Username1, I'm not sure what you are getting at. We are already fusing a new piece of glass to the old shell, and we are planning to rebuild the gun, since the supply of guns is very limited. The only materials we need are neck glass, stems, cathodes, heaters, and spacers. Neck glass and stems will never be a problem - there will always be glass fabricators that will do custom jobs. Cathodes and heaters may become problems at some time in the future, but are available now. If it appears that the supplier may stop making them, we will order a large quantity.

So, I don't see any need to ever change our rebuilding method and evacuate the entire oven.

As for cleanliness, Hawkeye was very un-clean, and yet they produced high quality rebuilds. Our facilities and oven will be much cleaner.

Pyrex CRTs present special problems. Apart from the higher temperature needed to work Pyrex it's likely that a soft glass pinch will need to be used. There are no new Pyrex pinches and re-using the old one has been regarded as not feasible. Though I suppose that could change if driven by desperation.

You can't fuse pyrex directly to soft glass as the differing coefficients of expansion will ensure the result will crack. Hence you need a graded seal, with several intermediate grades of glass between the pyrex and soft glass. The special glasses are available but it requires a high level of glassblowing skill to do it. My understanding is that RACS subcontracted that part of the work.

I haven't studied the structure of guns in detail but I think there are 2 main ways they were assembled. Older ones were spot welded while newer ones had their elements fused into glass rods. The former can be dismantled with appropriate skill and care, the latter can't. I hope that the HK assembly can be removed from later guns without the need for extensive dismantling.

As for cleanliness it's just commonsense to keep things decently clean, even if oldtimers' shops were a bit mucky. The only rebuilding shop I've seen was Display Electronics, near Heathrow airport. This was fairly mucky but they turned out good work. They didn't do any work on guns, just fitted new ones as was always the case back then. The 6/6 they rebuilt under my direction in 1986 had its gun rebuilt by (long defunct) Thorn-EMI Electron Tubes. As others have said, there is no dirt problem when pumping and baking the CRT. the muck is outside and the vacuum is inside. However a tiny speck of dirt in a gun assembly will likely cause trouble. Hence if the oven is in the same room as the gun rebuilding then the whole place needs to be kept clean.

We're not talking about microchip fabrication here, no need for clean room facilities, but it's always easier to make a clean place dirty than a dirty place clean:D

As pppenguin says, prewar tubes present many more problems. RACS finally gave up on doing them because of problems with cracks near the anode connector, though tubes have been successfully been rebuild by others.

Our focus is now on two types of tubes: late 40s/early 50s black and white tubes, and early color tubes (15GP22 and 21 inch round). I am concentrating on the black and white tubes, while two other collectors (with their own lathes and ovens) are working on the color tubes.

It is our hope that one day we will tackle prewar tubes.

I don't know if this has been asked but. Will that also include electrostatic tubes like the 7JP4?

I forgot to say that Steve and his team are doing a great job for all of us interested in keeping these old sets alive.

In the (good) old days you could set up a tube regunning shop in an old garage. All the guns were readily available and the skills were quite widespread. Results were a bit variable, the best rebuilders got results that were the equal of the manufacturers, the bad ones went out of business. Now new guns are not available and the skills are not so well known. All makes it much harder.

7JP4s create some additional problems. We would have to have stems made. Shipping of these tubes is an issue since the guns are so delicate. And, I don't know of anyone who has done an electrostatic tube. It is probably possible, and we will investigate it, since there would be a huge demand for that tube.

Here is a progress report on the project:

http://www.earlytelevision.org/crt_project.html

Fired up the oven yesterday. After 1 1/2 hours it was only up to 450 deg F. Another collector has the same oven, but with different insulation. His reaches 775 deg F. in less than an hour. He is using a total of 5000 w for heating, I have 8400. Today I will add another layer of ceramic blanket insulation and try again.

