It's been years since I've done analog design (I do digital design full-time) but for some reason I can't pinpoint what's going on with this circuit.
I'm troubleshooting an old Nintendo arcade game (Space Firebird) and there's a problem with some of the analog sounds.
Attached is a snippet of the schematics.
What I've deduced is that the shooting sounds and enemy explosion sounds are in need of the pre-amp that involves IC26. So this looks to me like a simple OR type structure that enables IC26 when either of these events occur. IS23 (74ls05) receives these 2 inputs. Pin 13 goes low when you shoot and pin 11 goes low when an enemy explodes. Lets start with the shooting....
When you shoot, the inverter turns on and since it's open collector, lets the 10k pullup at R28 take over. Looking at the base of TR6 it appears to start at -5V due to the 200k pulldown at R5 and then when you shoot, I would expect it to ramps up to around 4.5V because of the resistor divider between R28 and R5. But it doesn't... it climbs to 0v and stays there.
Another thing I noticed was that the emitter starts at -5V and then climbs only a few hundred millivolts and stays there. That tells me T6 is at least turning on. Shouldn't it be reaching close to 5V when T6 turns on because the collector is at 5V?
Let me know what I'm missing.....
Thanks!