Roland Jazz Chorus

There are many, many different models of Jazz Chorus amps in the world. I have worked on a number of them, but unfortunately I don't have a lot of notes to share... yet. Eventually it would be nice to organize all of the model info and schematics into one place. The JC120, the main amp, has quite a few variations. They have revised the insides several times while the outside remains largely the same.

One tip I would like to stress is to use the proper size screwdriver when disassembling these amps. While the vast majority of amplifiers can be taken apart with #2 Philips size, these amp use larger screw heads and I find the #3 Philips head is required for the main chassis screws. I often see these amps with chewed up screws from people using #2 screwdrivers. Another screw issue is that they are long and easily bent. Be mindful of the stress you may be placing on the remaining screws as you remove some of them. When taking apart logically, there should not be any stress to cause bending. One way to avoid stressing the screws is to put the amp on its side to disassemble. Whatever works for you.

JC-77 (2x10, 1980s?)

For my initial entry into my JC series notes, I had a peculiar problem with a JC-77 reverb.

The JC-77 is a model that seems to have only existed in the '80s. It is a 2x10 version, MN3101 chorus chip with a "CHORUS: MANUAL/OFF/FIXED" selector switch.

Reverb drive amplifier section from the JC-77

This particular unit had a reverb tank with all the wires broken inside, a common reverb tank problem. Fortunately, the tank worked after reconnecting the wires... except for a very odd problem of howling when I played a low "G" (about 92Hz) on an electric guitar while I was testing. After playing with the mechanical setup for a moment, and the fact that it would howl at any volume setting, I started to suspect something electronic was at work. Sure enough, the send signal was terribly distorted at and below 100Hz. I scoped C5, a local filter cap for the reverb drive, and there was plenty of ripple coming through on the low notes. The ripple was modulating the low notes and creating the "howling" effect. Jumping in a fresh 47µF cap across C5 removed the howl.

JC-120

The original Jazz Chorus, the 2x12 JC120, debuted in 1975 and is notable for its early use of the bucket bridge delay technology for its chorus effect. The chip used, the MN3002, had just been released, and the JC120 is probably the first amp produced with a built-in solid state chorus effect. Roland followed this up in 1976 by creating the Boss Chorus Ensemble CE-1 as a standalone MN3002 chorus.