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Old 17th Oct 2019, 11:41 am   #47
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Originally Posted by Oldmadham View Post

The electron beam, for that short while, didn't really do much harm.
Actually it did and is a very significant problem, that is on CRT's that have non-aluminized screens.

Not only did the aluminization protect from ion burns but it made the phosphor much more resistant to high beam current burns at switch off with an intense central spot.

I have evaluated a large number of non-aluminised pre WW2 CRT's and I can assure you the phosphor is extremely easily damaged by the turn off spot. I assume it is better with aluminization due to the thermal conductivity of the thin layer of aluminium.

For example I was able to detect screen phosphor desensitization in my 904 after only 3 or four episodes of turn off spot in an NOS 5FP4, which is why I modified the circuit to suppress the beam current at turn off.
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