I just bought a used sterling mic st51 its fading in the sound pickup

Kenneth, the ST51 may no longer be in your possession, but if it's gathering dust somewhere, rather than try to repair what is really an entry level mic, a couple of options: (1) buy a new (old stock, because the mic no longer in production) ST51 for about $100 if you can find one, or (2) think about using the body to build an entirely new, far superior audiophile mic.

For the latter option parts would run about $200 for pro audio grade components and transformer, and $175 for a primo K47-style cardioid condenser capsule. You would end up with the audio equivalent of a Neumann U-47 FET, a $4,400 mic (new). You do need to be handy with a soldering iron or have someone with the knowledge and skill to do the work (too complicated for beginners).

If neither is an option and you just want to get rid of it, a DIYer would be interested in buying it just to use the body to build a completely new mic.