Weird Lisa CRT reverse pincushioning

[wottle]

I was looking at FB Marketplace for anything interesting and came across a Lisa. The price is too high, but what really struck me is the video showing it powered on and working. The CRT seems to have an issue where the viewable area on the screen is limited to a circle (cut off at the top and bottom). It's not pincushioning (at lease I don't think it is) because it seems to be a perfect circle.

Anyone seen anything like this before? Is it just the worse form of reverse pincushioning ever on a CRT?

 

[warmech]

That's... weird. And not normally how pin-cushioning should look. Poorly adjusted pin-cushioning should yield an image whose contents are warped proportionally to the pincushion (think: how light warping around a black hole looks). See this post over on arcadecontrols. This image looks like it's just... masked into a circle. There's no distortion, it's just cut off. It's not externally masked, so that's on the CRT phosphors themselves.

I am so confused by this image.

EDIT: Sorry, this wasn't meant to sound condescending - this was just me thinking out loud. I genuinely have no idea what's going on here, lol. You're completely correct, though, there's no way that's pin-cushioning!

 

[stepleton]

+1 --- I've never seen anything like this before.

The raster itself looks fairly linear (so not pincushioning or similar distortion) but the picture (or the part of it we can see) is bigger than it should be, almost "zoomed in". Meanwhile the extreme "vignetting" almost looks like a shadow --- and I tend to suspect that it's probably just that, a physical thing that's blocking the electron beam. My best guess is that the trouble lies in the CRT itself, and that one of those metal bits inside the CRT neck that's involved in shaping and focusing the beam has become dislodged.

Again that's a guess; I really don't know but would love to find out. We'll probably never get to inspect this Lisa itself, but maybe ask on a forum with a lot of CRT or TV repair people?

 

[wottle]

+1 --- I've never seen anything like this before.

The raster itself looks fairly linear (so not pincushioning or similar distortion) but the picture (or the part of it we can see) is bigger than it should be, almost "zoomed in". Meanwhile the extreme "vignetting" almost looks like a shadow --- and I tend to suspect that it's probably just that, a physical thing that's blocking the electron beam. My best guess is that the trouble lies in the CRT itself, and that one of those metal bits inside the CRT neck that's involved in shaping and focusing the beam has become dislodged.

Again that's a guess; I really don't know but would love to find out. We'll probably never get to inspect this Lisa itself, but maybe ask on a forum with a lot of CRT or TV repair people?

Yeah, I tried searching general CRT for “severe tunnel vision” and “spotlight like picture on CRT” and didn’t turn up anything. I’ll probably post in a CRT / TV forum and see if anyone has any ideas.
But the list price right now is $4k, so unlikely anyone will be able to verify if we determine a cause.

 

[techknight]

I was looking at FB Marketplace for anything interesting and came across a Lisa. The price is too high, but what really struck me is the video showing it powered on and working. The CRT seems to have an issue where the viewable area on the screen is limited to a circle (cut off at the top and bottom). It's not pincushioning (at lease I don't think it is) because it seems to be a perfect circle.

View attachment 27737

Anyone seen anything like this before? Is it just the worse form of reverse pincushioning ever on a CRT?

Yoke is slid too far back. the CRT neck is blocking the beam, or the wrong yoke is installed and it has the incorrect deflection angle.

 

[wottle]

Yoke is slid too far back. the CRT neck is blocking the beam, or the wrong yoke is installed and it has the incorrect deflection angle.

Ooh. I didn’t think about the yoke position. I bet it’s slipped back like you said.

I kind of want to test this with an old Mac Plus CRT I have laying around to see if I can replicate it.

I did message the seller but they don’t seem the type who wants to be messing around inside the machine. But maybe they can add that context to the listing to help the future owner have a good place to start

Appreciate the ideas!

 

[techknight]

Ooh. I didn’t think about the yoke position. I bet it’s slipped back like you said.

I kind of want to test this with an old Mac Plus CRT I have laying around to see if I can replicate it.

I did message the seller but they don’t seem the type who wants to be messing around inside the machine. But maybe they can add that context to the listing to help the future owner have a good place to start

Appreciate the ideas!

CRTs have a maximum deflection angle. usually from 70 to 110 degrees or so. The yokes are matched to the CRT, because if not youd get strange behavior.

For example, using a narrow deflection yoke on a wide deflection CRT would result in the picture being too small and not fill the screen. However, using a wide deflection yoke on a narrow CRT, would result in the exact behavior demonstrated above.

A yoke being slid too far back will replicate the same behavior as it will cause the deflection angle to widen as it moves the centerpoint further back into the gun assembly on a CRT that doesnt support it, so therefore the beam will collide in the rounded neck of the electron gun assembly, or even run into the round hole of G3.