Turn it off. Open, and check for shorted resistors, capacitors, or rectifiers, etc.
If you are not comfortable doing this, or the problem does not make itself obvious, you may want more experienced help.
Service manual is here:
This project may be beyond my paygrade. Is there a resource for repairs or a new board? I'll ask around on the local level. A good friend and Linn TT mechanic doesn't feel the Pioneer is worth the trouble. This is a second TT, but one I'd love to keep running. My other TT is a Heybrook TT2.