If it were me, slower heating would be better, but using more energy to do it, is not
that impressive..... Sounds like my winter heating bills..... I guess I better check to
see if you swiped my heater.... :)

Is the temp meter working right? Is it digital.... is it set right...?

.

Very daft question. Is your temperature readout set to celsius (as used everywhere except the USA) rather than fahrenheit? 450C is about 840F.

I'm using an analog thermometer that reads 0-1000 deg F.

I've added another inch of insulation on 4 of the 6 surfaces. I'll fire it up today and see how much that helps.

Here here.
I'd also like to add that I feel this rebuilding project, much like the sets on display at the museum, is a way to keep television history alive. I've heard many people say how ubiquitous picture tube rebuilding operations were in the past and how that's all gone now. Having a picture tube rebuilding operation at the museum is another " functioning exhibit" on display.

The insulation helped a little, but still the max. temperature is 500 deg F.

The problem is leaks, allowing hot air to rush out the top and cold air to rush in the bottom. I will make a gasket for the door and plug all the holes in the inside of the oven.

Yep, 50' of stove-gasket and a pail of refractory cement will serve you well in this situation. Also consider throwing a test cadaver jug in while you're still in the test and measurement phase. (Unless I've missed the part where you may have already mentioned including one.) It may have beneficial or detrimental effect on the convection conditions. I'm certain that an unoccupied oven will behave quite differently to an oven with a large hollow glass object inside.


Another way to look at things is you're going to have a sizeable portion of the volume of the oven occupied by a vacuum, so on paper things will heat up faster with less energy input. Obviously in reality the thermal inertia of the glass envelope more than offsets the 35+ liters of vacuum in terms of required heat input.

Are you planning to measure the cool-down time as well? Definitely do this with some sort of test mass of glass. If it is too well insulated you might wind up waiting 6 hours for things to come down to the point where you can safely retrieve them to ambient conditions without catastrophic thermal shock. My experience with glasswork consists mostly of laboratory style jointery in pyrex, and some mucking around with soda-lime pieces. I'm unfamiliar with the whole hot-to-not side of the thermal processing on CRTs and the thermal down-ramp might not be anything critical other than "Don't open door above 160C". Still worth considering in terms of "how long will things take on a per-tube basis" since the oven is a one-at-a-time bottleneck. (Or maybe three if they're 8" tubes and the evacuation system is engineered for it.) In the event that some enthusiast¹ drives a van loaded with all of his dead tubes up from California to be a part of rebuilding them, if each tube requires ~8 hours to oven process he's going to have a LOT of free time to kill given any realistic number of tubes that can concurrently occupy a van.




The literature on the "Champion" plant has all the hallmarks of something you'd find in the text only advertisements at the back of magazines. "Make Money Rebuilding Picture Tubes In Your Garage! Send $5 for our full brochure and catalog to:" type of thing. Sure, we know it works, but its all very rough-and-ready procedures. It does mention "One tube per hour" for the oven, but I think that's a bit generous, even in ideal conditions. I can't find any specific documentation of how long it "should" take. Obviously, "When the vacuum is good enough you can tip-off" is precisely "how long" it takes, but they don't even give estimates for the various size tubes.


On a related note, what are the plans for servicing the pumps prior to the first live-fire testing? High vacuum pumps left idle for long periods, especially if exposed to atmosphere along the way, need servicing before they're ready to suck once more. At the very least the oil will need flushed. That is to say not just a quick drain and fill job. More like drain and fill followed by a run for a few hours, then drain and fill again, pump to hard vacuum, maintain for 90 minutes, and keep the system airtight from that point on. An outright overhaul wouldn't be totally uncalled for, depending on the condition of the pumps.



I hope that no one here is interpreting my posts as doom-and-gloom nay-sayer "born to fail" etc... To the contrary, I very much want this project to succeed beyond all of our wildest dreams. I'm merely trying to ensure that things are considered from all angles so that the project doesn't wind up dying from a self inflicted foot-wound. If I'm pointing out something that might not be wise, I'm going to do my best to provide a proper explanation, and also a workable solution where applicable. I'm not just sitting on the sidelines trying to shoot everything and anything down for kicks.












