Hi all and thanks in advance for your help!

I just purchased a used Sennheiser 421-U5 off fleabay (I know, what can I say?). It works, but it sounds like it might not be 100% ok. I have a little bit of experience with other 421s and know that they take a fair amount of gain normally, but this one seems to need even a little more and then the sound is a bit thin and brittle on the top. I've compared it to a 421-U4 we have at work and it definitely sounds thinner than that one.

So, first any ideas on simple fixes - cleaning, easy way to open and adjust something, etc.? Secondly, what's the deal on repairing these? My other option of course, is to try to return it to the seller, which I may consider.

Again, thanks for the help!

Rob