Hey guys I've got something that I can't figure out. About a year back I bought this marshall and it worked perfectly fine. Minus the loose input jack and dirty pots (which were all dealt with). Now I finally got it back from a buddy that was borrowing it and turned it on to find this.
https://youtu.be/JE-wYeK9R9g (https://youtu.be/JE-wYeK9R9g)
Any ideas?
First off, unless there is insulation I cannot see, it is VERY dangerous to have the live board sitting like that. If you intend to operate the board out of the chassis like that, at least lay a towel or newspaper across the front edge of the chassis so the board cannot touch the metal.
Your amp is oscillating. does it still do this while all assembled?
Just a thought, is the reverb pan connected? And if it is, did you check to see if either end has an open transducer? Some Marshall reverb circuits are sensitive to open circuits and will oscillate.
Yea I have it suspended so it doesn't touch the steel. As far as the oscillating goes it does that all the time. Put together or not. In this video I didn't have the reverb hooked up, but when I connect it the result is the same.
Did you check the pan transducers for opens, or does the reverb work, aside from the oscillation?
Otherwise, start at the start. Is this oscillation riding any of the power rails? Can you isolate the power amp and see if it still exists?
you say you sorted the dirty pots;did you replace them or just clean them?
the reason i ask is i had a hughes and kettner tubeman hybrid pre-amp (which is their take on the valvestate) which made some horrible noises i suspected of being the tube;only when i turned the gain pot several times it cleared up;cleaning it cured it completely.
a similar problem happened on my valvestate VS-65R;but this time it was a combination of 2 faults!;the pot's had been cleaned by an idiot using spray anti rust oil,this created a conductive mess across the PCB,especially when the amp stood around getting damp.it made the amp produce a speaker-killer buzz.problem 2 the tube had gone faulty;bizzarely,although the tube is used in clean and overdrive circuits, it only showed a fault when in overdrive (presumeably because of higher gain through it)
the VS amps are old enough now to have dried up smoothing caps (maybe a cause of the supply rail borne noise hinted at) another possibilty is a dying op-amp;the switching ones used in the VS can and do fail;there is an updated version which marshall recommend;as the ones used are now obsolete.
At the very end of the video you linked to when you are putting pressure
on the plug (and input jack) there is audio from a radio or television program coming
from the speakers. (at least it sounds that way to me...)
This is called rectification, and it typically happens when the input to an audio amplification stage
becomes ungrounded. In addition to the other suggestions made here, I'd check the input
jack itself and it's connection to the board, as well as all other ground connections near
the input jack.
Does the amp make any motor boating sounds with the guitar cable disconnected from the amp? I agree with amp_MD that you should start looking right at the input jack connections, but don't forget about the connections inside the actual jack itself, encased in plastic.