Cathode Ray Tubes: am I wrong? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

John_BS · 5th Jul 2013, 11:22 pm

OK guys: I'm trying to resist taking the entire tube assembly out for a second time until such time as I'm pretty sure that a degauss has a sporting chance of working.

Now, I may not have the theory quite right in my fuddled mind, but let's look again at the 'scope's performance:

-the scan is nice and linear in the x direction, and sits parallel to the centre-line of the graticule.
-y deflection +/- two divisions is linear within spec.
-y deflection beyond this on the LHS shows more "gain" than on the RHS of the screen.

Surely, in order for a magnetic field to produce such distortion, it would have to have symmetry about the "zero" y axis, i.e. it deflects the beam up when the scan is positive Y, and down when -Y. This requires the magnetic field to reverse direction in the region between the y-plates, which start out about 2mm apart

Is this possible, or am I wrong?

John

Radio Wrangler · 6th Jul 2013, 7:47 am

I don't believe it's a magnetic field. It would have to be a rather specially shaped one and rather coincidentally located.

David

6th Jul 2013, 8:43 am

Quote:

It would have to be a rather specially shaped one and rather coincidentally located

Perhaps it is.

Radio Wrangler · 6th Jul 2013, 9:42 am

Considering William of Occam to be a smart guy, I'll go for a good de-magging and then we're left with no other practical outcome than a new tube, or just living with it.

Certainly HP CRTs were carefully made of non-magnetic material, and I can't see Tek being otherwise.

Would they have anything to do trapezoidal adjustment in an electrostatic deflection tube?

Even in magnetic deflection tubes they had to play around with transductors to modify scan size down the tube.

David

John_BS · 7th Jul 2013, 1:44 pm

OK so do I degauss the tube + screen, or seperately, or just the screen? Ball-park ampere-turns required @ 50Hz?

John

maninashed · 7th Jul 2013, 1:53 pm

John, just try the tube without the mu-metal shield, just in case its that which has become magnetized.
Bill

HamishBoxer · 7th Jul 2013, 1:58 pm

Never had to degauss a mono crt in all my years in servicing.

7th Jul 2013, 4:57 pm

Quote:

just in case its that which has become magnetized

Thats the bit that could be magnetised, and...

Quote:

Never had to degauss a mono crt in all my years in servicing.

They don't usually have a mu metal sheild, QED.

5th Jul 2013, 11:22 pm	#21
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,782	Re: Cathode Ray Tubes: am I wrong? OK guys: I'm trying to resist taking the entire tube assembly out for a second time *until such time as* I'm pretty sure that a degauss has a sporting chance of working. Now, I may not have the theory quite right in my fuddled mind, but let's look again at the 'scope's performance: -the scan is nice and linear in the x direction, and sits parallel to the centre-line of the graticule. -y deflection +/- two divisions is linear within spec. -y deflection beyond this on the LHS shows more "gain" than on the RHS of the screen. Surely, in order for a magnetic field to produce such distortion, it would have to have symmetry about the "zero" y axis, i.e. it deflects the beam up when the scan is positive Y, and down when -Y. This requires the magnetic field to reverse direction in the region between the y-plates, which start out about 2mm apart Is this possible, or am I wrong? John

6th Jul 2013, 7:47 am	#22
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Cathode Ray Tubes: am I wrong? I don't believe it's a magnetic field. It would have to be a rather specially shaped one and rather coincidentally located. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Jul 2013, 9:42 am	#24
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Cathode Ray Tubes: am I wrong? Considering William of Occam to be a smart guy, I'll go for a good de-magging and then we're left with no other practical outcome than a new tube, or just living with it. Certainly HP CRTs were carefully made of non-magnetic material, and I can't see Tek being otherwise. Would they have anything to do trapezoidal adjustment in an electrostatic deflection tube? Even in magnetic deflection tubes they had to play around with transductors to modify scan size down the tube. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

7th Jul 2013, 1:44 pm	#25
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,782	Re: Cathode Ray Tubes: am I wrong? OK so do I degauss the tube + screen, or seperately, or just the screen? Ball-park ampere-turns required @ 50Hz? John

7th Jul 2013, 1:53 pm	#26
maninashed Rest in Peace Join Date: May 2011 Location: Preston, Lancashire UK Posts: 955	Re: Cathode Ray Tubes: am I wrong? John, just try the tube without the mu-metal shield, just in case its that which has become magnetized. Bill

7th Jul 2013, 1:58 pm	#27
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Cathode Ray Tubes: am I wrong? Never had to degauss a mono crt in all my years in servicing. __________________ G8JET BVWS Archivist and Member V.M.A.R.S