Bench-test a -51/-56 sound board with a test EPROM

Steps

• Burn the sound board test EPROM (2716 or 2532/2732 format) and fit it at U4. Set CPU jumpers for the 6808 or 6802 fitted (a -56 with 6808 and 2732: B, C, E, G, H).
• Feed +5 V to J1 pin 5 and ground to J1 pin 6, plug a speaker into J2, and jumper TP1 to TP2 so the amplifier runs from 5 V (page 66).
• Wire an LED in series with a 150 ohm resistor from U3 pin 15 to ground (resistor end to ground).
• At power-on the LED should blink and the speaker give a faint fast heartbeat if the PIA at U2 is fitted.
• No blink: U3 pins 2, 3, 4 and 40 should be +5 V, pin 37 about 2.5 V and pin 39 about 1.1-1.5 V (clock). Then pull U2 and U10 and look for activity of 0.5-2 V on address pins 9-25 (pin 21 is ground) and on data pins 26-33. Pin 19 should be about 0.5 V and pin 20 about 4.5 V.
• Missing signals with clock and 5 V present: broken trace or bad CPU; suspect sockets, and lift the pin of the chip driving a dead line to find a short.

Systems

• Bally MPU AS-2518-35

Related tips

• Sound module -32 and -50 test points
• Identifying AS-2518-51 and AS-2518-56 sound boards
• AS-2518-51/-56 sound board jumpers
• Sound Plus -56 and vocalizer test points
• No sound at all: start with the amplifier hiss
• Sound board chip 86L93 can be replaced by a 74LS93
• Spare sound board works on the bench but is silent in the game: no -5 V, bad 7905
• Background sound works in test but not in games: check DIPs 29-30, then reseat connectors

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