Jc-120 — Schematic Verified

Here’s a practical guide to finding, reading, and using the Roland JC-120 (Jazz Chorus) schematic.

3. Identifying Your JC-120 Version

Roland made several PCB revisions. The schematic differs significantly:

| Era | Key features | Common ICs | |-----|--------------|-------------| | 1975–1979 (Silver knob) | Discrete preamp, no reverb | 2SC458, BA662 (rare) | | 1980–1989 (Black knob) | Reverb added, FET input stage | MN3004, uPC1251 | | 1990–2006 (Mid boost) | Switchable mid, updated op-amps | NJM4558, MN3204 | | 2007–present (Reissue) | SMD components, no STK modules | TDA7293 (power amp) | jc-120 schematic

✅ Always check the serial number / date code on the chassis. The power amp ICs (STK-2030, STK-2050, or TDA7293) will tell you the era immediately.

6. How to Read the JC-120 Schematic Efficiently

1. Start at the input jack. Follow signal through Q1/Q2 (JFETs) → IC1a → tone stack.
2. Split point: After volume control, signal splits to:
   • Dry path (straight to power amp)
   • Chorus path (BBD → LPF → mix amp)
3. Power amp input from chorus mix + dry mix.
4. Protection circuitry (diodes, thermistors) near output.

💡 Find the chorus trimmer resistor on the schematic – it’s usually a 10k–50k trimpot between BBD output and ground. That’s your bias adjust point. Here’s a practical guide to finding, reading, and

3. The Effect Loop and The Famous Chorus/Vibrato

This is the heart of the JC-120’s identity. The schematic for the Chorus/Vibrato section is complex, involving analog delay lines.

Part 5: How to Find the Right Schematic

Given that Roland still supports (to a limited degree) these legacy amps, you have a few options for obtaining a clean JC-120 schematic: ✅ Always check the serial number / date

1. Official Source (Schematic Heavens): Roland’s European support site has a massive archive. Search for "Roland JC-120 service notes."
2. The Community (Elektrotanya & Synthnerd): This is the best resource. Look for the "JC-120 (JCR)" and "JC-120 (MCR)" PDFs. Ensure the file includes the parts list, as the resistor values are often smudged on the physical board.
3. Avoid the Fakes: Many websites charge $15 for a "download." Do not pay. The JC-120 schematic is legally available as a free service document due to its age.

The Tone Stack

Following the gain stages, the signal enters the Equalization (EQ) section. The schematic reveals a Baxandall-style tone control circuit (or a variation thereof).

• High Control: Unlike a tube amp’s "Presence" control which boosts upper mids, the JC-120 High control boosts actual treble frequencies (often 2kHz and up). In the schematic, you will see capacitors in the feedback loop of an op-amp that shunt low frequencies to ground, allowing only highs to pass.
• Bright Switch: A small capacitor across the volume potentiometer (visible in the schematic as a "bright cap") allows high frequencies to bypass the resistance of the volume knob. This results in the "sparkle" associated with the amp’s clean channel.

A. Preamp (IC1 – IC3 typically)

• Discrete JFET input (Q1,Q2) → high impedance
• Op-amp gain stages (NJM4558 or similar)
• Voicing filters (famous “JC” scoop around 1kHz)

The Complementary Pair

The JC-120 uses a quasi-complementary or fully complementary symmetry output stage, typically utilizing power transistors on large aluminum heat sinks.

• The Drivers: Transistors (often Toshiba 2SCxxxx series in vintage units) drive the current.
• The Output: The schematic shows transistors arranged to handle the "push" (positive voltage) and "pull" (negative voltage) of the AC audio signal.

5. Troubleshooting Using the Schematic

| Symptom | Check on schematic | |---------|--------------------| | No chorus | Q9/Q10 (MN3101 clock), Q11/Q12 (MN3004/3007), IC7 output | | Hum in one channel | Ground trace between preamp and power amp (common break) | | Distortion at low volume | Biasing of differential pair Q1/Q2 (2SC2240 or similar) | | One channel dead | RY1 (relay) contacts or switching IC (IC1) |