space invaders intermittent garbage on screen

cleverlyj

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when playing my space invaders it will occasionally give me a line of 4 or 5 dots up the very left side of the screen, and sometimes a period left of the s in score one, however it will go away when you beat a wave and the wave resets. voltages are good, and i have cleaned all the edge connectors and also the main edge connector between the two boards. is this a normal thing cause i know sometimes you'll get the bottom and top of the levels where it says credit shot up from the invaders shooting down at you.

jim
 
Yeah sounds like bad ram. The way to figure out which one, is while you have the extra garbage on the screen, using a small clip lead to ground pin 7 of each pair of rams. That will usually produce a line (or lines) on the screen. When you see the line go through or otherwise correct the garbage, its one of those 2 rams thats bad.

(Each pair of rams share pin 7 so grounding one really grounds both)
 
getting back from camping and what not, i wanted to revisit this and explain it a bit better...

using the elektron forge guide to check edge connectors/power, i corrected the voltage as it was slightly off. revisiting the issue, what i get is 1-3 dots up the very left side of the screen. it comes on when i turn the machine on and goes away the second the attract mode starts "playing the game". doesn't come on after that, but i may occasionally (every few games) get a single dot on the very left side of the screen from where the topmost left side invader was when he shifts down to the next row. but that also goes away after you beat the level and it resets. does that still sound like a ram issue or could that be a board interconnection issue?

i've still gotta pull the roms and do the ram test, but being that this is an intermittent issue like this, i didn't know if the ram test would show the problem or if i have to get lucky with it due to the dots not appearing except at the beginning during the point value screen. after that it will not come back till the game is powered down and turned back on again, and then it goes away again after the game play portion of attract mode starts. is this worth going after or should i let the issue "develop" and get worse before i bother with it.

here's a pic - in it the dot in question is right under the s in score. the dot to the left of the s in score is always there.

DSC03174.jpg
 
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Sent PM but figured I"d put this out there as well....

I doubt the ram test would show it. The manual ram test is the way to go for stuff like that, but the dots have to be on the screen for you to figure out which ram is bad.

With the dots on the screen, ground pin 7 of each ram pair. This may produce white or black lines. If you ground a ram and the line goes through the dots, or gets rid of them, thats the culprit (pair). One or both of those rams can be bad. (If looking at the board with the daughterboard to the left and the ram to the back, each column of chips (2 chips) is a pair.

Unfortunately since pin 7 is shared between each pair you can't narrow it down to the single chip without either piggybacking the chips in the pair or just replacing them one by one.

Make sense?
 
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