Because the power amps are fully independent, a failure in one output stage will leave the other speaker running perfectly. If one side goes silent, compare voltages between the functional power amp board and the dead power amp board. To help narrow down your look at this circuit, let me know: What specific year or era is your JC-120?
Typically a failure in the MN3007/MN3101 chips or the LFO (Low-Frequency Oscillator) circuit, which controls the speed of modulation. jc-120 schematic
carries the completely dry, unmodulated guitar signal. Because the power amps are fully independent, a
"Roland JC-120 service manual pdf" "JC-120 schematic full" "Roland Jazz Chorus 120 schematic diagram" Typically a failure in the MN3007/MN3101 chips or
Use an oscilloscope to check if the LFO is generating a clean sine or triangle wave.
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Roland did not make just one JC-120. Production ran from 1975 through the early 2000s (and later reissues). The schematic changed significantly over time. Searching for a "JC-120 schematic" requires knowing your specific version.