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Old 1st Jan 2014, 5:36 pm   #19
GMB
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Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 2,422
Default Re: Rejuvenating Magic eye Valves?

Quote:
Originally Posted by boxdoctor View Post
I wonder if the loss of brightness is always entirely due to loss of emission by the cathode.
No, it hardly ever is that.

The thing that is really interesting about magic eyes is that it is the ones with metal luminescent targets that fail badly.
The EM84 and similar last way longer.

The anode current tells you that it isn't a cathode problem.

The same type of failure occurs in the radioactive dials on old military equipment. They also used ZnS phosphor but this time activated by the radiation. They usually don't glow at all by now (but are still radioactive).

There are really only two possible ways that phosphors stop working. It is either because the "activator" is not working, or because they are contaminated by something that takes the energy away better than the ZnS. The common green ones are probably ZnS and copper and I think they work by the copper catching the energy and passing it to the ZnS to emit the light.

There is unfortunately a real possibility that the damaging mechanism is metal evaporating from the cathode and embedding in the target causing the energy to be diverted away from the emitter. This may be hard to fix although I wondered about creating a high voltage discharge with the target as cathode. The other reason, which the heat suggestion addresses, is that the copper activator diffuses into the ZnS and the structure no longer works properly - but can be restored by heat.

The microwave suggestion could work in either of these ways - but a more controlled method might give clearer results. But there is little to loose.
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