS or rebuild CRT for this set as well, the one I have in there now is ok with a brightener, but retrace lines are visible when the brightness and contrast are set at what I would consider to be a comfortable level for a dimly lit room,

I will continue to update this thread with pictures and possibly some more questions should they arise.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sun 16, 2019 10:29 pm 
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Some sets do require by design juggling the hold along with height and linearity...feel free to juggle.

The retrace lines may be a fault of the circuitry... Many sets have a retrace blanking circuit that samples the vertical output transformer with a high voltage cap that feeds a pulse to one of the video amp stages through a resistor... You can try to add one if it is not present.
Back in the 50s the vertical and horizontal blanking intervals were clean and completely black, but since then color burst and closed captioning, macrovision and many other things were crammed in and that set was not designed specifically to deal with.


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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sun 16, 2019 11:13 pm 
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Yes, this set has neither DC restoration nor a retrace blanking circuit. I looked into adding the latter, but most additions you see which tap a signal off the vertical output are applied to sets in which the video signal is sent to the cathode of the CRT and the brightness control is tied to the grid. The idea being that the positive voltage pulse from the vertical output will raise the voltage difference between the cathode and grid and drive the CRT into cutoff.

This set has the video output fed to the grid of the CRT and the brightness control connected to the cathode. Thus you need a pulse of the opposite polarity to drive the grid more negative and the tube into cutoff. I have seen this done with an additional phase inverter tube, but I don't know if I want to add that addition.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 17, 2019 3:10 pm 
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Pro tip: the polarity of a signal on a signal tube grid gets inverted on its way to the plate....if you can't inject it after the last video amp try injecting it before and attenuate it more.


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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 20, 2019 12:26 am 
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I have attempted a brief alignment, and I have found that I can get the linearity to be pretty good. I do have one issue with the picture that I have been unable to resolve though, and haven't been able to see the same issue documented elsewhere.

There are 4-5 vertical white lines visible on the left edge of the picture. I have adjusted the brightness and contrast to accentuate this issue, but it cannot be fully removed by means of these controls. Adjusting the horizontal drive and oscillator controls does not affect this issue either. Adjusting the width allows me to over expand the raster to hide but not resolve the lines. Adjusting the horizontal linearity control makes the lines wave a bit, but does not make them less noticeable.

I have also tried different damper and HO tubes, neither of which made any difference in this issue.

Attachment:
SAM_5628 (640x480).jpg
SAM_5628 (640x480).jpg [ 114.4 KiB | Viewed 824 times ]


Is this horizontal foldover? I do not believe that it is, considering that by adjusting the horizontal control I can create horizontal foldover which seems distinct from this issue.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 20, 2019 3:03 am 
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I remember problems like that when I worked on cameras for RCA. It's ringing in the horizontal deflection yoke. For cameras the solution was usually improved grounding, but I think TV set yokes also sometime have an RC suppression network across the yoke. Usually they're close to/hidden in the yoke assembly.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 20, 2019 3:10 am 
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Ah, yoke ringing was the magic keywords, the thread here describes an issue very similar to mine:

https://www.antiqueradios.com/forums/vi ... hp?t=12817

Sounds like I need to dig into the yoke and check the components connected therein. Thanks for the clarification on what this issue is called.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 20, 2019 4:36 am 
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Sometimes that's about as good as you can get it. Many sets were adjusted to slightly overscan on purpose to hide issues with ringing or linearity. Worth checking the components inside the yoke though just in case.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 20, 2019 4:16 pm 
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Another possibility is that (given there appears to be 9 cycles or less) the chroma burst might be passing through the video chain and on to screen. If you disconnect your video source from the set and some or all of the bars disappear then it is at least partially burst into on screen. The easiest way to fix it is to filter your video source before the modulator with a LC circuit that cuts off 3.5MHz and up.


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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Thu 20, 2019 10:34 pm 
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A 3.58MHz trap can be made from a 47uH coil in series with a 43pf cap (or any other convenient LC combination that resonates at the burst frequency). A series LC is a low impedance at resonance, so it can be inserted almost anywhere in a tube-type video amp chain to "suck out" the burst without doing terrible harm to the rest of the video.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Fri 21, 2019 3:19 am 
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I should have mentioned earlier that the vertical lines on the left side of the screen occur when multiple different signal sources are used, as well as slightly visible (but much less so) when both the tuner is switched out and no external source is fed into the set.

