Sound is fine for about five minutes, then cuts in and out.

Pre-DCS sound cuts out after the game warms up

Cause: The LM1875 main amplifier at U1 is failing from heat.

Checks

• Put a logic probe on the LM1875 leads; if its pulses follow the dropouts on one lead, the chip is bad.

Fix

• Replace the LM1875 and renew its heat sink compound.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics

Related tips

• Pre-DCS sound board A-12738
• Pre-DCS sound boards are tuned per game
• Sound board beep codes before DCS
• Pre-DCS speakers pop and get damaged, LM1875 runs hot
• Volume stuck at full and buttons do nothing (pre-DCS)
• Adding a line out to the A-12738 sound board
• Sound board fuse F501 blows at power-on (A-12738)
• Fitting a manual volume pot to the A-12738

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)