¹ We're all mad here! :D

There is an established ramp up and ramp down procedure for tubes - different for glass and for metal tubes. Our controller allows up to 64 time/temperature periods. We have documentation from Hawkeye and RACS for this.

I'm not sure having a tube in the oven will make much difference. The oven is about 20 cu ft. A typical 12 inch CRT has a volume of about 1.3 cu ft, while a 21 inch color tube (the largest we will do) is about 5 cu ft.

Our production will be very limited - probably one a day at most.

I agree on your thoughts on volume production, 1 a day is a good idea.

And by all means seal the leaks ! Just think about a house with energy costs...
Seal the lower end, it will be cooler....

You might think about making a "hat" with ear flaps you can just pull down outside
the oven, it may just be easier to deal with.... And you won't give up any inside volume.
Close the door, pull the hat down....

You may find you need to add a heating element too.....

.

It is possible to pump a CRT without baking. This was done for the 6/6 rebuild at Display Electronics in 1986. They gave 2 reasons. One was worries about the state of the pyrex, fears that it might implode due to all the imperfections in the glass. The other was the purely practical point that the CRT was too long for their oven.

The CRT worked very well after rebuild but I don't know how it's doing a few years on. The set with this CRT is currently in store at the National Media Museum, Bradford.

I know that the CRT wasn't left down to air for very long. While the gun was away for rebuilding they fitted the graded seal and rough pumped it. They also fitted a generous amount of getter.

Interesting, Jeff. When we get around to playing with prewar tubes I'll keep that in mind.

As I understand it, the reason you bake a CRT during pumping is to drive out any air and volatiles that might be retained in the glass and gun. I don't know enough about the process to judge what might cause problems with air and volatiles. I also don't know if it's possible to use the RF heater's work coil to heat and outgas the gun assembly without inadvertently firing the getters.

Just curious, would it be possible to get an adequate clean vacuum without bakeing the tube? I know it sound like a strange idea, but is it even remotely possible using modern equipment?

It's a question of process time and the dislodging of adsorbed contaminants, not the modernity of the equipment. A vacuum pump cannot draw contaminants in the tube from a distance, it can only trap them when they travel randomly to the pump.

More progress on the oven, though it still isn't finished. I bought some high temperature caulk (good to 750 deg C.) and sealed all the joints between the steel and frame on the inside of the oven. The caulk hardens like clay.

I was able to get the oven to 700 deg F, which is about what we need. However, when the oven had cooled down a good deal of the caulk had separated from the metal and fallen onto the floor of the oven. This is because of warping of the steel that occurs when it reaches a high temperature. I have the steel attached to the frame with bolts spaced 12-18 inches apart. By putting additional bolts along each edge of the steel I think I can contain the warping and that the caulk will stay in place.

I'm very glad to hear all the progress reports.. It seems like everything is falling into place.. A few years ago many were worried if it will ever happen, and now we are most certain it will..

I worry about the mixed units. Didn't a space mission once fail because one part of the team was working in metric units while another was in feet and inches..

Quite a few space missions have ended in failure for this type of reason. To my knowledge the Hubble is the only one to have overcome it. Everything else crashed and burned.

Can't you also disassemble the frame-steel interface and put caulk in the actual joint before squishing it together to help seal the joints all the way through?

Yeah, it's a bit of a problem when you don't notice your problem until it crashes... :sigh:

For Hubble, I'm assuming you're meaning the lens sag issue because they forgot to account for gravity bending the lens while it was sitting on Earth pre-launch?

That was not the Hubble problem.

The Hubble problem was idiotic stupid pigheaded project leaders.

There were two different test devices used to measure it.
One, the simpler one, correctly said that it was exactly as it was:
very very wrong. The more complicated, expensive one, was simply
wrong. No one ever checked either test device. No one did a trivial
by-eye test that would have verified which was right, for
two reasons:

1) that doing so required a human to climb a scaffold and that was
deemed both too dangerous and too expensive (building the scaffold,
the by-eye test apparatus was free ... several workers personally owned one, as do I).

