In display test one score display briefly shows only the 5s and 6s and none of the other numbers, and it does the same in another display position.

Score display shows only a few numbers in display test: failed decoder chip

Cause: A failed decoder chip on the display board.

Checks

• Swap the display to another position: if the fault follows it, the display board is at fault.
• Inspect its connector and solder joints before replacing parts.

Fix

• Replace the decoder chip on the display board, ideally fitting a socket.

Bally displays run on about 190 V DC. Power off and let the supply discharge before handling them.

Systems

• Bally MPU AS-2518-35

Games

• Eight Ball Deluxe

Related tips

• Bally 6-digit display: the decoder chip should be E-620-38 (4543)
• Only even-numbered score displays show numbers
• All displays show strange scores (one bad decoder)
• Bally 7-digit display misbehaves wherever it is fitted: replace the decoder chip
• Score display digit parts by position
• Bally -35: same digit positions dark on every display: open digit drive resistors
• One display digit glitches and the fault follows the display: bad resistors
• Bally 6-digit display: one segment out on every digit, find the bad driver by swapping

Source: Pinball Repair help group!! (Facebook group)