Mesa Boogie 50 Caliber + head noise problem

[beatnik]

Hi everybody

I have a Mesa Boogie 50 caliber + head that recently developed noise problems. The amp is quiet with no cable inserted but gets noisy with the guitar plugged in

The first thing I have done is replacing all preamp tubes with other tubes selected for low noise in other amps.

This unfortunately didn't solve the problem. I even tried replacing the power tubes, they are not the cause.

There is a very weird kind of noise, it sounds like hum but also has a flanging/phasing component to it. Also it suddenly would go up and down in volume

I have recorded a short sample of the noise problem. You can hear at 00:15 the noise going up in level, then it turns down again at 01:04

https://www.dropbox.com/s/oze8kogyki...01_01.mp3?dl=0

I absolutely have no clue what is the cause of this noise problem. I checked the pcb for dry joints and bad connections but everything seems to be fine and looks like these amps are very well built

I really hope someone could give any troubleshooting suggestions. I have access to a scope but I am kind of lost, would be great if anybody recommend which parts of the circuit to check

[Chuck H]

That sounds like electro magnetic interference to me. Nothing inside the amp will shift around like that. If this is a single coil equipped guitar you could try a humbucker equipped guitar or, if your guitar is functioning properly, just turn down the guitar volume control. If the problem goes away then it's outside interference. Not much you can do about that other than identifying the source and eliminating it when you want to play or moving your gear as far from it as possible.

Is the steady buzz/hum a result of your usual gain level? It seems excessive, but I don't have any context to judge by. There may be a ground fault in the amp input jack, the guitar output jack or the cable connecting them. If there is a ground fault in the guitar it's possible that turning the guitar volume down won't eliminate the problem.

P.S. You really shouldn't jump straight into changing all the tubes at the first sign of a problem. It can cause more problems and confuse diagnosis.