Alesis QS8-1 Distorted Output

[NighthawkJW]

I"m troubleshooting an Alesis QS8-1. It is as if the output sound is overdriven (not so bad at very low levels, but about half-way up distorts like NUTS). I did a global reset, hoping that would be an easy fix...
I think the problem is either the DAC, or failing JFETs. I replaced the op-amps and have checked all caps & diodes in the analog audio output circuit. The JFETs are J111A, the DAC is an AK43192 (obsolete?). I'm not sure about scoping it to trace the distortion (what voice for the best waveform?).

Anyone been down this road before?

[g1]

If you have a schematic please attach it.
If the jfet's are for muting, for troubleshooting purposes you can just remove them.

[NighthawkJW]

If you have a schematic please attach it.
If the jfet's are for muting, for troubleshooting purposes you can just remove them.

No, I don't have a schematic. Not sure what function they serve, but they are by the output jacks.

[g1]

Then they are probably mute fet's. I would try removing them to see if it gets rid of the distortion. Beware of turn on/off thumps, that is likely what they are there to prevent.

[Enzo]

I would also check your power supply. ICs like op amps are probably running on split supplies, like +15 and -15v (or 12v or 9v). So look at the power pins of them and see if both supplies are present and both up to about the same voltage (but of course reverse polarity).

If you had a 15v supply that had collapsed to zero or even just to 8 volts, then the entire signal path would be skewed, and you'd run out of headroom very quickly.

That is just a thought, and can be checked out with a volt meter in just seconds.

[doctor]

I would also check your power supply. ICs like op amps are probably running on split supplies, like +15 and -15v (or 12v or 9v). So look at the power pins of them and see if both supplies are present and both up to about the same voltage (but of course reverse polarity).

If you had a 15v supply that had collapsed to zero or even just to 8 volts, then the entire signal path would be skewed, and you'd run out of headroom very quickly.

That is just a thought, and can be checked out with a volt meter in just seconds.

Can the scheme help you?

[Enzo]

I see that indeed the op amps run on +/-15vDC.

And i also see that g-one's mute JFETs are present on the output jacks.

[NighthawkJW]

Many thanks to all (esp Doctor for the Schematic!). I pulled the JFETs and no change. Voltages look good - clean DC.

I am hoping it is something simple like the V-regulators, but suspicious of the DACs. Shouldn't there be an output from the Aux jacks? There seems to be nothing there.

The DACs are both AK4319 - output is very small and hard to scope going into the op-amps. The output to the jack scopes clean. The way it sounds, I would expect the piano sound to have a square wave form, but it makes a nice rounded sine wave. WTF? It sounds like it is playing through a cheap distortion pedal.

Does anyone have experience with those DACs going bad?

[Enzo]

I think the AUX out jacks only have signal if they are programmed to. There is a separate data stream for each DAC.

Nothing is failure proof, but I see VERY few failures of DACs in anything.

[Jazz P Bass]

Kind of difficult accessing the soundness of a DAC.
You can't make heads or tails out of the Digital input but if the Analog output looks bad on a scope, then the DAC is suspect.
I did have a few go bad.
Mostly really old stuff.
A Presonus synth & a NAD surround amp.