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The Set: Pete Deksnis's Site about the CT-100

Restoring a Vintage Color Television Set

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info exchange

3-3-00 Thanks for your web site. I too have a CT-100 which is in need of some fairly serious rebuilding.

I have had the set nowhere nearly as long as you -- only about six years. Mine came from a retired New England television station employee -- the set had served as the station's first studio color monitor. Later when broadcast-type monitors were purchased, the CT-100 found a new home at the transmitter site. Another upgrade there freed up the set and it became the employee's first color tv.

As might be expected, with the moves (was it B. Franklin who said that "... three moves are as good as a fire"?) and less than ideal maintenance and storage, the set is missing several parts -- knobs, back, wood grained filler under the escutcheon, and the escutcheon itself.

I purchased the set with the altruistic idea of restoring it and then passing it on to a radio-television museum that I had contributed to before. This never came to pass as the museum's founder left the picture not long after I acquired the set and the establishment fell on hard times.

I did some initial restoration work -- removing rust from tube shields, ballast resistor, etc. and lacquering so as to prevent its return; checking and renewing tubes where necessary; and generally cleaning some 40 years of accumulated dirt from the beast. A change of job duties cut back greatly on my hobby time. This, coupled with the acquisition of quite a few other vintage television and radio sets which just about filled my work space, caused me to put aside my restoration efforts until very recently.

I purchased a much larger home a few months ago and now have a work space to die for. The CT-100 has been safely transported once more and after a bit more unpacking and putting away, I expect to start the project again.

3-12-00 I pulled Peter Keller's book "The Cathode Ray Tube", copyrighted 1991. Contained therein is a very interesting chapter on the evolution of the color crt.

Mr. Keller describes the RCA crash program to produce a tri-color tube for their electronic color system and along with some information on their "hand built prototypes", spends some amount of time in describing the construction of the 15GP22.

On a slightly discouraging note, Keller states that, "Comparatively small production quantities of the 15GP22 were manufactured."

5-26-00 I would like to say that your website (and the nice large shop in my new home) spurred me to get back on the restoration project.

I too am a firm believer in getting rid of all of the paper caps and have been working to that end for the last three weekends or so. There are only about a half dozen left and these are ones that I didn't have replacements for. I'll get them changed out and finish the job as soon as the order from DigiKey arrives. I'll probably devote part of this long [Memorial day] weekend to cleaning more of the 45 years of filth from the chassis and checking and replacing tubes where necessary.

Speaking of caps, I noted in the correspondence that Mike was having trouble getting a replacement for the high-voltage focus line filter. I had a 0.01 uF @ 10,000V unit in my capacitor kit, originally purchased for another project, but it substituted for the focus filter rather nicely. You might refer Mike and anyone else needing a replacement for this cap to Frontier Capacitor in South Dakota. I've bought a goodly number of HV units from them for restoration of early tvs with electrostatically deflected crts. Their business address is: Frontier Capacitor, P.O. Box 218, Lehr, SD 58460.

Speaking of replacements, has anyone reported any trouble with the CT-100 selenium rectifiers? I've yet to run up the B+ voltage in my set, but do remember from my teenage servicing days (and some of my early broadcast career) that seleniums were not exactly trouble free and with those in the CT-100 fast approaching the half century mark, I can't help but wonder if I'll be smelling that rather pungent odor associated with selenium rectifiers going west.

On a parting note, I'd like to thank you again for referring my wants to Steve. He came through in a big way with the wooden filler for the front of the set. It really makes a difference in the appearance (if only I could find the metal door control cover now!). I was planning to cobble up some sort of filler for the space below the metal control door, but he saved me a lot of trouble. I'm sure mine would not have fit as well either.

Speaking of fit, I did have to do one little bit of alteration to the part Steve supplied and this has got me wondering about production runs, cabinet suppliers, etc. for the CT-100. As you know (and I didn't until I received Steve's contribution), the filler is held in place by the engagement of snaps on the rear of the panel with holes in the two vertical metal strips mounted to the cabinet. The snaps on Steve's piece weren't even close to mating with the holes. I looked very carefully at the metal strips and determined to my satisfaction that they had never been moved (and could not move much, if at all). To make things fit, I had to remove the snaps from the filler and relocate them over half an inch -- well beyond any normal manufacturing tolerances. To me, this would almost appear to be the result of different production runs or different cabinet suppliers. Any thoughts on this one? [
Interesting puzzle. Let's see if we get a response. --Pete ]

That's it for now. I plan to reform the can electrolytics as soon as I'm finished with the under-chassis caps. My next move will of course depend my success with the reforming process. I'll keep you posted.

Again, good luck with the CRT rebuilding operation. I've gotten somewhat nervous about 15GP22's since reading of your and the other experiences. I didn't mention it before, but I did get a supposedly NOS 15GP22 when I bought my set. It was contained in a GE box which had seen better days. I built a sealed wooden box to store the GP22 and the remains of the GE box in, but before I sealed up the crate I did a visual inspection of the tube and took several pictures -- no white getter or other apparent damage to the tube. I have not had the crate open in a number of years, so I'm hoping the tube is in the same condition now.

