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.

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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.

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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.