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Hello, I have a Marshall jcm 900 2100slx that is quiet until a cord is plugged into the input jack. Then a pretty loud buzz or hiss kicks in that will turn microphonic/squeal if the 1/4" plug on the other end of the cord gets close to the speaker output jacks. Acts like a pretty big noise antenna with a guitar or not.
The buzz goes away with V1 pulled, or turning the pre gain pot down, so the noise should be in the input stage. The V1 pin2 screen is also extremely sensitive. probing for voltage or signal will squeal. Tried 4 different tubes and all have the same noise and all sound microphonic when tapping on them.
I have pretty much tried everything I can think of to track it down further.
looked over all the grounds and wiring, tried new input jack with different methods of grounding it, ground to circuit board and right to the chassis.
new shield wire to the v1 pin 2 grid.
flip-flopped the gain pots and made sure those were grounded and functioning.
tried a new tube socket, the V1-V2-V3 tubes are dc heated and I tried normal AC with 2x100ohm center tap. also replaced the power filter caps and the 10k dropping resistors.
I have replaced all of the components in the V1 section (plate/cathode/screen coupling resistors/capacitors and tried different layouts in the process. keeping them off the board and trying to make sure I am not missing a broken wire or solder joint.
The power transformer makes an audible mechanical sounding buzz that is similar.
Really kind of stumped and must be overlooking something. Any ideas on narrowing it down better? more likely to be layout, bad component or solder contact? something outside the V1 section? I'm kind of just starting to guess and shotgun it now.
Thanks for the help! this board is a great resource
Ryan
The buzz goes away with V1 pulled, or turning the pre gain pot down, so the noise should be in the input stage. The V1 pin2 screen is also extremely sensitive. probing for voltage or signal will squeal. Tried 4 different tubes and all have the same noise and all sound microphonic when tapping on them.
I have pretty much tried everything I can think of to track it down further.
looked over all the grounds and wiring, tried new input jack with different methods of grounding it, ground to circuit board and right to the chassis.
new shield wire to the v1 pin 2 grid.
flip-flopped the gain pots and made sure those were grounded and functioning.
tried a new tube socket, the V1-V2-V3 tubes are dc heated and I tried normal AC with 2x100ohm center tap. also replaced the power filter caps and the 10k dropping resistors.
I have replaced all of the components in the V1 section (plate/cathode/screen coupling resistors/capacitors and tried different layouts in the process. keeping them off the board and trying to make sure I am not missing a broken wire or solder joint.
The power transformer makes an audible mechanical sounding buzz that is similar.
Really kind of stumped and must be overlooking something. Any ideas on narrowing it down better? more likely to be layout, bad component or solder contact? something outside the V1 section? I'm kind of just starting to guess and shotgun it now.
Thanks for the help! this board is a great resource
Ryan