Star Castle Sound Board Repair, Part 2
I received my repaired Star Castle MPU board from Mark Shostak, and plugged it in with the sound board I had repaired. After installing the speaker I mentioned in my last post on this subject, I noticed that a few sounds were missing. Ugh! So, I busted out the Star Castle Operation and Maintenance Manual. The sounds that were missing were the “Soft Explosion,” “Loud Explosion,” “Thrust,” “Sheild,” and “Fireball.” Looking at Block Diagram One, I noticed that all these sound effects are related. Also, the “Background Sound” was staying at a constant high tone, instead of lowering its frequency to a low pitched tone when you begin the game.
The first rule of thumb (which I stubbornly ignored this time, unfortunately) is to check all the voltages. I wish I did this first, as it would have saved me tons of time. The sound effects above use a few ICs in common, one of which being the Noise generator, IC14. It looked like the Noise generator wasn’t supplying the needed noise for these The Noise generator is one of the few ICs on the sound board that uses +15V. I checked IC4 (the +15V voltage regulator) and sure enough, it was registering +30V! IC4 was bad. I ordered a replacement 7815 from Mouser, and this fixed those sound effects.
I looked at the “Background Sound” circuitry, but more closely at where +15V feed is. If the +15V had been bad, maybe it fried something in that part of the Background Sound circuits. Sure enough R2 (a 1/4W 160 Ohm resistor) was burned and registering 0 Ohms with my digital multimeter. I replaced this resistor, and now the Background Sound works. The Star Castle is now fully operational again!