Zenith 29JC20 21"--cataract and convergence
 

First big challenge is getting the cataract off the tube (21FKP22). The blear covers all but about the central six inches. I have watched a few youtube videos on cataract removal, mainly using heat. The tube is now out and sitting in the top of a garbage can. The plan is to set it out on a RILLY hot day, then if that isn't enough hit it with the heat gun. (I am in TN, so yes we have days in the upper 90's.) I would appreciate any advice or warnings concerning this operation.

The second big challenge is that the plastic frame for the convergence coils (and magnets) (looks like a 3-leaf clover) is busted in two places, so the coils are kinda ski-wacky. Suggestions for repair? I am guessing something like JBWeld. Or by some miracle are these available? What do I know?

Thanks HUGELY
Techguy46

There are two types of cataracts the green Halo and the moldy white. Green you can cut cold with a guitar string. The white you either heat or soak in water. If it is white and that far in it should be an easy fix.

You may be able to glue the clover. On my 29jc20 they just installed a newer style clover and spliced the wiring to the board.

For the "white" cataract, I think the water method is safest but it can take weeks. I've done this a few times; it doesn't work well with the "green" type.

I've done a number with heat, best with a decent heat gun. I actually did one on a dud (aired tube) using a propane torch and it seemed to work just as well though I'd be cautious doing it with a "live" tube. I also tried infrared bulbs on a dud one time and it worked well but the crt cracked underneath-too hot!

The set must have the original CRT. The CRT is RCA sourced, but it was numbered 21FK instead of 21FJ. Why? :scratch2:

Because they had to buy it from the one 'FK'ing supplier they never wanted to give business to. :D

Cloverleaf
 

I have a NOS cloverleaf. Might reproduction be a possibility with 3D printers? It would definitely have to be flexible to work.

As mentioned earlier, installing the hard plastic convergence assembly is the way to go.

Phil

Sure it should be possible. There are 3D scanners and printers around, and a variety of types of plastic filaments available...Once it has been scanned and any needed adjustments have been made to the CAD file then it should only be a matter of selecting the best-suited filament and figuring out how many copies to make.

Zenith 29JC20
 

Anyone know if Zenith 29JC20 (First Zenith Color) Has as nice picture as a later roundy like 26KC20?
Perry

It was on par with my 25MC33 before I sold the MC...Granted I'm using direct video injection since the tuner IF needed work and I was not prepared to open that kettle of fish.

Zenith 29JC20
 

Where did and how you connect the direct color video input?
Perry

There was a thread on here where someone else explained it. IIRC (it has been ~3 years since I did it...) all I had to do for the video was unhook the outputs of the two video detector diodes from the set and connect the unhooked connections downstream of them directly to the center conductor of the video cable I used...This also defeats the RF sound circuits IIRC.

Progress!!
 

Nice hot day, set the tube out with a black plastic bag over it around 8 am. By noon it was around 150 deg all over the face (used one of those laser temperature meter thingies) and the lens popped right off! Easy peasy. I will try to post some pics.

Next step is thorough cleaning and sealing.

Nice work! When a cataract is that far in it is easy to fix...When one has just started forming to the point that 1-2" of the edge is cloudy they are not as fun to fix.

BTW is that a 5" SONY 1/2" EIAJ RTR videotape in the background of the first pic?

The 29JC20 chassis delivered very nice color pictures with a sulphide (green) screen. Back in the '60's I used to go to the Zenith schools when they would come to Knoxville and Graybar Electric would host these as they were the regional Zenith distributor. The Zenith field engineer we called "the Dutchman" would tutor these classes. He would always refer to the 29JC20 and his young son was touted to be a 29JC20 expert!


As for the "cloverleaf" convergence plastic mount, it was always a problem even way back then. Its' main drawback was that is was not "adjustable" with a screw clamp mount and it was the dickens to reinsert the coil & magnet components in this plastic ring that was not flexible. In doing so it always felt that you were going to break it. The worst part was to replace the completed assembly on the neck of the tube as it was VERY VERY TIGHT! If you had a loose CRT socket, you could also crack the tube or break one or more of the wires underneath to further the hindrance. Time, age and heat would always make it brittle and break as it had a short life span.


