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 Post subject: RCA 21-S-510N - Restoration Round Two
PostPosted: Dec Wed 26, 2018 6:42 am 
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My how time flies. I have had this television sitting in my basement for almost three years now, and feel I have gained the knowledge to get it working properly again, hopefully with the help of the folks here.

As of January 2016, I had acquired this set, and replaced its capacitors and out of tolerance resistors, but was chasing an issue with a lack of picture, but good raster and sound. At some point, the television lost the raster entirely, and I became discouraged and my interest lapsed, so I let it sit in anguish.

Since then, I have gained more experience working on radios, and have completed one other successful television restoration, and now have renewed interest in getting this set working again.

The chassis was much as I left it three years ago, some dust needed to be removed and items cleaned. I went ahead and retested the tubes, found a 6SN7 with a H-K short and replaced it (although, since that cathode and one side of the heater were grounded it shouldn't have caused any serious issues). I rechecked my previous work against the schematic and found nothing amiss. I cautiously powered up the set, and found that it was in much the same condition that I had left it, good sound but no raster. (The ion trap was in place as well.) There was high voltage present, and I could hear the horizontal oscillator running, so I proceeded to think of the worst and checked the CRT. I checked the voltages and found that all was as expected, except that the cathode had 0 volts present.

I do not have a picture tube tester, so I used the quick and dirty method to check emission, and thus the cathode described here: viewtopic.php?f=3&t=331576

Using the above method, I found that there was practically no current flowing between K and G1. If I increase the heather voltage to about 7.0V, I can see about 0.02V between the cathode and ground, which is much less than expected, and probably is showing filament emission. The conclusion I have arrived at is that this CRT (a 21AMP4A) likely has an open connection to the cathode sleeve. I removed the base and re-soldered the connections to the tube and rapped on the tube neck a couple of times, but found no improvement.

My question is with regard to re-welding this tube. I understand that some testers had a function to do this, would it be possible to replicate this by capacitor discharge or some other means, or should I just start looking for a new 21AMP4 or equivalent picture tube?

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Last edited by Hcompton79 on Jun Sat 01, 2019 9:51 pm, edited 1 time in total.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Wed 26, 2018 2:03 pm 
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Before assuming that the cathode is open for sure, maybe you can get someone local with a tester to do a complete test. You may be right about open. But I may not be tempted to do something drastic first. Just a thought though.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Wed 26, 2018 3:20 pm 
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I'd start looking for another CRT. That was a really common type and you should be able to get one pretty easily, although the ETF is currently not showing any in their inventory. I might even have one for you, but mine are all in a storage area and unfortunately I don't have a list of what's there.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 3:46 am 
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I believe I have a new old stock RCA Silverama 21AMP4A. I have shipped 21" tubes successfully, and know how to pack them properly, however I'm not too keen on the idea when it's freezing cold in this part of the country.

Dennis, what are your thoughts on shipping these in the winter time? I suspect they were commonly shipped back in the day, but then again there were probably warranties that covered things such as defects caused by the cold?


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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 4:13 am 
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I expect they were shipped all day long back then regardless of the season. We used to order replacement CRT's direct from the local distributors and get same day-next day delivery so that would indicate the factory was shipping to the distributor all year around.

If I were Hunter, I'd be thinking of driving to meet you somewhere to pick the tube up in person though, as I don't trust shippers to be gentle with something so fragile today.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 4:17 am 
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If that CRT has hollow pins I would suggest resoldering all of them, even if they appear to be OK.


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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 4:18 am 
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I hadn't thought about the issues associated with shipping a CRT in freezing temperatures, however I am glad that Kevin brought such up. I too would like to hear from Dennis, as he has a great amount of experience here, but my thoughts are that that the transition temperatures would provide much more problem than the cold temperatures themselves.

On the seller and buyer's ends this could be minimized by gradually accumulating the CRT to differing temperatures such as transitioning from the house to the basement to the garage and then into the outdoors, but there would of course be no guarantees of the condition of the CRT when it is in the hands of the shipping company.

If this is a sufficient concern, then one could of course work out a deal with the premise that the CRT would be shipped during a time of the year with more favorable weather.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 4:21 am 
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Johnnysan wrote:
If that CRT has hollow pins I would suggest resoldering all of them, even if they appear to be OK.


What exactly are hollow pins in your context? This particular example has pins much like an octal tube, where a wire is run down the center and then soldered in place. I did try re-soldering them to no avail. I do have the tube base soaking in alcohol right now to dissolve the adhesive, so that I can pull off the base and make sure that there is no broken connection to the envelope.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 4:33 am 
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Mr. Detrola wrote:

If I were Hunter, I'd be thinking of driving to meet you somewhere to pick the tube up in person though, as I don't trust shippers to be gentle with something so fragile today.


