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I've spent a lot of time fixing an odd sound problem on my 'Round The Town' Scorpion 2 board, posting some info here which may help others. I found that when I turned the machine on in the morning it had no sound although the game was all working fine. The sound would start working usually within a few minutes or sometimes after 30 - 60 minutes, after that it would be fine for the rest of the day. Whenever I started to try and trace the fault the sound would come good and then I had to wait until the next morning, which made the whole fault finding very long winded. I checked the AUD5V and AUD12V and all the sound chips had their supply and their 0V feed. One morning, while the fault was on, I touched pin 6 of the audio amplifier chip IC12 type LM380 with my test meter probe and noticed the sound immediately came good. Pin 6 is the 'inverting input', the game sound uses pin 2 the 'non-inverting' input.. For a typical LM380 circuit the unused input is pulled towards 0V often with a 1K resistor. The circuit on a Scorpion 2 board has a capacitor between pin 2 and pin 6 (C21 2n2F) and a capacitor from pin 6 to ground (C22 10nF). I took out C22 and tested it for resistance, it was 100K rising to 350K, the new replacement measured way over 1Meg. After replacing C22 the sound is fine, I've tested it for five mornings and it's been there immediately each time. So I guess C22 being leaky was the problem and that after the machine was switched on each day it gradually recovered and was then ok for the rest of the day. When I touched pin 6 with my test probe the tiny bit of capacitance was enough to make IC 12 work. I'd like to understand better the C21/C22 arrangement, could anyone explain exactly how it works?
