Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, does the 2mtr TX functions as expected with the Yaesu FT-857D? Have you tried a RESET?
If yes to both, then it would be an internal hardware or more particularly, a potential fault on the front end circuit; i.e. :
If comfortably able and feel safe to do so, are you then considering a DIY (do-it-yourself) repair approach? If yes, do you have access to and familiar with a multi-meter and even a soldering iron?
Equally important, could you make use of the repair/Service Manual or at the very least, the schematic diagrams?