Roland keyboard has bum audio-out circuit

[BHA1]

I have a discontinued digital piano, model Roland HP-245. Its speakers put out sound as designed. The two 1/4" TS output jacks are such that one puts out absolutely nothing, the other puts out only LOUD static, no useful signals at all. No parts are available, no tech is willing to work on it (even house-call places!) and Roland does not permit even the slightest of tech manuals to be available except to their certified repair shops. I know where the circuit board is for it, and suspect that possibly there are issues with the units' op-amps. However, without a schematic, I am uncertain as how to start troubleshooting. On the fence: do I leave it alone and give up any hope of finding the circuit's faults, or are there safe ways to inspect the last elements of an audio signal chain on an unknown circuit? I am experienced in troubleshooting PCs and such, and can assemble basic audio circuits on a breadboard; that it where my component-level skills are at this time. I have a good multimeter and good oscilloscope, and am reasonably comfortable using them.

Comments are invited.