Atari Centipede arcade pcb repair

The failed AR2 power/amp unit for this cabinet was rebuilt and tested fine but the logic board had also developed a graphics fault.  The test mode gave some clues as to what was going on as the character test only displayed odd numbered characters (A C E G).  This suggested the low bit of whatever selected the character was stuck.  The screen full of A’s was because the space/blank character was stuck as an A also.

I used MAME to clarify the layout of the character ROM – each character is 8×8 pixels in size and each row of 8 bits is stored as a byte, then 8 sequential bytes for the 8 rows.  So 8 bytes a character means 3 address lines (2 to the power 3 is 8), or A0-A2.  That means that if every alternate character in the range was bad, the next highest address line or A3 is the place to look.

The addresses track back from the ROM to a LS86 at E7 – this was removed from the board and tested – and yep, it had failed in exactly 1 bit.  When replaced with a new chip the game was fine.