that happened to me once only with a "boom box" of my DD's at Radio Shack. they insisted they fixed the broken part, broken since purchased from them, kept it two weeks and didn't touch it. It was so obvious and they didn't back down until I demanded a regional or whatever manager meet me at the store to view it. It was in their possession throughout. Sure enough, it was obvious to the manager I was right. They replaced it on the spot. I was nice at each step, making sure I would not be "wrong" anywhere. So, you should see what they have to say but if there was dust, it should not have been there upon return. at the very least, a maintenance involves oiling and to oil, there should not be any dust.