How does this mute switch work

[aron]

When there is no current on base, then the transistor passes signal from collector to emitter?

Data Sheet



Just wondering if there can be a case where the transistor could be partially on. This synthesizer has a bad capacitor somewhere and it takes 20 mins to get full output. I'm assuming that the mute transistors are partially muting, but I'm not sure.

[R.G.]

When there is no current on base, then the transistor passes signal from collector to emitter?

I can't tell exactly from the photo, but is the setup so that signal is running through the horizontal line across the schemo, through R37, and the on/off signal to the transistor is through R36?

If so, and you're SURE there is no base current, then the transistor is bad, or there is an external defect (solder short or other problem).  This circuit works the obvious way: if there is no current in the base, the transistor doesn't shunt audio to ground. If the base has current, the transistor turns on and sucks the current down to ground.

Just wondering if there can be a case where the transistor could be partially on.

Yes. If the voltage on the non-transistor side of R36 doesn't go below 0.5V when not muting, the transistor could be being told to be on a little bit. One good way to test this out is to temporarily short the base and emitter. If the transistor still partially mutes the signal, either the transistor is bad or there is a short on the collector-emitter.

Note that you have to be sure there is signal there to mute. If signal is not coming through from the left hand side to R37 and the collector, Q1 doesn't matter much.








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[aron]

Thanks RG. I assume that this is for anti-pop when the keyboard is turned off. If it requires voltage to mute, then how does something like this work when the keyboard is turned off? When you press the off switch? Thanks for the tip. I know how to proceed now.

Aron

[boogietone]

Thanks RG. I assume that this is for anti-pop when the keyboard is turned off. If it requires voltage to mute, then how does something like this work when the keyboard is turned off? When you press the off switch? Thanks for the tip. I know how to proceed now.

Aron

Would the caps from the power supply (or elsewhere) still need to drain giving enough power to "mute" the transistor on power off?

[aron]

I don't know, that's what I was trying to figure out.

> If the voltage on the non-transistor side of R36 doesn't go below 0.5V when not muting

I bet this is what is happening, but I need time to check it out. For now, just leave it on for 6 minutes and then play.