Jc-120 Schematic

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

. Whether you’re restoring a vintage 70s unit or troubleshooting a modern reissue, understanding the JC-120 schematic is key to maintaining this legendary amp. The "King of Clean" Circuitry

Unlike the tube stacks that dominated the 1970s, the JC-120 was built on a stable, solid-state design. Its core power comes from two independent 60-watt power amplifiers —one for each 12-inch speaker. Dimensional Space Chorus: jc-120 schematic

This isn't just a standard mono effect. The circuit creates "true" stereo by sending the dry signal to one speaker while the other receives a pitch-modulated signal. Bucket Brigade Devices (BBD):

Vintage JC-120 schematics feature the famous BBD chips (like the MN3002 or MN3007), which provide the warm, liquid-like analog chorus that modern digital recreations still strive to emulate. Independent Channels: Since its debut in 1975, the Roland JC-120

Channel 1 provides a pure, direct signal path, while Channel 2 adds the famous effects, including a spring reverb and the controversial "distortion" knob. Finding the Right Schematic

The JC-120 has undergone several revisions over its 40+ year history. To ensure you have the correct diagram, check your serial number and physical features: The ultimate JC-120 thread Input impedance : 1MΩ (R1, R2) Tone stack

Some pics that display cosmetic differences of JC-120's of different pedigrees: From top to bottom. - Very old version of the amp: Solid State Guitar Amp Forum jazz chorus jc-120 - Roland

3. Critical Component Values (from schematic)

• Input impedance: 1MΩ (R1, R2)
• Tone stack: Bass (100k log), Middle (50k log), Treble (100k log) – passive FMV style but with different cap values (C9, C10, C11 ~ 0.047µF, 0.022µF, 470pF)
• Chorus BBD: Usually MN3004 (512-stage) for delay A, MN3007 (1024-stage) for delay B – creates two-phase modulation
• Output transistors: 2SD718 (NPN) / 2SB688 (PNP) – TO-3P case. Replace with MJL3281A/MJL1302A if unavailable.

The Safety Triangle

In the output section, you will see smaller components like diodes and resistors protecting the power transistors. These prevent voltage spikes (from inductive speaker loads) from destroying the output devices.

2. The Preamplifier Stage (The "Glass" Tone)

The input stage of the JC-120 is where the characteristic "chime" is born. Looking at the schematic for the input section (often centered around the input jack and the first few transistors or op-amps depending on the revision), we see a design focused on fidelity.