As for removing the faceplate on a bonded face, a rebuilding plant that I would work at on nights and weekends, we would place a clean scrubbed tube facing a large (2' X 2') natural gas radiant heater (about two feet away) and after about 10-15 minutes we would place it face up on a wooden box with a one foot hole and take piano wire with a wood dowel on each end then place your knee against the tube and pull the wire between the faceplate and tube and pull. Be sure to wear heavy gloves when doing this not to mention handling the hot tube from the radiant heater. It would slice through with ease. I DO NOT recommend using a hair dryer or heat gun as this will create hot spots on the glass as the heat applied must be uniform throughout the whole glass area and can be dangerous as if you get a very hot spot on the tube itself as it likely can cause the tube to implode depending on the ambient temperature of the area. Moving the heat gun around on the face is usually a losing battle as when you move it around, the spot you just heated will cool rather quickly as there is a large mass area on the face.

I hope this answers a few questions here.

Julian,
You nailed it. I had forgotten what a "joy" those hard plastic convergence assemblies were! The original cloverleaf was much easier to work with, for sure.
Phil

Tom, you have sharp eyes, yup that is a 1/2" r-to-r SONY videotape. Also might note not one but TWO HP-35 calculators (1974) and a Schaub-Lorenz Weekend T50K radio (1964). I have a serious old technology Jones.

The convergence frame is indeed a very brittle plastic and it seems as if without the break in the collar I could never have gotten it off the tube. In addition to the collar break there is a break in one of the clover leaves such that the coil and magnet are twisted kinda sideways. I will post pictures in a while. The suggestion of making a new frame with printing is good (I do have access to a 3D printer), although I don't know how flexible that stuff is once it has cooled, and flexibility is a real issue in stretching it over the tube socket.

Techguy46

That was probably true for CRTs that were less than 10 years old. My experience with 40+ year old CRTs tells me that the laminate has dried out completely and is much harder now, and a wire will need a lot more force making it a tough pull, and a very risky one. And you definitely need to wear more than just gloves to use a wire on a hot tube.

.

If you can't get suitably flexible plastic to make it out of then perhaps you could edit the 3D file such that there is a gap between pedals of the clover then extend each half of the gap towards the rear of the CRT into a pair of sturdy tabs that can be drilled. Then a bolt and wing nut through the tabs would make for an adjustable neck clamp built onto the clover...Just thinking out loud.

You could always cut the gap and drill the holes as needed in a finished printing. You could also make it thicker where it needs to be for more durability.

.

Definitely true of white cataracts the OP has fixed. The PVA of the white cataracts is very hard, especially where it has discolored...Pulling a wire thought it does not seem possible without causing damage to the glass. maybe when new and hot, but even that seems a bit off to me.

The green cataracts on later Zenith tubes, however, are very soft especially where discolored and I have been able to pull a cold wire through those on a chilly fall's night with only moderate effort.

I may have missed it but I do not recall Julian specifying which type of cateract was being dealt with so perhaps he was discussing the green type more common on Zeniths rather than the white in the set the OP has.

That is a good idea. My suggestion was to make it so that the clover could be made not to squeeze the CRT neck during the installation process to make that process (which was noted to be very difficult) easier...Some later rectangular CRT clovers have a construction like I mentioned which Seems to lend it's self well to serviceability. Though I may be making a poor suggestion as my 29JC20 has a later clover installed and I only have a vague memory of pictures of the correct original clover.

Always interested to hear people discussing construction of replacement Zenith clovers - hope something finally happens, as a number of people including me would be very happy to obtain one.

By the way, some of the small front panel control knobs are made of equally fragile plastic and could be worth the trouble of remaking.

I have forgotten about the resin being 50 years old by now. It might make the resin harder to cut through. Not saying that it won't, but the wire used to cut through the resin should not scratch the faceplate. I have never seen a case of this.

BTW, somewhere I have two NOS Zenith cloverleafs and if someone out there is capable of copying one without damage, I would loan it to you, provided I might get a few copies!

