cwilbar
Well-known member
Working on a board. There is a general static/hiss to the audio background when no sounds are occuring.
I find Galaxian is like that a little, but this one seems to have a bit more than the usual to me.
I have verified it is coming from the Fire audio circuit. The static seems to be the frequency of the free running oscillator (7S). I determined this by grounding the control of the analog switch. There is a small amount of voltage on it (which is roughly in line with another working board) I've compared it to.
So far I have tried swapping the following:
7R 4066 analog switch
7T LM324 quad diff op amp
C26 0.01MF 50V which is in series with the output of the LM324 for the fire sound
C25 1MF 16V Tantalum which sits between control of the analog switch and ground and provides for the duration of sound production of the fire sound
I have blindly tried C29 (0.01MF) between the control of 555 timer 7S and ground.
The C28 cap had already been done along with the other electrolytics.
I'm unsure why there is a difference. A good board (low hiss) and this one both have similar voltages on the analog switch control line for that sound. But as there is a slight + voltage here, I think the analog switch is allowing some to get through. I did not have a new 4066, so I did have to test this with a used 4066 pull of unknown condition.
Looking for any recommendations of those familiar with the Galaxian sound circuits.
If there is a marginal part in there I'd eventually hit it (there are only 7 resistors, 5 capacitors, 1 diode, a 555, a 4066, and a LM324 in that circuit. Though, maybe some of the caps in the audio output final stages of amplification might serve as a noise filter.... and it might be normal, but is filtered out on a 'happy' board.
Hoping someone can shed some light on this.
I find Galaxian is like that a little, but this one seems to have a bit more than the usual to me.
I have verified it is coming from the Fire audio circuit. The static seems to be the frequency of the free running oscillator (7S). I determined this by grounding the control of the analog switch. There is a small amount of voltage on it (which is roughly in line with another working board) I've compared it to.
So far I have tried swapping the following:
7R 4066 analog switch
7T LM324 quad diff op amp
C26 0.01MF 50V which is in series with the output of the LM324 for the fire sound
C25 1MF 16V Tantalum which sits between control of the analog switch and ground and provides for the duration of sound production of the fire sound
I have blindly tried C29 (0.01MF) between the control of 555 timer 7S and ground.
The C28 cap had already been done along with the other electrolytics.
I'm unsure why there is a difference. A good board (low hiss) and this one both have similar voltages on the analog switch control line for that sound. But as there is a slight + voltage here, I think the analog switch is allowing some to get through. I did not have a new 4066, so I did have to test this with a used 4066 pull of unknown condition.
Looking for any recommendations of those familiar with the Galaxian sound circuits.
If there is a marginal part in there I'd eventually hit it (there are only 7 resistors, 5 capacitors, 1 diode, a 555, a 4066, and a LM324 in that circuit. Though, maybe some of the caps in the audio output final stages of amplification might serve as a noise filter.... and it might be normal, but is filtered out on a 'happy' board.
Hoping someone can shed some light on this.