Asteroids Vector problems

MitchO

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So my Asteroids is nearly fixed. I have replaced pretty much everything on the monitor and it seems to be performing well.
However I still have a slight vector problem at the top of the screen (see pic) that I can't trace down. The problem only affects horizontal lines and happens primarily at the top of the screen. The further down the screen you get the less noticeable it becomes. The bottom of the screen looks fine.

I have done some basic troubleshooting on the board, replaced the electrolytic caps, cleaned and replaced the pins on the edge connector and cleaned the legs of the socketed chips.

It also has a slight hum in the audio but I don't think it's related.

Any ideas are appreciated.

Thanks,
Mitch
 

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I finally got a chance to get back to this and was doing another visual on the board. I noticed that pins 3 & 4 of chip C5 74LS161 had been cut, presumably a previous owner had attempted a speed up mod. I went ahead and reattached them and not surprising nothing had changed.

At this point I will probably need to dig out the logic analyzer and check out the vector logic. If anyone has any ideas of where would be a good place to start I'm all ears.

Mitch
 
Thanks for the tip.

I replaced all of the electrolytic capacitors on the game board a couple of months ago, which accounts for C93 and C117. C94 appears to be a deglitching cap, not something I would normally suspect but I supose I could swap it out. Since you mentioned that area I decided to meter the output of the 7815, it came back at 14.7. Anyone know what the tolerance is on the Asteroids board for the 15V circuit? Though, I may want to replace it anyway just to be on the safe side.

Mitch
 
So, I decided to try to narrow down the problem board and picked up another working monitor deflection board. When I plugged it in the original problem went away but now I have a different problem. Some of the vectors sort of "shimmer". You can see a video of the shimmer on the test grid by going here: http://www.atari7800.org/arcade/ and clicking on the 100_0364.mov link.

I talked to the seller and he didn't have this problem when he tested it before shipping.

Mitch
 
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