Fun with Video Centering Rings & Linearity

In Asteroids
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After having some time to play the game, I noticed an issue. The display seems to favor the top part of the screen. I adjusted the X & Y size pots, but the bottom is still far off, maybe an inch or two. The high-score text isn’t near the high-score text that’s burned in on the tube.

Also, towards the top 1/4″ of the screen, objects seem to scale down as they approach the top, and get larger as they approach the center of the screen. I know I’ve seen info on this on the forums and elsewhere so hopefully it’s not too involved. And after some research, it really wasn’t involved as much as patient tweaking of pots and rings…once I replaced the pots that were bad.


To fix the horizontal and vertical positioning, I needed to adjust the centering rings on the neck of the tube. There are two metal tabs that are glued to prevent movement that rotate around. They each pull the beam a bit off center, so if they are directly opposite each other the effect is nullified, or if they are pointed in the same direction they will pull it the most in that direction. Or, they can be offset to pull it partly one way and partly another. I mocked up a quick photo of where these are.

vector monitor centering ring adjustment

I did not adjust these while it was on, instead there was a lot of turning the game off, move things, then back on.

Once I got the positioning looking good, I needed to tackle the linearity control pots found on the deflection board itself. And I had to do this twice, because I discovered one of the pots was incredible touchy, and the slightest breeze or vibration caused the image to jiggle, wiggle and bounce. New 1K potentiometers were ordered from Arcade Parts & Repair and the adjustment had to be done again.

electrohome deflection board linearity pots

One hand adjust pots on the board, the other on either the x or y scaling pot on the pcb. Flashlight in mouth. Floor standing mirror in front of the game, control panel lowered so I could see through the game to the mirror from the back. That was fun. Got it pretty good, not perfect, but no more odd ball warping of graphics as they float about.