Check all of the solder and traces for the three color transistors on the neck board (ones with the heat sinks).
This neck board is known for having a lot of problems in that area.
If that doesn't get it try swapping the blue transistor (Q503) with either the green (Q504) or red (Q505) and see if the problem moves.
If you need it repaired I repair this chassis. Cost to repair $40.00 plus parts and return shipping.