Centipede main board - obscure fault

Solder

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Been working on a Centipede board for a few hours and can't seem to isolate a really obscure fault, as follows:

Game plays okay and attract mode works, but on power up/game start TWO centipedes appear - one starts at the top of the display as it should, BUT it's red in colour instead of green and has no legs.

A couple of inches below it is another centipede but this one is white, and HAS legs!

The white centipede exactly copies the movements of the red one (ie the 'real' one) but the white one kind of 'exits' the screen about half way down.

About a quarter of the way down the screen the red one then becomes the green one WITH legs at the same time as the white one vanishes.

Also on startup a white flea appears in a semi random position roughly in the high scores area.

Everything else is okay.

Thought it may be a fault with the horizontal motion object circuit but can't find anything wrong, and poking around everywhere else hasn't helped either.

RAMs and ROMs are fine.


Any ideas please?
 
I find that with these freakish problems the first thing I do is get out my magnifying light and go over the component side of the board - the side with no solder mask and look for any scratches that could be causing two traces to short together. They could be a very fine scratch and pull just enough solder over to make the short. I keep my continuity tester handy to remove any doubt if two traces are shorted. I just repaired a Millipede the other day that was giving me fits and it was a board short - the scratch was so fine that brushing it with a toothbrush remove the scratch.

I recall that I've seen a similar problem to yours several years ago - if I think of what it was or get a chance to crack open my old repair notes I will pm you.

Bill
 
I find that with these freakish problems the first thing I do is get out my magnifying light and go over the component side of the board - the side with no solder mask and look for any scratches that could be causing two traces to short together. They could be a very fine scratch and pull just enough solder over to make the short. I keep my continuity tester handy to remove any doubt if two traces are shorted. I just repaired a Millipede the other day that was giving me fits and it was a board short - the scratch was so fine that brushing it with a toothbrush remove the scratch.

Already thoroughly checked it over twice with my illuminated magnifier - guess i'll have to check again. :)

I recall that I've seen a similar problem to yours several years ago - if I think of what it was or get a chance to crack open my old repair notes I will pm you.

Bill

I'd REALLY appreciate that, thanks. :)
 
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