Jc-120 Schematic -

The schematic relies on a Bucket Brigade Device (historically the Panasonic MN3007) driven by an MN3101 clock generator. This chip physically delays the audio signal by milliseconds.

Let’s walk through the schematic like a signal tracer. With a copy of the diagram in front of you, follow these stages:

: This is the heart of the amp. It uses Bucket Brigade Device (BBD) chips—traditionally the MN3007 or MN3002—to create a short, modulated delay. The schematic reveals a unique "dry/wet" split where one speaker carries the dry signal and the other carries the modulated signal, creating an acoustic chorus effect in the air.

: Best for units produced in the 70s and 80s using discrete components. jc-120 schematic

Mid-vintage and modern versions transitioned entirely to integrated circuits, incorporating operational amplifiers (op-amps) like the or TL072 for the input and tone stacks. Additionally, the head model variant, the JC-120H , utilizes its own distinct single-channel schematic that features a unique "Hi-Treble" control instead of standard bright switch circuitry. Key Architectural Sections of the Schematic

JC-120 UT / JT (2nd Edition - these are the newest JC-120 amps) - Gain stages are discrete, FETs or combinations of FETs and BJTs. Solid State Guitar Amp Forum JC-120 | Jazz Chorus Guitar Amplifier - Roland

Purists ignore it, but the JC-120 includes a distortion circuit. The schematic shows a separate gain stage using an op-amp with clipping diodes (usually 1S1588 or 1N4148) in the feedback loop. The schematic relies on a Bucket Brigade Device

The LFO creates a smooth sine or triangle wave. In "Chorus" mode, the LFO speed and depth are fixed to Roland’s factory-optimized sweet spot. Switch the amp to "Vibrato," and the schematic routes control back to the front-panel Speed and Depth potentiometers, allowing the user to adjust the frequency of the LFO modulation. 4. Power Amplifier and Output Section

Working on a JC-120 involves high voltages, specifically on the main filter capacitors (up to 70V DC) and mains AC. While the schematic is invaluable for low-voltage work, always discharge capacitors and unplug the amplifier before probing. If you are unsure, take the schematic to a professional technician.

Before diving into any repair or modification, the very first step is identifying which version of the JC-120 you own. The schematic for a 1978 model is different from a 1990 model. As noted by experts on repair forums, "there were many revisions" of the JC-120 over its long production run. A single schematic is not universal, and using the wrong one can lead to endless confusion. With a copy of the diagram in front

Two independent input channels (Normal and Effect). Speakers: Dual 30cm (12-inch) "silver cone" drivers.

[ Effect Preamp Signal ] | +----> [ Direct Signal Path ] -------------------------> [ Left Power Amp ] | +----> [ MN3007 BBD Delay Line ] ^ | (Modulated Clock) [ MN3101 Clock Driver ] <--- [ LFO Circuit ] | v [ Modulated Signal Path ] ----------------------> [ Right Power Amp ] Bucket Brigade Device (BBD)

A high-headroom, ultra-clean path with EQ.

The JC-120 is a 120-watt, two-channel guitar amplifier head designed by Marshall Amplification. Introduced in the late 1980s, it quickly gained popularity among guitarists for its versatility, reliability, and rich tone. The JC-120 is part of Marshall's JCM800 series, which revolutionized the amplifier market with its master volume control and versatile channel switching.