I have a falling-apart one on my 25LC20 and yes, of course a spare from a 25MC33. I love the look of the troublesome ones and have inquired if mine could be a template.
I was told that ".stl file" was needed and that involves either a scan or some other type of prototype mapping operation.

Re "a bolt and wing nut" on the cloverleaf--or any metal patching for that matter--I would worry that this would disturb the fields from the convergence coils. Would patching a cloverleaf with say, small bits of brass or aluminum mess up the convergence?? Anyone try it? Just askin'....

Convergence assembly picture
 

OK, here is a closeup of the 29jc20 assembly. As described, one break in the collar and one break in a leaf. While the shape is pretty complex, I think it can be reasonably approximated with the 3D printers I have access to. I will do some research on the most pliable material available and get back if I think it is practical. I work (or used to) work at an engineering school and may be able to use a laser scanner that they have.

I will be out for a week--please be patient.

Techguy46

I've seen later design convergence clovers with a wing nut bolt to adjust how snug/loose it fits the neck...As long as you don't magnetize the bolt it will not affect things.

Post 91 of this forum thread compares my Zenith 29JC20 picture (first 3 pictures) to my RCA 21CT55 (last 2 pictures):

http://videokarma.org/showthread.php...th6S321&page=7


Dave

Regarding color, The first one most likely has a sulfide tube (unless it was replaced). I don't know if the later one came from the factory with a rare earth tube, but if so, that would make a difference, with both brighter and truer reds.

Also, at some time (I don't know when, maybe not until rectangular tubes), Zenith went from the original RCA "9300+27MPCD" white point (really meaning, the red is not strong enough to make 6500K daylight white), to a unique 8000 Kelvin white point that no one else used. Moving toward a daylight white meant that variations in transmission were not so strongly reflected in skin tone variations.

Also, tube sets varied from one set to another more than solid state, so comparing any two roundie tube sets, you may get very different results from the average engineering aim point. Variations could be not only in the gray scale set up, but also in tolerances for the gains and phase angles of the color demodulators.

In terms of picture sharpness comparisons, I have no idea. Because of the lack of real high definition detail in analog TV, receivers always used luminance peaking to enhance the contrast of edges and small details. The amount of peaking and the peak video frequency designed into a chassis was strictly a matter of taste, and you could see differences in different manufacturers. Peaking was affected by both video circuits and tuner/IF response shape. These parameters were always a matter of discussion in product engineering and marketing, with side-by-side viewing of different proposals. So, besides differences between manufacturers, you could see the choice of parameters drift from year to year within a single manufacturer as internal opinions changed. Of course, there were also set-to set differences due to alignment. Much later, alignment variations were reduced drastically by the introduction of surface wave IF's.

I'm afraid I haven't said much that's definitive about the two sets you asked about; maybe someone who saw a lot of both chassis could comment on the average trend.

Progress report
 

Tube (21FKP22) is clean, lens is attached with silicone goo, remounted in cabinet without catastrophe. So far so good.

I tested the tube with my B & K 465 CRT checker. OK emission from all three guns, fairly well balanced. The life test was not too optimistic, the needle bounced up and then headed down fairly quickly, again on all three guns. Not surprising, this set was used heavily for about ten years before it got retired. I am reluctant to do any electronic "restoration" at this point, it should make a picture.

Best to power it up on a variac. Mine did produce a vertically shrunken color picture on all original caps for a while, but bad lytics drawing too much current caused the 3DG4s to short and a panic shutdown not long after...These are getting to the age where new lytics are mandatory, and recapping the sweep is good practice,

There are enough Black Beauties in the chassis to be really scary. I have replaced hundreds of them in my radios, and they were all leaky to some extent. Beats me why the audiophools pay big bux for them, sigh. In any case, BB and electrolytic replacement is in order. Just have to clear the bench, that is a LARGE chassis!

Actually, I believe it's guitar people who love "Black Beauties"...something about tone. Dunno...as an audiophile myself I don't want any of those leaky buggers around.

At least there is money to be made taking the bees we'd normally round file and selling them to the guitar people.