Driving could be problematic, considering I'm located in northwest WI, and there's a rather large body of water separating our two states. Either way it will be costly. Bottom line is you need to be very serious about wanting to restore and watching the old TV. Obviously it would be much more cost effective if he can find one locally around the greater Detroit area.


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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 5:01 am 
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As long as you let the temperature gradually change from one extreme to the other, it ought to be OK.

When delivering new or repaired TV's that had been in the delivery truck for a while in below freezing weather, we always informed the customer that the set would need to warm up for several hours, preferably overnight, before applying power. Our shop never had any issues with frozen CRT's cracking at the neck, but some of the other local TV shops reported that they had an occasional failure by applying power too soon, as the heat from the filament warms up that area of the neck faster than the rest of the neck can get warm from just sitting in the room for a few minutes.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Dec Thu 27, 2018 5:18 am 
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Kevin Kuehn wrote:
Mr. Detrola wrote:

If I were Hunter, I'd be thinking of driving to meet you somewhere to pick the tube up in person though, as I don't trust shippers to be gentle with something so fragile today.


Driving could be problematic, considering I'm located in northwest WI, and there's a rather large body of water separating our two states. Either way it will be costly. Bottom line is you need to be very serious about wanting to restore and watching the old TV. Obviously it would be much more cost effective if he can find one locally around the greater Detroit area.


I agree with Kevin. This set is not a particularly rare or valuable model, but it is in very nice condition and has family history to it. As such, I would not be opposed to spending what is necessary to have it working again. I would like to pursue more local options at the present time though, and if I come up dry I may get in touch with Kevin in the future. I do not require immediate gratification to get this set working, it has been sitting for three years and it can sit a bit more, but I do not want to let it sit in perpetuity.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Tue 01, 2019 4:26 am 
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Yes, well if all socket voltages are present and the cathode reads zero volts, I assume that the brightness is set to max.
If there is an OC cathode, you should still see some light (usually retrace lines) on the screen in a dark room.

I got caught out recently with a broken connection on my test socket using that test and I thought I had an OC cathode. Check your wiring carefully!

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Tue 01, 2019 5:55 am 
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Well, I was waiting for parts for my Zenith L403 portable so I took another look at the CRT. I have had it soaking in alcohol for the past couple of days and the adhesive had softened enough to pull the base off. I did not see any broken connections with the base off, so I re-soldered the base on and performed the test described above again.

The results I got were much the same as before, however, for the heck of it I tried again and left out the 1000 ohm resistor. This time the reading started at about 0.3V and shot up to about 3.5V after about 10 minutes. Since this test is basically testing the tubes ability to boil electrons off the cathode, this new result confuses me as it indicates that the cathode is doing its job, but just extremely poorly.

Irob, the brightness control was turned up all the way, I can vary it and read 0-100V or so on the cathode pin, but I have no raster on the screen under any lighting or illumination conditions.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Tue 01, 2019 10:07 pm 
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The 1K resistor converts the current to voltage. 1V equals 1 ma. Without the resistor just a bit of
leakage would result in a voltage reading.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Thu 03, 2019 12:52 am 
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One thing that hasn't been mentioned is the position of the ion trap. The original CRT requires one, and if it's even a little bit out of adjustment, there would be no light on the screen. On some sets just a few degrees of rotation, or 1/4" front or back on the neck, and you would see nothing.

Also I'd expect that even a nearly dead tube would show a characteristic tiny white dot in the center of the screen when power is removed, maybe not very bright though. Do you have a way to verify that the HV on the CRT is reasonably close to what it's supposed to be? Having HV isn't enough if you only have half what it calls for.

At this point, you might want to borrow a test CRT like a 5AXP4 from someone, or another tube with the same neck size and basing as your original, and verify that your chassis is actually as functional as you are thinking it is.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Thu 03, 2019 1:05 am 
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I was thinking a test CRT would be beneficial. I believe a 8XP4 would be the appropriate unit for this set with its 90 degree deflection angle.

I probably also need to acquire some more test equipment to properly diagnose some issues here. I have ways to determine if high voltage is present, but not to quantify how high it is. I will probably be on the lookout for a scope, a HV measurement probe and some other things at the upcoming MARC meet this month.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Mon 07, 2019 7:13 am 
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Any reading at all with that test equates to some light on the screen (even if in a dark room) so the ion trap position sounds like the likely culprit. You should indeed get something. If you do, and the brightness control does not change it, an O/C cathode is suspected.

