I bought a used sp 404 a few months ago that had a faulty volume pot for $40. So I ordered a new volume pot from Roland and had my cousin solder on. Well that fixed it and it was working perfect for a couple months. Then all of a sudden I switched it on and all I hear when I press any of the pads is a crackle/static sound that lasts the duration of each sample. It makes the sound through the headphone jack and the outputs up top. I thought it might be the volume pot but both left and right channels are working fine and whenever I raise the volume the crackle just gets louder. I tried deleting the factory sounds, and updating the OS and then loading the factory sounds back on there and it's STILL making the crackle/static sound. Has anybody else had this problem? Can anyone help?

I've heard that on my 303, usually right before it corrupts all my samples completely as you mentioned. I just erase the card and the problem resolves itself until the next sneak attack I would try a new memory card and hope that is faulty and not the unit, shit maybe the card is even placed in the 404 upside down on accident, did you check that out? I'm fairly certain I've put my 303 memory card in like that before and thought my sampler was broken. bonne chance!

I don't keep the CF card in it. I just used it to load the new OS with the stock sounds. So it's just the sounds that are the stock sounds on bank A and B that I can test. Even when I record samples in and play them back they also sound staticy/crackly, but it sounds like that through every output.

That;s no bueno, can you open it up and take pictures of the circuit boards, maybe the issue can be narrowed down with a visual inspection. Maybe the soldering done previously has shorted something out.