Topics

CRT static charge


Dave Peterson
 

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.

Dave


 

On Mon, Dec 7, 2020 at 07:29 PM, Dave Peterson wrote:


Is this normal?

Could it be an indication of anything untoward?
Short answer to your two questions: Yes, it is, and No, it isn't.

Raymond


Dave Peterson
 

Thanks Raymond. I figured when dealing with voltages with a "k" suffix it's probably wise to be safer than sorry-er.

Dave

On Monday, December 7, 2020, 10:32:23 AM PST, Raymond Domp Frank <hewpatek@gmail.com> wrote:

On Mon, Dec  7, 2020 at 07:29 PM, Dave Peterson wrote:


Is this normal?

Could it be an indication of anything untoward?
Short answer to your two questions: Yes, it is, and No, it isn't.

Raymond


Eric
 

IMHO that is very normal and a product of the accelerating voltage in the CRT. If it was arcing out of the tube there would be no vacuum left in the tube and the tube would be bad. All my 7000 series scopes do it and the old tube TV’s used to do it. I used to check the high voltage section of a tv by just running the back of my hand over the the tube about and inch off the face, no dissemble needed. If the hairs on my arm stood up the HV section of the TV was good.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Dave Peterson via groups.io
Sent: Monday, December 7, 2020 1:36 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] CRT static charge

Thanks Raymond. I figured when dealing with voltages with a "k" suffix it's probably wise to be safer than sorry-er.

Dave
On Monday, December 7, 2020, 10:32:23 AM PST, Raymond Domp Frank <hewpatek@gmail.com> wrote:

On Mon, Dec 7, 2020 at 07:29 PM, Dave Peterson wrote:


Is this normal?

Could it be an indication of anything untoward?
Short answer to your two questions: Yes, it is, and No, it isn't.

Raymond


Dave Peterson
 

OMG, that's so funny. I totally forgot about that. As soon as my eyes landed on that bit about old TVs my brain jumped back to 1970 and the crackle that accompanied twisting the power/volume knob on the TV flooded back into my mind. Thanks for that old memory! Oh yeah! CRTs! It's been a long while. Perhaps that's part of the nostalgia and joy of old oscilloscopes.
Dave

On Monday, December 7, 2020, 10:44:27 AM PST, Eric <ericsp@gmail.com> wrote:

IMHO that is very normal and a product of the accelerating voltage in the CRT. If it was arcing out of the tube there would be no vacuum left in the tube and  the tube would be bad. All my 7000 series scopes do it and the old tube TV’s used to do it. I used to check the high voltage section of a tv by just running the back of my hand over the the tube about and inch off the face, no dissemble needed. If the hairs on my arm stood up the HV section of the TV was good.

Eric
-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Dave Peterson via groups.io
Sent: Monday, December 7, 2020 1:36 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] CRT static charge

Thanks Raymond. I figured when dealing with voltages with a "k" suffix it's probably wise to be safer than sorry-er.

Dave
    On Monday, December 7, 2020, 10:32:23 AM PST, Raymond Domp Frank <hewpatek@gmail.com> wrote:

On Mon, Dec  7, 2020 at 07:29 PM, Dave Peterson wrote:


Is this normal?

Could it be an indication of anything untoward?
Short answer to your two questions: Yes, it is, and No, it isn't.

Raymond


 

Dave,

You might also be noticing it now because of a change in humidity. Higher humidity air is better at bleeding off the static charge from the face of the tube than low humidity air, so you are more likely that get that crackling noise in the winter than in the summer.

This is an effect that was very well known in the days of CRT TVs, and I remember it as one of the distinctive auditory experiences of using TVs back then (that and the sound of the flyback transformer). I just tried one of my Tek scopes, and I didn't get any crackling on power up, nor do I feel any static field holding the back of my hand near the tube face, but it may not be very dry yet here in Maryland, and the implosion shield may block some of the static field that I would otherwise be able to detect by hairs standing up on the back of my hand.

-- Jeff Dutky


Ed Breya
 

I second Eric's suggestion, especially regarding the "back of the hand" test, which makes a great static charge detector. With a small CRT like in a scope, you can just place your knuckles against the CRT face.

If the CRT lights up and seems to operate normally, then eventually the sound should go away as the static charges in the materials equalize. The root cause may be that you did too good of a job cleaning the face parts. I have often said that dirt and grime can be your friend, when it comes to static electricity problems. After a CRT has been in service for a while, ambient moisture and particles form a natural static discharge path on the surfaces. If you've used an anti-static wipe on the plastic shield, that should have similar effect. Maybe try the same on the CRT glass face too, and be sure both sides of the plastic shield are treated.

Of course, this all assumes the sound really is just discharge at the face. If it's something else inside, then it's a different story. Try looking at the scope, opened up, in the dark. Sometimes you can see discharges around the HV parts. Also, if there's enough, you can smell ozone and nitrogen oxides in the vicinity.

Ed


stevenhorii
 

Isn't the buildup of static charge the reason CRTs get dusty - attracting
dust - and also why sitting in front of a CRT TV or computer monitor bad
for those with allergies?


On Mon, Dec 7, 2020, 14:18 Ed Breya via groups.io <edbreya=
yahoo.com@groups.io> wrote:

I second Eric's suggestion, especially regarding the "back of the hand"
test, which makes a great static charge detector. With a small CRT like in
a scope, you can just place your knuckles against the CRT face.

If the CRT lights up and seems to operate normally, then eventually the
sound should go away as the static charges in the materials equalize. The
root cause may be that you did too good of a job cleaning the face parts. I
have often said that dirt and grime can be your friend, when it comes to
static electricity problems. After a CRT has been in service for a while,
ambient moisture and particles form a natural static discharge path on the
surfaces. If you've used an anti-static wipe on the plastic shield, that
should have similar effect. Maybe try the same on the CRT glass face too,
and be sure both sides of the plastic shield are treated.

Of course, this all assumes the sound really is just discharge at the
face. If it's something else inside, then it's a different story. Try
looking at the scope, opened up, in the dark. Sometimes you can see
discharges around the HV parts. Also, if there's enough, you can smell
ozone and nitrogen oxides in the vicinity.

Ed






Richard Knoppow
 

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.
   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.

Dave



--
Richard Knoppow
dickburk@ix.netcom.com
WB6KBL