Need help with a sick ES150P

Hi All, I picked this sub up for less than a pack of smokes from a garage sale to use in my home office. When I first turned it on, it worked just fine. A week later when the green light is on the first thing that happens is the woofer come out slowly then goes back in to its resting position then it makes what I can only describe is a low frequency crackle. This happens when it is playing music, not playing music, RCA plugged in or not plugged in.

I pulled the amp this morning and saw no issues with a visual inspection. No bulging caps or burned PCBs.

Any ideas what the problem might be?

Even with the crackling, it is a fine sounding sub which fills in the 70hz and lower that my desk top speaker can produce.

Its not a sub that justifies any real expenditure for repairs. The shortest path to fixing it is to replace the amp with a generic plate amp, or just wire up an external amp.

It might not be top of the line, but if I can repair it myself it will work great in my little office system......an justify my shop to my wife:D