The manual shows a cross-section of the key mechanism. Each key has a pivot pin and a return spring. You will see a white plastic strip running the length of the keybed—this is the "key slip" prevention bar. Remove it. Then, gently lift the faulty key from the front edge, angling it toward the back.
This could be a failed op-amp (often a NJM4580 or similar), a broken headphone jack that disconnects the main outs, or cracked solder joints on the RCA jacks. roland juno di service manual work
Service manual solution: The schematics allow you to trace the audio signal path from the DAC (Digital-to-Analog Converter) through the mixing stage and out to the jacks. You can probe test points (TP1, TP2, etc.) with an oscilloscope. The manual shows a cross-section of the key mechanism
Once open, do not immediately desolder anything. Use a multimeter. Run the full test sequence
Before opening the synth, use the manual to enter Self-Test Mode:
Run the full test sequence. Note exactly which test fails. This isolates the problem to a subsystem.