Help needed - Noisy mic preamp channel.

[Cableaddict]

Hey guys.

I know my way around a soldering iron & have a basic understanding of electronics, but I'm not an EE & I don't own a scope.

Still, I'm hoping I might be able to fix this problem myself, if you guys can point me to the most likely source:

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I have a True Systems Precision8 mic preamp. AFAIK, it's all discrete.
One channel (only) has suddenly started outputing a fairly high level of steady HF noise. Everything still works: The frequency response seems correct, the volume pot works as expected, etc.

The other 7 channels are dead silent.

(And no, it's not the wiring, etc.)

I don't have the schematic yet, but I'm searching.....
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Any ideas or suggestions?

 

[Douglas Blake]

Without a schematic and a scope the best I can suggest is that you need to get someone to sit down with you in person and run it through a scope for you.

In the mean time you can look for shorts, water damage etc...

 

[ubergeeknz]

Does the volume control affect the volume of noise? That should provide a clue to the noisy component.

 

[Douglas Blake]

Does the volume control affect the volume of noise? That should provide a clue to the noisy component.

He might also try simply cleaning the pot with some alcohol....

 

[Cableaddict]

Does the volume control affect the volume of noise? That should provide a clue to the noisy component.

Good thought!

And no, it does not affect the volume of this residual noise at all.
I assume that means the noise is on the output path, yes?

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I'll be opening it up this weekend. Maybe I'll get lucky I find a bad resistor.

Or maybe this is a chip design after all, and I can try replacing the one op amp. - But if it were an op amp, the noise would likely follow the volume knob, no?

If it's discrete, maybe a bad output transistor?

 

[ubergeeknz]

Any of the above. Happy hunting. A signal tracer or oscilloscope will help, you can use another channel for comparison.

Another trick is heat/cold on the individual components, see where it makes a difference. Fingers work for heat. Cold usually freeze spray or an inverted compressed air can.