If the chroma burst were the cause, would the lines not be removed by getting rid of the chrominance signal? I fed the set a signal from the luminance output of a component video source and noticed no difference between this and a standard composite video signal in terms of the vertical lines.

To me, this would indicate that the issue is not chroma related, and since it is still sightly visible when the set is fed no signal would make yoke ringing a much more plausible cause, no?

I still have not yet dug into the yoke, I suspect that I will have to have to remove it completely from the picture tube neck in order to access the bypass components based on the method of attachment used to secure it and the focus magnet to the set and stabilize the picture tube.

The horizontal deflection coils are bypassed by only two components, a 120pF capacitor and a 1000 ohm resistor:

Attachment:
Deflection Yoke.jpg
Deflection Yoke.jpg [ 78.94 KiB | Viewed 781 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sun 23, 2019 1:30 am 
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Got the yoke off of the set today. I replaced the 1000 ohm resistor which had drifted slightly out of tolerance. The 120pF capacitor looked fine, but was only marked "120HV" on the casing, and the service documentation only describes that component as "Part of Yoke Assembly". Not knowing the working voltage or its construction (looks like coated tubular ceramic, but not sure) I elected to leave it in place. It did not test shorted and I have no capacitor checker of the correct working voltage, and even if I did I am not sure what the working voltage was originally.

The vertical lines on the side of the screen looked much the same as before. Although, they really aren't that noticeable when playing live content and not a test pattern. I believe I will just run out the width to allow a bit of over-scan and live with the effect, unless you know what to look for I don't think one would pick up on it.

Attachment:
SAM_5632 (640x480).jpg
SAM_5632 (640x480).jpg [ 223.36 KiB | Viewed 760 times ]

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Mon 24, 2019 5:45 am 
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Some British designs used to have a variable capacitor that would tune those lines out....

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jun Sat 29, 2019 4:14 am 
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I won't deny it, but I have been on pins and needles this past week awaiting this shipment. Fortunately, everything arrived intact and I am happy to say I have a NOS CRT to complete this restoration and replace my CRT which is on its last legs.

I was made aware of this CRT being included in the ETF classifieds list a few weeks ago, so I got in touch with Steve McVoy and arranged for its purchase. It was shipped out within a few days and arrived a little bit earlier this week. It was double boxed and shipped via Fedex Ground. I did immediately check to make sure it was not broken in transit upon receiving it (hence the box being taped in the first photo), but was not able to do anything further until today.

The original shipping crate was used to house the CRT and was then wrapped in large bubble wrap and placed within another box. The only thing interesting to me was the lack of any markings indicating glass on the outside of the shipping box. There was a small puncture to the outer casing as seen below, but this did not disturb the contents.

Attachment:
SAM_5633 (640x625).jpg
SAM_5633 (640x625).jpg [ 212.55 KiB | Viewed 647 times ]


Attachment:
SAM_5635 (640x480).jpg
SAM_5635 (640x480).jpg [ 194.16 KiB | Viewed 647 times ]


This tube is a General Electric rebuild 21AMP4B with an aluminized screen.

Attachment:
SAM_5636 (640x362).jpg
SAM_5636 (640x362).jpg [ 157.25 KiB | Viewed 647 times ]


Attachment:
SAM_5637 (640x480).jpg
SAM_5637 (640x480).jpg [ 179.65 KiB | Viewed 647 times ]


Look at this interesting orange coating around the second anode button, presumably some kind of anti-corona coating?

Attachment:
SAM_5638 (640x480).jpg
SAM_5638 (640x480).jpg [ 193.01 KiB | Viewed 647 times ]


The gun in this CRT has what appears to be a date written in black ink on it. Looks like "23-1-70" to me. Perhaps either the gun was made or the CRT rebuilt on the 23rd of January 1970.

Attachment:
SAM_5640 (640x480).jpg
SAM_5640 (640x480).jpg [ 166.54 KiB | Viewed 647 times ]


This tag was found in the bottom of the box, it appears to have been attached to the neck but the glue dried and fell off. This and the straight gun visible in the previous photo means that an ion trap should not be necessary.

