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  #46  
Old 10-09-2014, 05:37 AM
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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

Last edited by Steve McVoy; 10-09-2014 at 07:22 AM.
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  #47  
Old 10-10-2014, 07:45 AM
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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.
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  #48  
Old 10-10-2014, 03:52 PM
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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...?

.
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Old 10-11-2014, 02:11 AM
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Very daft question. Is your temperature readout set to celsius (as used everywhere except the USA) rather than fahrenheit? 450C is about 840F.
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  #50  
Old 10-11-2014, 07:03 AM
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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.
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  #51  
Old 10-11-2014, 12:13 PM
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Originally Posted by ppppenguin View Post
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...
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.
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  #52  
Old 10-11-2014, 04:02 PM
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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.
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  #53  
Old 10-11-2014, 08:55 PM
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Quote:
Originally Posted by Steve McVoy View Post
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 enthusiast 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.












We're all mad here!
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  #54  
Old 10-12-2014, 07:04 AM
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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.
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  #55  
Old 10-12-2014, 08:25 AM
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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.....

.
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Last edited by Username1; 10-12-2014 at 08:31 AM.
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  #56  
Old 10-13-2014, 02:07 AM
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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.
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  #57  
Old 10-13-2014, 05:45 AM
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Interesting, Jeff. When we get around to playing with prewar tubes I'll keep that in mind.
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  #58  
Old 10-13-2014, 01:34 PM
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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.
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  #59  
Old 10-13-2014, 06:13 PM
Bill R Bill R is offline
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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?
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  #60  
Old 10-13-2014, 07:31 PM
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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.
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