I am in the process of restoring a vintage electro-mechanical arcade game from 1973. It has a small sound board that makes two sounds for a crude simulation of crowd cheering. One sound is just filtered white noise (triggered by putting 18v B+ to pin 3), and the other sound is the same white noise but with an additional whistle added (triggered by pin 4).
The issue I am having is with the whistle sound. If I power the board at full voltage (30v AC) the whistle makes a short chirp noise when first triggered, and another chirp when the trigger input is released. It should make a continuous tone that starts shortly after the crowd noise, and fades out as the trigger is released.
If I either remove the D5 zener, lower the input voltage so that the zener is not regulating, or even just add a 470 ohm resistor between the zener/R26 junction and the rest of the circuit, it will work perfectly fine. It's like the B+ power rail has to be "soft" to allow for the oscillator to work. It also will not work if I use my bench power supply to input clean DC directly to the rail.
I'm pretty sure the problem is due to the original "X16A5332" transistors being replaced with 2N3904, but I really just want to understand why it behaves this way. Why would it need a soft/dirty B+ rail to operate correctly? And again, all functions are working correctly except for the whistle noise.
Note that there is an error in the schematic. Q4/Q5 emitters are missing their ground symbol.
The whistle sound is generated by Q3, Q6 and Q8. From what I can tell, Q6 and Q8 are a monostable multivibrator which is being triggered through Q3. C15 charges to a given amount depending on which trigger input is used, which in turn puts a positive voltage on the emitter of Q3, and also enables the white noise output from Q4 to pass to the audio amp. The higher the charge on C15, the longer the white noise stays active. VR2 adjusts the duration. The whistle will stop very shortly after B+ is removed from the trigger input, while the white noise will continue a bit longer.
A game with the same board can be seen/heard here, at around 43 seconds into the video (not my machine, just an example of how it should work):
Note that the game triggers the whistle 4 times. On the board itself, the whistle will go as long as the input is held to B+.