Neo Geo Arcade board problems

I have about four Neo-Geo six slot arcade game boards with Video RAM errors. It is not the actual video RAM, that much I have determined. It has a lot of large surface mount chips with upwards over over a hundred pins each. It uses a Motorola 68000 chip. Can someone give me some clues on how to track this problem down? With this kind of error am I going to find an address or data line that is stuck high or low? Or could it be another pin. Nothing on the M68000 seems wrong, The chip checks out fine on a tester. I have replaced the video RAMS and this M68000 so it must be in another chip or in a trace. If I probe each and every pin of all of the components would I for sure find the error or can this type of error not show up as a hardware difference? Can this be a software error? Lets say I find the pin that is not working, What would that pin look like on a scope? Just a high 5 volt or low ground, or could it be at a logic high or low, that it is just not transitioning from one state to the next as it supposed to? Could it be more then one pin? There is no documentation for this board at all. I am not sure where to began and what to do next. Thanks very much Russ

Then why do you conclude that the problem is "Video RAM errors"?

What are the symptoms?

If you are getting graphical glitches, it could be the video RAM, or the circuits which write data to the video RAM, or the circuits which take the data from the video RAM and generate the video signal.

Any error will ultimately manifest itself through the signals which appear on the tracks. Whether or not you can recognise the error on a scope trace is a different matter. If a track carries a stream of 0s and 1s from one chip to another, you probably aren't going to be able to recognise the right data from the wrong data from a trace (and not necessarily from a capture unless you understand the functioning of the chips involved).

An error could be a signal which is 011100110 instead of 010100110. You aren't likely to recognise such an error without at least a good understanding of correct behaviour and a logic analyser.

Personally, I think it's somewhere between "optimistic" and "futile" to locate faults on such a complex board if you have neither documentation nor a general understanding of how such systems function. Even if you fully understand the board, it may be hard to identify a fault without a logic analyser.

However, there are some types of fault which might be simple to identify. If you get a response to shaking or flexing the board, that suggests dry joints (finding them is a different matter). Also, try scoping the supply voltage (Vcc) pins on all of the chips; if the decoupling capacitors have failed, you would expect to see ripple at high (MHz) frequencies.