10-30-00 Hello again! It's been a while - summer activities have taken their toll on my CT-100 restoration project. It's still on the bench, fully recapped now and a lot cleaner than it was. Yes, I did discover my share of open peaking coils. These are being ordered at long last, and I hope to be applying voltages before too much longer.

Your speculation [
refer to my Restoration Log Menu link of 10-1-00 -- Pete ] about the crt's copper exhaust stem got me thinking about a tour of a major transmitting tube rebuilding facility I took several years ago. The company specialized in rebuilding just about any large tube (and a few small ones -- reflex klystrons). One of the bits I recall was the tubulation used for exhaust purposes. I was told that even if the tube came in with a glass exhaust stem (and most did), a copper one was fitted (Housekeeper sealed I presume) to the core in every case. This had nothing to do with the vacuum ultimately acheived, nor did the stem act as a getter (I've never seen copper listed among active gettering materials). The fact of the matter there was that the company preferred to seal off the vacuum by crimping rather than heating and constricting. I did not push the matter, but suppose that crimping might make for more uniform results, even though all tubes were being rebuilt one at a time.

I do not claim any especial expertise in this area, but might speculate that as the 15GP22 was apparently built in rather limited quantities, the crimping operation was easier and could be performed manually in a more limited area than a glass tubulation seal.

(To seal a glass tubulation by hand, a small tipping torch is applied to the tubulation as the vacuum is being drawn. When the glass softens sufficiently, the tubulation is constricted by the vacuum. Once this happens, to make a neat and even seal, the glassworker has to manually pull the sealed object away from the pumping station (or otherwise apply some mechanical intervention) to separate the molten glass ends. The open end of the tubulation is attached to the vacuum system via a gland type of seal -- usually right above or even a part of the difusion pump and this is not moveable).

A crimp and a cut seem much easier when you think about it.

Further, even though the crt would be in a bakeout oven for the final vacuum processing, the localized high heat applied by the tipping torch could liberate some gas or water vapor molecules bound up in the glass tubulation. These would not be pumped away, as the glass's constriction effectively isolates that crt from the vacuum pumps. While this might not amount to much impurity, it is just that much more that the tube's gettering agent (not yet fired at this point) would have to contend with. Since the 15GPs were a limited production item, maybe RCA wanted to build them to as high a standard as possible. Also, and this is admittedly fairly remote speculation, perhaps it would have been easier to open up and re-exhaust a copper tubulated crt than a glass tubulation in the even that the tube didn't test out and had to be re-evacuated. (This would assume that the tube was tested before the getter was fired.)

1-12-01 [posted 2-4-01]The tube and wrappings I crated were not from General Electric as I had originally reported. I have similarly crated several other crts for safekeeping and confused a rather mundane GE crate with the 15GP22.

I did look carefully and somewhat nervously at the tube's getter. It was still very silvery. I am somewhat encouraged after reading the other tales of white getters.

I took a week off around Christmas and finally finished the recapping and replacement of open peaking coils.

I formed up the original electrolytics successfully and then brought up the unit with a Variac (high voltage circuitry disabled for this trial run). No problems, smoke or otherwise, were encountered and I was rewarded with good audio. I observed (via oscilloscope) video and chroma signals, checking periodically for hot spots, excessive current draw, etc. before shutting down.

Before I hauled the chassis, yoke, etc. upstairs to the cabinet and 15GP22, I made a final test with the high voltage circuit activated. Unfortunately, I had only about 7 kV at the HV connector. Voltages looked good around the 6CD6, so I am probing further and have ordered a new sweep tube.

I am greatly encouraged that the set (seleniums included) passed the smoke test. Hopefully I will have some more good news in my next communication on the set.

2-5-01 I did make time last weekend to get back to the case of the missing 13 kV or so in my CT-100's HV circuitry. My troubleshooting efforts have led to another mystery. In the process of diagnosing the HV fault, I began to disconnect some of the loads from the flyback.

Interestingly enough, at least one of the loads on terminal 1 is the vertical sweep circuit. The plate of at least the half of the 12BH7 used as the vertical oscillator does not get its B+ from the 375V or 400V (Sams or RCA) dc power supply tap as shown on both Sams and the RCA drawings. It is supplied via flyback terminal 1. (I did not take time to trace the source of the vertical output half of the 12BH7, but I suspect that it may not go to the dc power supply tap either.)

If you look at the HV/sweep circuitry, flyback terminal 1 would be the source of "boosted B+" in a B&W receiver. It is labeled on the Sams CT-100 schematic as "boost 720V", yet it appears to go nowhere. I would have thought 375/400V would have been enough plate potential for the vertical sweep stage. (Sams only calls for 105V on the vertical oscillator.) However, what I have in my set looks factory wired.

