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There is a bizzare device called picture tube charger or CRT-Charger. Here I attach the some diagrams of it from a circuit-book.

picture tube charger

Now, what is its function? what changes it create inside the CRT?

such- as, Is it a method to reverse the metal-evaporation from CRT-electrodes? or is it a method to increase the electric conductance of the low-pressure-gas kept inside the CRT? or something else?

Also, What is the principle behind automatic turning-off of the larger-bulb and glowing-up the small-bulb at completion of the so-called rejuvenation process? (though in these circuit-diagrams there is no mention of any automatic timer-circuit, and as-if it is implying the picture-tube changes its electrical behavior at the end of that rejuvenation process)

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  • \$\begingroup\$ Surprisingly, in our vocational school (under government), this circuit has-been taught theoretically. however i never seen the device being used, but heard they are still used. \$\endgroup\$ – Always Confused Jun 29 '16 at 8:32
  • \$\begingroup\$ Oh a "surface phenomenon" take place inside the tube \$\endgroup\$ – Always Confused Jul 1 '16 at 13:24
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I was thinking "just use google", but then I saw a comment suggesting that many people wouldn't even know what to look for...

A common kind of vacuum tube failure is when the gain just gradually goes to zero. This happens when the "emitter" stops emitting electrons. That happens when the "emissivity" falls. That happens when the emitter is "poisoned", that is, when the emitting surface is contaminated.

When the tube is manufactured, the envelope is evacuated, then the tube is heated. The "getter" in the tube reacts with residual gas and off-gassing, and removes remaining contaminants from the system. This process is never perfect, and in old tubes wasn't very good.

So as the tube is used, contaminants react with the warm emitter surface (less so with the cooler collector or grid or screen surfaces, and it doesn't matter anyway)

It is sometimes possible to "rejuvinate" these tubes, by getting the emmitter hot enough to boil off the contamination. The only way to do that is get the heater very hot, much hotter than normal. But -- that's the other way tubes fail:

Another common kind of tube failure is when the heater burns out. If you are very lucky, you may be able to swing the filiment back into contact, as with a light bulb, but ... that trick never works.

And what kind of a circuit would you use to get the heater so hot? Not a normal radio circuit: a special piece of equipment, designed to run as much current through the heater as possible, while being careful not to overheat it and burn it out. A tube rejuvinator. Originally and often called a tube charger. Often made specifically for a specific tube, or a specific family of tubes.

CRT tubes are special, in that they were very expensive. If one was getting old, and the picture (or later, one color) was fading, you'd connect it to your Picture Tube Charger, and try to get a couple more years of life out of it.

A tube charger normally runs constant current. Better ones had an automatic timer (so it didn't depend on your finger). The one you have uses a light bulb as the current limiter: light bulbs have non-linear resistance and were often used a current regulators.

edit: On further examiniaton, it looks like the incadescent bulb is in the anode circuit, and is being used to indicate (and limit) anode current, not heater current. When the tube starts working (as a diode in this circuit?), you'll get some emitter/collector anode/cathode drain/source current. The fact that they claim it's safe for up to 10 minutes indicates that they are trying to over-heat fairly gently.

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    \$\begingroup\$ Nice answer. Worth mentioning that the "getter" is a highly chemically active metal like magnesium, which evaporates and condenses on the glass envelope, causing the silvery patches you see on many tubes. If that turns white, the vacuum got out... \$\endgroup\$ – Brian Drummond Jun 29 '16 at 11:05
  • \$\begingroup\$ According to the description, the 2 bulbs are being used as automatic timer, indicating the completion of process. (Maybe also they are being used as current limiter... because definite watts bulbs are being used). But what's the principle ormechanism behind automatic turn-off the larger bulb and glowing-up of smaller bulb as an indication of rejuvination? \$\endgroup\$ – Always Confused Jun 29 '16 at 12:58
  • \$\begingroup\$ Valuable informations. \$\endgroup\$ – Always Confused Jun 30 '16 at 18:51
  • \$\begingroup\$ A huge thanks for that edit. The text-description (important notes section) in above photograph, also states , ... Current flowing... will produce various degrees of fireworks ... depending upon... age and condition. You now may watch the 15W bulb ... flash intermittently while the bombardment in the neck of picture tube is taking place. As soon it glows brightly and continuously, ... ready for use" - that means; the 15W bulb is directly related to main current through tube. \$\endgroup\$ – Always Confused Jul 1 '16 at 14:46
  • \$\begingroup\$ I've still not taught the pinout of CRT Base. When I'll learn them in accurate- (and safe-)- way, I could logically analyze those circuit diagrams. \$\endgroup\$ – Always Confused Jul 1 '16 at 14:52

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