I wonder if the blue filter has a static filter on it or in it. Also, there are laundry softening tissues that go in the dryer that have anti-static stuff in them Wiping one on a meter face or CRT will often get rid of static charge. The tissues are often called anti-cling.
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If you have an ohm meter that will read very high resistance try it on the two filters to see if there is any difference in surface conduction.
On 12/7/2020 10:29 AM, Dave Peterson via groups.io wrote:
Something interesting noted on restored parts scope: when powered up the CRT seems to create a static charge crackling noise.
That is, my perception is that the sound is coming from the face of the CRT. This scope currently has a clear shield on it. My existing 465 with a blue shield does not crackle when started. Every component of the face of the scope has been freshly cleaned, so is presumably very dry and free of any oils from handling. The screen shield wiped with a glasses cloth you get from the optometrist to get rid of the static cling. Otherwise lots of dust would obviously accumulate. So presumably the shield has been discharged. The scope is properly grounded, including the ground plug on the face of the scope.
It sounds like fresh laundry being separated, but much quieter. It barely audible, but it's there.
Is this normal? I don't ever recall it from my previous experiences, but it's so innocuous sounding that I'm sure it wouldn't arouse much suspicion otherwise. I'm just very attentive to every sound, and very wary of anything with a scope of completely unknown origin and state. Could it be an indication of anything untoward? Haven't gone through a calibration yet. Getting ready to do so.