The speakers pop about once a second and eventually fail; the LM1875 heat sink runs hot.

Pre-DCS speakers pop and get damaged, LM1875 runs hot

Cause: DC is reaching the speakers through failing capacitors feeding the LM1875 (even 5 mV DC overheats it). In the example 40 mV DC was measured.

Checks

• Measure DC volts across the speaker leads: it should be zero.

Fix

• Replace C46 and C47 (1 mfd tantalum), C20 (10 mfd), C22 (22 mfd) and C23 (.22 mfd).

Systems

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

Related tips

• Pre-DCS sound cuts out after the game warms up
• Pre-DCS sound board A-12738
• Pre-DCS sound boards are tuned per game
• Volume stuck at full and buttons do nothing (pre-DCS)
• Sound board beep codes before DCS
• Adding a line out to the A-12738 sound board
• Fitting a manual volume pot to the A-12738
• Sound board fuse F501 blows at power-on (A-12738)

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