Attachment:
SAM_5644 (640x480).jpg
SAM_5644 (640x480).jpg [ 146.47 KiB | Viewed 647 times ]


The tube tested good for shorts, so I checked emissions and was quite pleased to find it tested well into the good zone. Testing for cutoff and life was even better, with the life test it took 21 seconds for the emissions to drop into the "bad" zone once the heater had been killed and a full 48 seconds until the emissions dropped to zero. This is substantially better than my current 21AMP4A which has had the cathode welded and the life test causes the meager emissions to drop to zero within a second or two.

Attachment:
SAM_5641 (640x480).jpg
SAM_5641 (640x480).jpg [ 217.96 KiB | Viewed 647 times ]


Attachment:
SAM_5642 (640x483).jpg
SAM_5642 (640x483).jpg [ 207.61 KiB | Viewed 647 times ]


I am very happy with this, and will work on placing this into my set as I am getting finished with the restoration.

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 Post subject: Re: RCA 21-S-510N - Restoration Round Two
PostPosted: Jul Mon 01, 2019 5:27 am 
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I ordered a 120pF 1600V mica capacitor and replaced the one in the yoke. It seemed to significantly reduce the lines visible in the left side of the picture. It did not completely remove them, however it is much less noticeable now. I don't think they would be visible unless one knew what they were looking for.

This did highlight another issue though, there was a slight drive line visible in the screen. However reducing the horizontal drive control and readjusting the horizontal frequency control completely resolved this.

I also added my hardwired A/V input adapter. The adapter itself is very similar to that shown on Phil Nelson's site, although I adjusted the value of the bridging resistors to 820 ohms and added a .1uF capacitor inside of the case of the adapter to feed the video amplifier grid. A DPDT switch chooses between the three RCA jacks mounted to the side of the adapter or the sets own tuner and IF stages. Coax runs to the associated circuits and back to the adapter. I routed the cables through the phonograph input hole in the chassis that this set lacks.

Attachment:
SAM_5650 (640x480).jpg
SAM_5650 (640x480).jpg [ 244.05 KiB | Viewed 566 times ]


There are two holes in the bottom of the adapter to mount it to the back of the cabinet through two of the many ventilation holes in the bottom.

Attachment:
SAM_5666 (640x480).jpg
SAM_5666 (640x480).jpg [ 255.43 KiB | Viewed 566 times ]


I then began the process of replacing the CRT. I removed the chassis and flipped the cabinet face down on some towels. The CRT is retained by a metal collar around the bell and a basket on the back holds the yoke and focus and centering magnet assembly. So I proceeded to remove the focus and centering magnet assembly and yoke once more.

Attachment:
SAM_5645 (640x480).jpg
SAM_5645 (640x480).jpg [ 198.74 KiB | Viewed 566 times ]


Attachment:
SAM_5647 (640x480).jpg
SAM_5647 (640x480).jpg [ 185.9 KiB | Viewed 566 times ]


Attachment:
SAM_5649 (640x480).jpg
SAM_5649 (640x480).jpg [ 155.78 KiB | Viewed 566 times ]


After removing those I loosened the bolts so that I could unhook the rear mounting bracket from the CRT collar. I then proceeded to unbolt the CRT collar from the cabinet and transfer it to the new 21AMP4B. I proceeded very carefully as I do not have great experience with handling CRTs of this vintage and wanted to avoid a possible implosion. I also took the opportunity to clean the inside of the cabinet while the CRT was out as the HV attracted a lot of light dust to the cabinet.

After this, I simply reversed the procedure and put everything back together. I performed the final picture adjustments and set the TV in its current place in my living room.

First, my comments on the new CRT. It is a substantial improvement. It has much more brightness than my old CRT did even with a brightener and can now be watched in daylight. I found that I had to reduce both contrast and brightness from where they were before by quite a bit. Also, retrace lines are not visible at a normal brightness level any more. I suspect this will give me many years of good viewing.

Now, Here is a photo of the finished TV:

Attachment:
SAM_5655 (614x640).jpg
SAM_5655 (614x640).jpg [ 228.19 KiB | Viewed 566 times ]


And a short video of the TV in operation (the skipping is due to the converter box, not the TV):

https://www.youtube.com/watch?v=m6x7L7Apvlg

I want to thank everyone who has assisted me in the restoration of this television. I know it is nothing particularly rare or collectible, but it does have sentimental value and I am sure some of my relatives will get a kick out of seeing something my great grandparents watched decades ago.

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