sega x board supermonaco gp little dots in sprites

wugly

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I am trying to repair a sega super monaco gp arcade game. I have two problems. the first problem is that all the sprites have little 1 pixel white dots on the screen. the dots go across the sprites both up and down so that the dots look like they are making up a matrix of cubes. all the other graphics are perfect with no problems. In diagnostics I ran the rom ram and special ic chip test and got back good test results. My second problem is that the volume is at a fixed setting and sometimes is not to well heard. the music is ok but the sound effects and voice are low. adjusting the two volume controls by the test switch seems to have no effect.

camera does not photograph what is truly happening but if you look closely in the blue field to the left of sega you can see the dots. actaually the swim suit models looks horrible with dots but camera hides most of problems and this is my best picture that clearly shows something
 

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I have been considering this problem for a couple of days and my thinking is that this problem is not something as simple as broken trace or burnt out component. the repeating pattern of dots is like the sprite is being generated with a single dot and every new sprite piece is being generated with that dot in the same place and then all these defective pieces are tiled together which makes this pattern. does anyone know if my thinking is possible and if it is would the problem be with the sega sprite generator chip???

looking for some knowledgeable advice because I am lost with this board
 
Looks like a RAM issue. Run your fingers across back of pcb and see if you get the dots to change. If so, you have the area. This is if theres an open on RAM. Common problem so thats where I would start. We had a lot of broken ground and 5vdc pins on the sockets to all those ROMS there too so it wouldn't hurt to verify they're ok too.
 
I put the board back into the cabinet and it ran perfectly for an hour and then the sprite video was corrupted again. I am trying to use the tap method to find where the problem is. I have determined if I rap in the vicinity of the second cpu, actually more to the left and down a little the video will clear up for awhile. thinking about reflowing some of the pins in that area and maybe poping some the socketed chips for a little contact cleaner spray. ubnfortuneatly I can not get back to it until the week end
 
well after reflowing the board in the area of the cpu did nothing for me. tried tap test again with a lighter touch and got no response. finally started trying to reseat the chips and the screen would go good for awhile then die. finally got it localized down to two eproms in the upper right corner. reflowed them and clean the sockets and still no go. when they were pressed though the game was perfect. now I start looking near the pcb mounting screw which is right there. Gave the pins and the traces a real thorough look and saw nothing at all. Got out my 40 power loupe and looked and didn't really catch anything except a little bit of oddness in the light but I could not really make anything wrong out. I decide to give the area of oddness another look but this time with my 100 power little scope. I now can see a very small pinpoint crater in the trace near one of the through the board holes that connect to all the pin 19 on the eproms.on the first column. I can only see it at 100 power at anything less I might see a little anomaly that could be dismissed and with the naked eye its invisible. such small traces. A little solder on the trace and the graphics are perfect now
 
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