Thank you Tom

I'm sorry, what do you mean by "I've got a sticky in one of the sections..."?

I will be waiting with bated breath to see if degaussing fixes this.

Magnetization of a 21AXP22 color CRT has noticeable effects on purity, but not such gross effects on geometry. On the other hand, the high voltage is lower here so magnetic effects could be stronger.

Has anyone seen such pincushion due to bell magnetization before?

Here is the effect again

https://imagizer.imageshack.com/v2/1...924/CP7Q3m.jpg

I had on a 16AP4. It was only one corner though.

Rational thought suggests the only thing that causes pincushioning is a wide deflection angle on a flat screen or stray magnetism near the screen.

You could test by locating a not too strong magnet near to the rim of the CRT. However the bell is at at voltage so you would need to be extremely careful.

I thought about pincushion but have never heard of anyone have that problem on this old of set. Would you degauss from the front or the back side, or no matter?

I'm not going to get within 5" of this CRT so doing anything inside the cabinet while it's on is something I can't risk...not with my unsteadiness (from medication). If I am going to do any demagnetizing, my only options are outside while it's on or inside when it's off.

Degaussing coils are still relatively available due to the prevalence of old arcade units with standard CRTs.

I guess I could put the chassis and yoke back in the unit with the weak CRT and see if it produces a proper image. That would all but confirm Penthode's suspicions of a magnetized CRT bell.

Monochrome tube does not have a shadow mask. In this case you want to run the degaussing coil all around the bell.

A thought: is it possible this set is intended to have some anti-pincushion magnets or pole pieces that are missing?

I don’t see any on either set. It does have permanent magnets in the focus ring but they are a ways away from the tube.

And there’s no mention of anything like that in the Rider’s service info which has an extensive setup section

The fixed anti pincushion magnets only appeared with the later wide deflection angle CRTs in the 1960s. I recall seeing them on later sets.

The 70 degree tubes like the 16GP4 generally did not suffer from pincushioning or raster distion unless there was a strong external magnetic field nearby.

My guess it is a magnetized cone are the corner distortions as the beam reaches a sweep extremity.

If that’s the case, maybe I should get the cheap wand style on eBay. I’d have to find a way to get 220V to it.

I haven't noticed any on the 2 or 3 sets I've had here with 16GP4's. If that was a thing wouldn't it be listed on the 16GP4 data? I have seen magnets on newer rectangular tube yoke assemblies.

I would think the best thing to do is to remove the tube from the set and go over the tube with a color TV degaussing coil.

It wouldn't normally be a problem with a 16GP4. Unless in the past the tube was subjected to a strong magnetic field.

One thing that bothers me about this set Chris is working on is that the raster apparently won't fill the mask. I would definitely be verifying the power supply voltages.

For some perspective here's my Sentinel with a 19AP4 crt. You can definitely see some outward bowing, which seems quite normal for one of these big flat face tubes. Some of it is distortion from the face curvature. Never mind the guy peeking out from inside the TV.

https://live.staticflickr.com/65535/...a4159cce_z.jpg

https://live.staticflickr.com/65535/...a835166e_z.jpg

https://live.staticflickr.com/65535/...84803dbb_z.jpg

Definitely going to do that Kevin. Like I said before I never checked it because the rectifier is new.

The picture on your Sentinel looks like the reciprocal of what I have on mine. Very interesting looking distortion.

It is opposite, but this is more normal of these vintage sets. The 19AP4 is set back behind the safety glass a significant distance. So there's some optical distortion from that. And the camera doesn't do it any justice. When watching a program it's not as obvious as these pictures indicate. I expect back in the day no one thought much of the shortcomings. There wasn't any alternative other than a movie theater. Technology advanced significantly over the next 10 years.

A sticky is a topic that stays at the top of the forum section despite other newer topics happening. In this case it's a sticky in the radio TV tech forum section. https://videokarma.org/showthread.php?t=264048

Chris,

Tonight I fired up my RCA TC-166(KCS 40A) chassis with the 5AXP4 test crt and put a DVD test pattern on it. To my surprise it shows the same concave edges you're experiencing. I don't remember it being like this, but I probably never used a test pattern when working with it earlier. This weekend I'll attempt to slide this back in it's cabinet and see if the 16GP4 makes any improvement. Right now I'm not sure what to make of it.

https://live.staticflickr.com/65535/...77f43e5d_z.jpg

Wow!

The slight bend is normal. It is due to the geometry of the CRT. The greater the deflection angle, the greater the bend.

The 15GP22 color CRT has a flat panel screen mounted internally within the tube. Because of this, the pincushion is very noticeable.

Wider deflection angle tube used pincushion correction. For monochrome sets, permanent magnets were used. For color, the introduction of the 25" rectangular CRT in 1965 required dynamic pincushion correction circuitry because permanent magnets would affect color purity.

I still contend the bell of the replacement 16GP4 has become slightly magnetized and removing the tube and carefully going over it with a degaussing coil will alleviate the problem.

Perhaps it was intentional to counteract the outward curve of the CRT face in metal cone CRTs?
https://live.staticflickr.com/65535/...25e2e196_c.jpg

So I should go over the metal bell correct…not do the withdraw circling from the screen

Considering the electron beam scans from a center point behind the crt face, it makes sense there will be distortion more so as it moves outward, predominantly in the horizontal scan. Back at this time RCA must not have considered it a serious enough problem to require correction. I'm not aware of any round screen monochrome set that implemented pincushion correction.

Problem for me is it can be seen when the image has straight lines in it. As you get towards the center it’s not noticeable but it is the last 2” or so of the vertical and horizontal limits of the tube

Is that image a zoomed image from the settings on your converter box Bob? Is that how you got it to fill the screen?

I'm feeding this set a standard 4:3 aspect ratio image. It's a Zenith porthole set in zoom mode. The set is designed to fill the entire screen in this mode by over scanning.

I’m going to try to demagnetize the metal crt bell. As good a place to start as any.

Yes the slight curvature to the faceplate help to reduce the pincushion effect
Only the much later monochrome sets with 110 degrees would you find pincushion magnets.

So the static magnetism to the metal bell is the only cause I can see to cause a pincushion effect on an old round metal CRT. The weird thing is that from Chris's photos the pincushion looked so uniform. Which would suggest the tube at some point in it's life must have gone through a strong uniform magnetic field.

When it was shipped, did it go through a parcel scanner to determine the box's contents?

I picked it up from an owner who bought it new almost 70 years ago. Tomorrow I’m going to do some yoke swaps to see if that makes a difference

Go over the metal bell, slowly withdrawing before switching off the current.

Chris last said he was persuing alternative yokes. I cannot see how a defective yoke could physically cause the pincushioning.

To review: could Chris recap the sequence of events?

Did the origal tube work and display non pincushion?

Did the pincushion only appear on the replacement tube?

Does Chris have a degaussing coil and has there been any update?

Hey guys...sorry I've been MIA. Life showed up really intense. I'll update everything shortly. Nothing has progressed since my last update. Picture is still slightly distorted with symmetrical bends in the vertical and horizontal aspect like it's being pinched top to bottom and side to side.