Mesa Boogie 50 Caliber + weird 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

 

[radardoug]

Are you sure its not what you are plugging into it? If you plug in a cable with nothing at the other end of the cable, is it noisy? If you then short the free end of the cable is it noisy still? If it is quiet, then its your guitar and/or effects.

 

[beatnik]

the noise is worse with the guitar plugged in, also the clean channel and lead channel are noisy in different way, the first is like hum, the other is more buzzing

i think i should try with a humbuckers guitar as well because maybe there is also some interference from the single coil pick ups

 

[beatnik]

power amp section

 

[Gus]

Have you checked the input jack? 
You posted it happens when there is an input to the amp
Broken solder?
Dirt?
Do the contacts have good pressure when the plug is installed?

Next it looks like it uses LEDs on LDRs to do some switching.  Note the extra stages that gets added at the input by the LED/LDRs
Could have something to do with that.

 

[radardoug]

If you aren't using humbucking pickups, then its your guitar that is picking up the noise.
DO the test I suggested!!! Why are you guitarists so stupid???

 

[PRR]

> DO the test I suggested!!!

If you plug in a cable with nothing at the other end of the cable, is it noisy?
If you then short the free end of the cable is it noisy still?

 

[beatnik]

If you aren't using humbucking pickups, then its your guitar that is picking up the noise.
DO the test I suggested!!! Why are you guitarists so stupid???

i did your test, that's why i said that most of the noise is caused by the single coil pick ups

but even with a different source there is still hum in the clean channel, but different on the lead channel, so i believe the amp is not right yet, but i won't risk being called stupid guitarist again so i won't bother you with this anymore !

 

[mjrippe]

i did your test, that's why i said that most of the noise is caused by the single coil pick ups

but even with a different source there is still hum in the clean channel, but different on the lead channel, so i believe the amp is not right yet, but i won't risk being called stupid guitarist again so i won't bother you with this anymore !

What was the result with the input cable shorted (connect tip to sleeve)?  Still hum or quieter than with the humbucker guitar?