Similar Marshall Noise (as seen in 496dart's post)

[thewheelman36]

Alright, so here is the new thread. For those who are just now reading, I have a G100R amp head with a really screwball input jack fix/replacement, I just can't tell. The amplifier emits an oscillation that increases in frequency as the contour knob is turned from 1/4 to full. Below the .25 mark it sounds fine, however, the sound I want lies above this range. As I understand what Enzo has written, this may be due to a ground path being shorted/cut/whatever that controls the mute when nothing is plugged in.

Sincerely
wheelman

[Enzo]

AS mentioned, straighten out the jack situation first. Mute line is just one possibility.

[thewheelman36]

This could take awhile. I'll send a pic soon, but this thing looks REALLY difficult to get out without almost destroying that side of the amp. I told you it was krazy glued or something. But I will try, thanks. Any other possibilities if that's not the problem?

[Enzo]

The boards should come free enough. I never completely remove a board, I just free it enough to flop it over for access.

Failing the jacks, I start scoping the stages to see where the oscillation occurs. I also watch the power rails to see if the unwanted signal is there. A lack of decoupling could cause such instability.


A broken gain control could as well. In op amp based circuits, the gain controls are usually in the feedback circuit of an op amp, and if they crak or go open, then the feedback loop of the IC is broken. Gain maxes.

[thewheelman36]

Right. I feel kinda stupid with some of the terms you used, as I am not a fluent electrician by any means, but I have some knowledge. Is there a diagram pointing to the things you mentioned anywhere so I know where to begin looking? Thanks

[Enzo]

Not aware of any drawings other than schematics. COntrols are just controls - the things the knobs go on. Demount the board so you can get at them. The solder under them is to be checked, and we want to make sure the actual control is not cracked. The controls is based on a flat piece of what looks like circuit board material. PArt of it sticks out the side of the control and has the three legs riveted to it. This part sometimes cracks.