Schematic | Jc-120

Since its debut in 1975, the Roland JC-120 Jazz Chorus has defined the "clean sound" for generations of guitarists

• EL34 (or KT77/6L6) Power Tubes: Four power tubes in a push-pull configuration, providing 120 watts of output power.
• Phase Inverter: A PI (phase inverter) circuit, built around a single 12AT7 tube, drives the power amp.
• Biasing: The power amp features a adjustable bias circuit, allowing for precise control over the power tubes' operating point.

Would you like a step-by-step guide to tracing the signal path on the JC-120 schematic for a specific symptom (e.g., “no chorus” or “hum in left channel”)? jc-120 schematic

: This is the version most commonly circulated online but uses op-amps for gain stages, whereas older combos use discrete transistors. Since its debut in 1975, the Roland JC-120

• The Internet Archive (archive.org) – Search "Roland JC-120 service notes" or "JC-120 schematic". Look for the 1980s "Service Notes" (e.g., PN 01030761).
• Synthxl.com – Free Roland JC-120 schematic PDF (clean, scanned from original).
• Elektrotanya.com – Free download after short wait; often has multiple revisions.
• Roland support (legacy) – No longer public, but service centers have them.

• Op-amp swapping: The schematic reveals that many sections rely on the slew rate of the original 4558. Burr-Brown op-amps are faster and can cause ultrasonic oscillation, which blows tweeters.
• Removing the distortion: The distortion circuit is physically tied to the power supply rails. Cutting it out changes the load on the ±15V regulators. The schematic shows a 1k resistor load; you must replace the circuit with an equal resistor to keep the voltage stable.

5. Common Repairs & Schematic Clues

Symptom: No chorus / weak chorus

• Check IC11 (MN3002) pin 7 (output) with scope.
• Verify clock at pin 6 of MN3101.
• Adjust VR5 (chorus level) and VR6 (BBD bias).

Have you restored a classic JC-120? Share your schematic war stories in the comments below. EL34 (or KT77/6L6) Power Tubes : Four power

Channel 1 (Normal): A high-headroom, ultra-clean path with EQ.