Had my Genz-Benz Shuttle 6.0 die on me at a recent rehearsal. It started making a loud white noise, and the signal that made it through was heavily distorted. Noise existed even with all the controls at zero, and the mute on. Luckily I was on upright, and the drummer just went to rods while I went acoustic.
With some quick troubleshooting, was able to determine that the signal out of the DI was fine both pre- and post-EQ, which isolated the issue to the power amp section. Noise also came through the headphone jack, which comes straight off the power amp through resistors. Also noted that the signal light would tun on with the noise, which according to the G-B block diagram, would isolate the issue to the master volume gain stage (before the switching power amp).
At this point, I took the amp over to my Dad's (retired electrical engineer, he loves to fix electronics). We pulled the lid off, and first checked for any obvious issues (broken components, solder joints, leaking caps, burn marks) and saw nothing. Next he poked around the circuit cards with a wood stick, and was able to make the noise come and go by tapping on the forward-most board. He tapped each of the components until we found the culprit, a small connector.
This is a short grey harness that connects near the master volume knob, and the middle of the same board. Not sure without a schematic, but we are pretty sure this is the feedback line for the master volume gain stage. A little contact cleaner and reseating both ends of the cable, and the amp is working perfect.
Don't really know the history of this amp, I bought it used at a pawn shop a few months ago (I already had another Shuttle 6.0, but couldn't pass up the deal on the second). So it might have already had the issue before. Luckily wasn't at a gig
Anyway, hope this helps if anyone else ever has this issue.