Fault-finding the original System 80 sound board

Steps

• With no sound at all, measure +5 V, +12 V and -12 V on the board first.
• Press the sound board test switch (with S1 and S2 set opposite) and watch DAC pin 4: a changing tone-like waveform should appear.
• If not, check the reset on U1 pin 1 (low at power-on, then high), the clock on U1 pin 28 and U2 pin 39, and activity on U2 PA0-PA7.
• If those are dead, check the read/write line, data bus and address bus; AB10 and AB11 are buffered on the board, so check both sides of the buffer.
• If DAC pin 4 is active, suspect the LM380: look for about 200 mV peak-to-peak on its pin 2 and an output swinging roughly +2 to +10 V on pin 8.
• If the DAC waveform is broken up, check PA0-PA7 and the -12 V. Replacing the DAC or the LM380 usually cures it.

Systems

• Gottlieb System 80

Related tips

• How the original System 80 sound board makes sound
• Supplies on the original System 80 sound board
• The original sound board self-test only works with S1 and S2 set opposite
• Original sound board option switches S1 and S2
• Fitting the original sound board PROM
• No sound at all
• Distorted sound
• The wrong sounds or tunes play

Source: System 80 Service Manual, Gottlieb