Also, I did mention that I obtained replacement peaking coils from Digi-Key, but neglected to provide the part numbers. I didn't need a complete set of coils, but here are the values that I did cross:

- 120 microhenries - pt # M7842-ND
- 180 uH - pt # M7840-ND
- 250 uH - closest is 270 uH - pt # M7842-ND
- 36 uH - closest is 39 uH - pt # M7832-ND

I hope that this helps.

2-16-01 As I noted before, there is definitely more than one connection on flyback pin 1. This is the source of boosted B+ and it is feeding the vertical stage in my chassis. The connections do look original. It's not a straight wire connection over to the vertical sweep area, indicative of someone's mod, either.

I'm planning to spend some amount of time on the set over the next few days, as today, coupled with the Monday holiday yields me a four-day weekend. With any luck at all, I'll have the HV up and running and maybe can carry the chassis upstairs where the cabinet and CRT are waiting. (Keep your fingers crossed that it's not the flyback! I dusted off my B&K sweep circuit & flyback tester two weekends ago, but didn't get very far due to time constraints.)

Since I have your number, I may give you a call. At the least, I'll be producing a schematic drawing of my findings and will see that you get a copy.

2-25-01 Here are the numbers as requested from my CT-100. I believe that these are all that there are!

below the Candohm resistor: B8002239
immediately to the right: 1103207 - 1SUB4 (these are stamped)
CTC-2 415L1 RVB274 (these last three are silk-screened)

removable top section - 46
on cabinet member just below top - 83

On another note, I think I mentioned previously that last weekend I had succeeded in producing a sound and an image on the set. However, I had no vertical sync. I traced this to a defective printed couplate vertical pulse integrator network (never saw one fail before) and made the parts house circuit here last Monday trying to find a replacement. I learned that not only did the houses not have the part, no one working that day had any idea of what I was looking for. It seems that if it's not related to computer networking or connectors, they don't carry it. I had to make two stops before finding some corona dope (at some time in the past R186 (50 megohm) had failed and was replaced with three resistors in series -- with the HV back up to the 20 kV level, there was a lot of spraying from this series string). Alas, I can remember when corona dope, corona putty, tuner cleaner, and even aquadag coating were staples at all such establishments. We're getting old.

Anyway, I constructed an integrator from discrete components and low and behold I had a locked-up picture! The picture tube focuses nicely and I have plenty of output from all three guns. This was good news indeed. The downside was that I didn't have any color.

I tried several things and was only able to get color by severely detuning the fine tuning control to favor the chroma information at the expense of picture sync. My guess is that someone along the way screw-drivered the alignment. Although I'm not looking forward to it, the plan is to put the set back on the bench for a complete alignment as soon as I can make time.

I should note that I was able to achieve fairly good purity and gray-scale tracking. The convergence leaves a lot to be desired, although the image is watchable at normal viewing distances. In rotating the "cross purity coil", I did manage to blow the 4.5-amp fuse, so this unit will be disassembled and checked for shorts. I am still playing with the D.C. convergence, however things (neck magnets) do not seem to work as effectively and precisely as they have in any of the "more modern" sets I've converged. I don't know if this is typical or perhaps I haven't found the "sweet spot" for the magnet assembly on the neck or maybe the magnets have weakened over the years. I'll attack this after the alignment procedure. No rest for the weary.

Even with compromised convergence and poor sync, I must say that it was quite a thrill to see a color image on the old girl this Saturday. I looked at a little off-air material.

I would like to add one more thought in parting. This pertains to the deflection yoke.

For some reason, the plastic cover on my yoke has become deformed (the body of the yoke is fine). The cover no longer fits very well. I didn't give this much thought until I attempted to move the thing during the purity setup.

Warning. I can certainly vouch for the health of the sweep circuits, even when overloaded with my body resistance! I would highly (make that VERY highly) recommend that the eyelets on the periphery of the yoke be covered with a couple of layers of Scotch #33 electrical tape and that any ill-fitting back covers be taped in place with the same material! Enough said.

You might want to air this yoke cover thing. I'm curious to know if others are deforming.

10-20-02 I've been busy since I got "the set" working last year and haven't gotten around to the required cabinet restoration that I promised myself after seeing pictures on the old girl. About the only things that have changed since I last wrote you was the pulling of the chassis in preparation for the promised cabinet resto work and snagging a "pencil box" on e-bay earlier this year to cover up the ugly hole in the front. (It was not the correct one -- came from a 21-CT-55. However, it does improve the looks of the thing and will have to do until the right piece comes along.) Maybe with the coming of cold weather and the cessation of outdoor activities, I'll be able to get the cabinet in decent enough shape for a photo or two or three. (I could offer those now, but the thing is pretty ugly with a lot of bare wood showing through what is left of the finish and some water damage to the lower extremities -- an artifact from wet basement storage in Connecticut).