A robust transformer and filtering section designed to minimize sag and preserve absolute clarity. 2. Preamp Circuit Analysis Input Stage and Gain Control
If you are working on a JC-120 on your test bench, keep these common schematic-related points in mind:
Uses 4558 or equivalent low-noise dual op-amps.
The defining feature of the JC-120 is its two-channel output. The chorus circuit splits the signal, sending a "dry" signal to one speaker and a "wet" (modulated) signal to the other, creating a spatial, 3D sound. 2. Breakdown of the JC-120 Schematic Sections
A low-frequency oscillator (LFO) drives the BBDs, creating the signature "wobble." jc-120 schematic
The Roland JC-120 Jazz Chorus schematic Go to product viewer dialog for this item.
To create the chorus effect in an era before digital microprocessors, Roland used analog (typically the MN3002 or MN3007 chips, driven by an MN3101 clock oscillator).
Your JC-120 is a classic that deserves proper care.
These feature a rotary power switch, a heavy toggle switch for chorus/vibrato selection, and internal wiring that can be notoriously difficult to navigate. They utilize older, often obsolete operational amplifiers (op-amps). A robust transformer and filtering section designed to
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The internet is full of mods claiming to "improve" the JC-120—bypassing the buffer caps, swapping op-amps for OPA2134s, or ripping out the distortion circuit.
Finding the correct schematic is often the first hurdle. Fortunately, the JC-120 is a popular amp with a dedicated community, and various resources exist for the determined seeker.
When diagnosing a broken Jazz Chorus using the schematic, technicians generally look at three common failure points: The defining feature of the JC-120 is its two-channel output
In "Chorus" mode, the dry signal is sent directly to one power amplifier and speaker, while the modulated (wet) signal is sent to the other. The two signals mix in the air, creating a rich, physical stereo field that cannot be replicated by mono amplifiers. 4. The Power Amplifier Section
Shifted to plastic-encapsulated power transistors or integrated power IC modules to streamline production.
Before the internet, if your JC-120 broke, you took it to an authorized Roland dealer. Today, the DIY repair community relies heavily on the and its accompanying schematics. Why?
Introduced in 1975, the JC-120 was a revolutionary product from Roland. At a time when tube amps dominated the market, Roland took a different approach, aiming to create the best clean amp ever made by leveraging the unique capabilities of solid-state technology.
If the chorus stops working but the vibrato works, look closely at the switching logic near the control jacks. If both fail, check the voltage supply pins on the MN3101 and MN3007 chips; these analog ICs are sensitive to voltage spikes.