The 520 tractors were sent to dealers with an incorrect lamp in the tractor's tachometer. This lamp was of too high a wattage such that the plastic housing gets burned to the point it is ruined. Suggest owners of 520 tractors from years 1991 to 1995 read the service bulletin.
Not specified in the bulletin is how to tell if your tractor has the incorrect lamp. Remove the lamp from it's holder and look for a number stamped on the brass base. It should read 53x in which case your tachometer is probably healthy. If it reads 1816 the tachometer will be burned by the extra heat from the lamp.