Look up the tube on https://frank.pocnet.net/. There might be an ion trap positioning guide.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Sun 13, 2019 1:27 am 
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As dumb as it sounds, I had a B&W 50s CRT set with an open cathode, yes it had no raster or brightness, and if you would tap the neck of the tube bright flashes would show up on the screen. Tested the CRT and it showed completely dead. This was back probably in the early 70s. Saw an ad in the news paper about a booster that would "cure" an open cathode. Was skeptical but bought one from a local TV repair shop that sold them. It was called a nulife CRT restorer. I think back then I paid a whopping $15. The directions showed which wires to plug into the terminal strip in order to "cure" an open cathode. Weird thing it brought back a raster and picture on the set. Although the picture was a bit smeared, it was watchable. Seen the exact same "booster" on e-bay and it's still for sale. No guarantees that it would work, but you might give it a try. Here is a link.

https://www.ebay.com/itm/NOS-NIB-Vintag ... Swz7NaWUKH

Here is another link to an advertisement of the nulife CRT restorer Go to page 87 in PF reporter magazine. Notice in the right side of the advertisement in the middle is claims to fix an open cathode defect

https://www.americanradiohistory.com/Ar ... 957-03.pdf


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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Sun 13, 2019 4:59 am 
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walterbeers wrote:
As dumb as it sounds, I had a B&W 50s CRT set with an open cathode, yes it had no raster or brightness, and if you would tap the neck of the tube bright flashes would show up on the screen. Tested the CRT and it showed completely dead. This was back probably in the early 70s. Saw an ad in the news paper about a booster that would "cure" an open cathode. Was skeptical but bought one from a local TV repair shop that sold them. It was called a nulife CRT restorer. I think back then I paid a whopping $15. The directions showed which wires to plug into the terminal strip in order to "cure" an open cathode. Weird thing it brought back a raster and picture on the set. Although the picture was a bit smeared, it was watchable. Seen the exact same "booster" on e-bay and it's still for sale. No guarantees that it would work, but you might give it a try. Here is a link.

https://www.ebay.com/itm/NOS-NIB-Vintag ... Swz7NaWUKH

Here is another link to an advertisement of the nulife CRT restorer Go to page 87 in PF reporter magazine. Notice in the right side of the advertisement in the middle is claims to fix an open cathode defect

https://www.americanradiohistory.com/Ar ... 957-03.pdf


Well that's an interesting thing. A cure all for picture tubes eh? While I find that of interest, I really would like to get this set working properly, meaning if it needs a new picture tube then it will get one. I really would like to avoid quick fixes as this set is both in really nice shape and has family history to it. I have been keeping an eye out for an 8XP4 test tube, so that I can make sure that the picture tube really is an issue, as well as to check other issues until I find a replacement 21" CRT. I hope to find some things necessary to continue this pursuit at an upcoming radio swapmeet, so i can continue on later in the month. I do appreciate the information though.

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 Post subject: Re: RCA 21-S-510N - CRT Open Cathode
PostPosted: Jan Mon 28, 2019 6:10 am 
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Well, I have made some progress on this set thanks to the help of Dennis (Mr. Detrola). I met him at the Michigan Antique Radio Club Vintage Electronics Expo this past Saturday, and in addition to buying a couple of things off of him, He was generous in letting me borrow a B&K 440 CRT tester.

I went ahead and tested the picture tube initially for shorts and continuity, and got the result I was expecting, none of the neon panel lamps illuminated indicating either extremely low emission or an open cathode.

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Testing for emission, I found that I could hardly get the needle to move on the display past its resting position. I let it cook with the filament set on 6.3V for about six hours yesterday and saw no change, then repeated the same but at 8.4V for another six hours today but again saw no change. This result, along with the previous testing and the fact that this tube did work in the past but suddenly went dark indicates definitively that it had an open cathode.

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I did note that the B&K 440 has a shorts clearing and element welding function, so I set it up for that and proceeded to press the shorts clearing button while rapping on the picture tube neck as described in the manual. After seeing several flashes in the tube neck and testing continuity and then repeating the process, I was able to get the neon lights to illuminate in a fashion that was proper for a normal tube.

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Testing emission, I found to still be weak, but indeed functional. I successfully re-welded the cathode.

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Finally, I reconnected the chassis and after powering up the set, and adjusting the ion trap, I was able to get a raster on the set. :D

It still has major issues, but it is a step in the right direction.

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