Korg i3 - faulty power on mute?

[drhayes]

Hi there,

I have a Korg i3 which is misbehaving. Directly up front: I may be an idiot, so please always assume that I don't really know what I'm talking about.

The symptom: all outputs are quiet. With extreme amplification you can hear digital noise and the correctly produced sounds. I think I have tracked the issue down to the power on mute circuit, but I seem to have a problem understanding, what is happening, or what is supposed to happen.

I assume that the Power On Mute rail is supposed to initially be high and then to drop to ground. Why do I think so? Because if I short POM to ground, all outputs work fine.



The muting of the signals is done with transistors - 6 of these units:



So, as much as I believe to understand, if the base of Q3 is positive, the signal is pulled to ground, if the base is ground, the transistor is inactive and the signal goes through.

But the power on mute goes to 4.2V and slowly drops (if forced to ground shortly) to 2.8V. If I look at the circuit, I have a hard time understanding how it would ever reach 0 V.  As much as I understand, the circuit does exactly the opposite. The 5 Volt charge C40 through R41 slowly raising the voltage on the base on Q7. While Q7 hasn't switched on, we have -12V on the power on mute rail. As soon as Q7 switches on, the POM rail goes up - and stays there.

Measuring all components in circuit shows that all are within specs, except for C40, which measures as 110 µF. Edit: I have desoldered C40 now, out-of-circuit my multimeter shows the capacity to expect.

Now my questions: what is it that I don't understand?

[Zoli]

Did you considered that Q7 is PNP(=raising base voltage will switch it off)?
As recommandation: measure the voltage on the positive sides of C39 and C40, once the system is up;
those voltages will give you a good indication where the fault lies.

[drhayes]

Hi Zoli,

apparently I have not considered the type of Q7. Thank you for pointing out where I had the mistake in my analysis.

C40 apparently was bleeding current to ground, so the voltage never went above 3.3V. Replaced that with a new cap, and the voltage rises to 4.7V a few seconds after turning the synth on. And then - as you correctly mentioned - Q7 switches off and I get a nice low POM (haven't measured it now, but all muting stops after a few seconds).

Cheers!