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Old 23rd Oct 2019, 5:15 am   #55
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 72
Default Re: Cretinous idea - The combined volume pot and mains SW.

Originally Posted by Argus25 View Post
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.
OK, prewar CRTs are certainly a different story.
They were mostly originally similar to, or in fact, were, CRTs designed for Oscilloscope use.
Even with much more modern 'scopes, I well remember the adjuration to "turn down the intensity!"

Pretty much anything recognisable as a "TV" CRT from the 1950s on will not normally have a problem.

The kind of "burn in" I have seen over the years, is when a tube is deliberately underscanned to ensure a correct 3:4 ratio picture, or those tubes which have displayed a "quad screen" display for a long time.

I had a big Philips which had the latter problem, although it was really only obvious when the picture background was static -----fairly rapidly moving backgrounds masked it enough to be tolerable.
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