Check the -51/-56 PIA, RAM and amplifier with the test EPROM running

Steps

• Make a second LED probe (resistor end probes, other end to +5 V). PIA U2 pins 2-17 and 19, 39 should toggle about once a second; pins 26-33 (data) pulse on a logic probe; pin 34 (reset) stays high.
• Press the board test button to start the RAM test: the CPU LED pauses, then blinks again roughly two seconds later when the RAM passes; with bad RAM it freezes.
• During the RAM test, U10 pins 1, 14, 15 are ground, pin 24 is +5 V and the rest show 1.5-3 V. Good readings but a failed test means a bad U10; otherwise suspect its socket.
• Touch C5 and then C2 with a finger: hum at C5 shows the TDA2002 amplifier U9 works, louder hum at C2 shows the LM3900 preamp U8 works.
• If the board still fails in the game, fit a sound ROM that answers the test button and press it; no sound means replace the AY-3-8910 at U1.
• If an NE555 is fitted at U7 (rare), U7 pin 3 should pulse continuously; if not, replace it.

Systems

• Bally MPU AS-2518-35

Related tips

• Leon test: type 2 sound board RAM IC11
• One flash only: U7 6810 RAM
• Seven flashes, then it starts over endlessly
• CPU chip types: 6800 on System 3/4, 6802 or 6808 on System 6/7
• Not even static from the speaker
• No sound at all: start with the amplifier hiss
• Game locks up after the seventh flash
• Sound module -32 and -50 test points

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)