Tempest - the Barber Pole effect

westal_sage

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Well, I've just about got my relatively newly acquired Tempest dialed in to shape. Only a few little issues remain.

1 of them is what I'll call the "barber pole" effect. If I go to the test screen color grids, you can see it plain as day - it happens only on the horizontal lines, and only for red and green (not blue).

As an example, looking at any given horizontal red line, when it paints on the screen it looks as if it's a line that's red on the front and black on the back and it's twisted. At this point, it's a non-moving barber pole, but if I make any purity adjustments, as I'm moving the control it looks like a moving barber pole.

Anybody seen this before or have suggestions on what might fix that?
 
I know what you are talking about. What's happening is the red gun in this case isn't exactly lining up with the grid on the front of the screen. I'm thinking it may be due to the yoke not being square with the grid. You can try rotating the yoke to better align it. Be fore warned you may only make it worse.
 
Ah thanks Talon! Yeah for now I might just leave it alone. Even though you can see the effect on the thin vectors in the test grid, you don't even really notice it that bad on the thicker vectors during gameplay.

I'm chalking this one up to just a little annoyance and not something I'm gonna dwell on too much. I *was* able to make some convergence adjustments that drastically improved things from when I first got the game. I had never done that before and was kinda scared to try, but once I got a feel for what each ring was doing, it wasn't that bad.

Thanks again for all your help!

- Wes
 
The test grid is about the only place you get nice long vertical or horizontal lines. During.g game place most everything is at a diagnol.
 
I first noticed this when I saw a Tempest game in the wild about 10 years ago.

I kept trying to convince myself that it was MAME or something running Tempest on a non-vector monitor, but then realized that it's just the effect you get from the staggered color pixels on the face of the tube.

It's funny- you don't get the same thing on a B/W vector game because the screen is painted with a single color phosphor.

A couple of things you could do to minimize the effect is to turn up the brightness a bit and defocus the beam slightly to get LESS CRISP vectors.
 
Thanks kstillin! Yeah ya know I messed around with the drive and bias pots and got it to look a little better. Sounds like a good thing to try to slightly "fuzz up" the vectors just a tad.
 
I posted about the same problem in the monitor section. I rotated the yoke and it got a bit better but is not yet perfect. I do need to adjust my brightness/bias/drive, so maybe that will make a difference.
 
Hey i86 - I'm interested to hear more about your experience with rotating the yoke. Can you elaborate on what effect it had on the wood grain (what I call barber pole) thing?

Like, as you rotated, could you see the effect get better/worse? Is it easy to explain *how* the effect changed - like was it that the spacing of the dark spots changes, or you get less faded out looking parts of the line, etc.?

I'm just kinda reluctant to try, 'cause I guess that would throw off the convergence as a whole and I'd have to redo it again. I got it dialed in pretty darn close so I might just leave well enough alone.
 
Well, I knew I had to do something with the yoke because on the test pattern screen I could clearly tell that the top right of the screen was higher than the left side. When I rotated the yoke for that, I could see it was cleaning up the barber pole problem somewhat- there were less of the dark 'stripes' in a given horizontal length. My wife was watching the screen when I was doing the final adjustment, so I had to take her word on what looked best. It definitely lessened the effect, but there was still the dark 'stripes.' In the center of the screen, all lines for all colors were perfectly straight and strong. In a couple spots off the center grids the vectors are dim for a stretch. From about 10+ ft away it looks like the vector just stops drawing for about an inch and then starts strong again, quite strange. This isn't noticeable in game, just bothersome to the perfectionist in me. I still need to adjust my drive and bias, so maybe that will clear that up.

In any event, don't worry about rotating the yoke. There's nothing you can't undo if you find that it doesn't help. The biggest thing is trying to loosen the yoke from the wedges. It feels like it's on there tight (should be after 30 years of static friction from the clamp), so take your time, moving it back and forth slowly.
 
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