[Techtoolslist] Another midway 8080 and pod (space invaders deluxe) question

[David Shoemaker]

Have a space invaders deluxe board.

 

Rom tests based on quarterarcade fluke crc's are fine on loop.  Game does
not run correctly.

 

Ram short fails all over the place.

 

So trying to narrow it down a touch I do a write 0x2000 = 0xff then a read
0x2000.

 

Comes back with 0xFF.  Hit loop and it starts to wander all over creation.

 

Thinking I have a bad bit I do some testing.  

Write 0x01, read & loop is stable=0x01.

Same with 0x02, 04, 08, 10, 20, etc etc.  In fact everything I tried came
back with a good read loop EXCEPT 0xFF.  With that value I get random
garbage.

 

WTF?????

 

Any ideas on why ram would fail this way?

 

Thanks,

David