Troubleshooting Soundcraft GB30 preamp

[drunton]

Hi everyone - I'm stuck and wondering if anyone has a suggestion...

I have a portable Soundcraft MFXi20 board for live use and it has a single channel that is noisy.  The noise sounds like a scratchy LP/record.  I believe the issue is in the preamp stage.  If I use an insert cable, the noise disappears from the channel, so I feel the EQ and bus sections are all fine.

Troubleshooting has been quite a pain.  The noise is visible on my scope with the scope channel at the highest gain selection, but it only shows up at the output of the opamp stage.  I believe the preamp is roughly of the "ESP, low noise balanced" (http://sound.westhost.com/project66.htm), except with some modifications that I haven't been able to fully trace out.

I don't see any noise on either sides of the compound diff pair, but I am starting to think the noise just can't be seen until the TL074 opamp stage provides the balanced to unbalanced conversion and gain.

I am at the phase now of just starting to replace parts in this order:
the two .01uF tantalum beads around the diff transistors
the three other .01uF/.1uF tantalum beads
the diff transistors
the electrolytics
the TL074

Anyone have a schematic or any suggestions?

Thanks,
Dave

 

[JohnRoberts]

You may want to check solder connections first, but yes, a noisy part in a high gain stage will probably only be visible at the opamp output.

Likely suspects are transistors, maybe the opamp, perhaps a leaky cap.

Good luck.

JR