2) the idiots said that the by-eye test was not sensitive enough
to detect any possible error. That was, however, assuming that their
already-decided idea that the complicated tester was right was indeed correct: they knew that the by-eye test would be sensitive enough to tell
which tester was correct.

drussell, yes I can put caulk between the steel sheets and the frame. However, the stuff cures like clay and probably wouldn't provide much of a seal.

Maybe you need to forget the calking and go to overlaping strips of sheet steel like
corner molding not tied to the larger sheets so they can all expand and contract
without warping. Maybe tied to the frame, and rasting on the larger sheets inside
the oven, possibly made of several layers covering the area fully, but not a single
piece end to end so expansion will not cause it to warp.

.

Any members with some TIG skills? Pre-heat the oven and weld the corners up, then let things cool down and see if anything shears off. Since it is just mild steel I don't think it would present much of a challenge even to an "only-occasionally" TIG operator. Stainless would raise the bar a little, but still doable by a non-pro operator.

Your other option is to widen the gaps somehow, and "caulk" them with stove gasket. Caulk here is used in the original nautical term of shoving greased rope in between the decking boards with a mallet and tamping tool, not the modern stuff that comes out of a tube.

Success with the oven. By using lots of bolts to attach the steel to the frame I was able to seal it well enough to get up to 700 deg F in about 2 hours. To improve this, I'll get some stove gasket material and put it between the steel and the frame.

The next issue is the controller. It doesn't work, and I think I may have damaged it by applying 120 vac to the wrong terminals. I might have to buy another one.

I'm going to try this:

http://www.amazon.com/Nomex-High-gas.../dp/B00CI7A69I

Check out the latest progress report:

www.earlytelevision.org/crt_project.html

1948 dumont WOW
 

Hi,
im just looking to find some info from some people who know what there talking about, I have what I believe to be a 1948 Dumont large screen tv, radio,and phonograph all in one mahogany entertainment center. The TV comes up in middle( very cool ) the unit is large approx. 5ft wide 4ft tall. from what info I do have is this was a high end unit back then.
If anyone can point me in the right direction or any help please contact me 516-707-3686

thank you
Rob
also have other small units ( help ) hate to see history go to waste

thanks again

You have a DuMont RA-101-1A:

http://www.myvintagetv.com/dumont_101a.htm

A better forum to find out stuff about old TVs is:

http://www.videokarma.org/forumdisplay.php?f=19

Steve-

I am happy to read about all the progress that you and others have made with the CRT project. One thought I had, now that I have read both the entire Web page as well as the discussion here: I noted that the heating element is from a convection oven. Would it be feasible to obtain and use the element(s) from a self-cleaning electric oven? During the self-cleaning process, those ovens go up to maybe 800 degrees, if I remember right. Such an element would need 220V wiring.

He is right. We have an electric self cleaning oven. It is 220v and the instructions
do say not to touch or open it, temperatures will reach 750 - 800 degrees....
I have never used the self cleaning option....

.

There is nothing unique about the heating element in self cleaning ovens. The one I'm using with the fan is from such an oven. Mine is 8400 watts at 240v.

The reason I am having more difficulty reaching 700 degrees relates to the size of the oven (4 times the volume of a typical oven) and air leakage.

I am in the process of using an automotive silicon sealer on all the interior joints of the oven. If this doesn't bring it up to the desired temperature I will add an additional heating element - like a stovetop element. I can add about 1400 watts and still power it from my 40 amp breaker.

One thought on 7JP4 rebuilding. There were many essentially identical scope tubes made so if a gun gets shattered in transport or something similar, robbing the parts from a less sought after scope tube may be a solution. It could also potentially be worth wile to rephosphor otherwise identical scope tube types to make new 7JP4s.