Jc-120 Schematic

[Carlito Austin]

whatchathink of it? I have 2 Vintage Rolands from the 80's I love. The cleans and the Chorus is amazing. Always wanted a Jazz Chorus. I have a 50-watt Roland Dac-50XD with built-in Chorus, Flanger, Short delay, long delay, etc. - it has 4 transducers (8" speakers) and just RIPS the finest clean I could ever imagine. A perfect amp for my Multi-effects pedals.

I also have a really early JC120, with the big rocker switches for the FX. Mine has a nasty hum but also has had the ground prong removed from the power cord. I've been planning to put in a new grounded cord, hoping that fixes the noise. That's the only complaint I have about the amp. It's a pretty spectacular piece of equipment!

I just scored one at a local pawn shop for $125 out the door. Has hum from the ground prong being broken off, one original speaker was replaced with a Pyle, and the chorus switch, reverb pot & distortion pot have no effect. Should all be easy enough to sort, thankfully the inside of the chassis looks like it's never seen the light of day! Guts definitely look old, it's pre-FX loop, made in Japan, etc.

Ah! Thanks so much for all the info & background - this is the kind of stuff I want to learn. There is SO much info on the net about most of the amps I own(ed) and I was surprised to get home after scoring this one and find relatively little. A site with gut-shots of the various versions, etc. would be cool. Once I have a chance, I'll get good pix. of mine and post them.

'nother update on this. Still no luck in diagnosing the issue with my chorus controls. I've spoken with Roland USA and they don't have any service manuals or schematics for the JC-120 earlier than the one dated '79. My amp appears to be earlier than this as the chorus circuit is different, but it still uses a single MN3002 which was replaced by a MN3004 in a different configuration on the later amps. Bummer.

's for models with ser. no 481650 - 502499 (JC-120) and 480550 -

500849 (JC-160) ...but that one doesn't have the "main in" inputs and

the ser. no. is too small so, I guess we can ignore it.However, there are about eight or nine different versions of that amp

(maybe even more) - some of them are WAY different from another. The

version you have should be indicated in the model number as: JC-120-

xxx, where xxx is a cryptic letter code Roland never bothered to

explain.So, I hope you realize that due to these circumstances it is rather

impossible to say anything specific. Anyway...All JC-120 versions have two power amplifiers: One for each speaker.

In schematic #7 the "Main In inputs" are indeed MONO inputs feeding

the two power amplifiers separately. Each input will also disable its

corresponding preamp channel when a jack is inserted. So, It's

basically a left-right channel setup - if you want to consider it that

way.

Ric 370/12 into the front. Xenyx 1202FX into the MAIN IN's

to mix in laptop-supplied tracks from a Strat and a cheap (really

cheap - found in a dumpster, no joke) Buzzo bass. Big fun

pretending to be the Byrds. :^)Niles Mills

niles...@gmail.com

After the great success of the bulky Chorus Ensemble CE-1 issued in 1976, which was based on the Roland Jazz Chorus JC-120 Amplifier integrated chorus effect, Boss decided to revise and release a similar sounding effect but in a compact size modifying the circuitry. The CE-2 builds on the legacy of the CE-1 with reduced features: mono output instead of stereo, no vibrato mode, no integrated power supply, no level and no intensity controls.

The CE-1 was designed more to be a multi-instrument effect, with a 50K low input impedance suitable for keyboards. The CE-2 has higher input impedance and boosted mid frequencies, both arrangements very suitable for electric guitars.

Roland stopped marketing the CE-2 in 1982, however still produced until 1990. There are basically 3 versions of the pedal:

The Chorus or Celeste effect was integrated into Hammond organs since the 1940s. Some early stand-alone stompboxes like Vibra-Chorus and Uni-Vibe by Shin-Ei were released in 1960s, creating some degree of phase shifting or chorus modulation.

As a matter of fact, the Roland/Boss CE-1 Chorus Ensemble taken from the JC-120 amp circuit is renowned as the Mother of Chorus, being the standard for the modern chorus sound. The CE-2 follows the same tonal response.

The CE-2 schematic can be broken down into some simpler blocks: JFET Bypass Switch, Power Supply, Input Buffer, Pre-De Emphasis Filters, Anti-Aliasing & Reconstruction Filters and Bucket Brigade Device Stage.

The circuit is designed around the MN3007 1024 stage BBD (IC3) and the MN3101 clock driver (IC4). The input and output circuits use the dual op-amp uPC4558C IC, while the LFO is implemented with the help of a TL022LP op-amp. Some additional active filtering is applied using Sallen-Key transistor stages.

The effect response is commanded using two controls:

In the original CE-2 pedal when the effect deactivated, the input signal will always pass through the Input Buffer and the Pre/De-Emphasis filters, despite symmetric, they might add some coloring to the guitar tone.

Considering the fact that the input impedance of the CE-2 is low, around 407KΩ, it might result in the loading of the guitar pickups. As a rule of thumb, the input impedance should be at least 1 MΩ.

For these reasons new CE-2 clones and old modded units, often include true bypass (Triple Pole Double Throw 3PDT switch) gives the cleanest possible path for the bypassed signal. It also carries other benefits like the number of components used in the pedal and using a light indicator and a power-on LED at the same time.

The JFET Bypass is made of one switch Q9 and a toggle block formed by Q6 and Q7 to manage the switch. This configuration can activate/deactivate the effect using a simple push button.

The JFET Bypass Switch enables two possible toggle configurations:

- The resistor voltage divider composed by R27, R28, and VR3), generates 4.5V to be used as a bias voltage in some stages. The resistors junction (+4.5V) is decoupled to ground with a large value electrolytic capacitor C18 47uF. The VR3 potentiometer is able to finely trim the 4.5 voltage: sometimes due to the loading of all stages, the 4.5 voltage might suffer some offset, the BBDs are sensitive to the bias, adjusting VR3 will bring maximum clean headroom.

The early Boss products like CE-2 with reduced power consumption used the ACA adapters which provide unregulated 12VDC (unregulated means that the output voltage drops when current consumption exceeds the maximum capacity, which is 250mA). This pedal was originally designed for an external 12V ACA supply adapter.

The series resistor R53 and diode D5 between the minus input on the power jack and ground are used to reduce the internal voltage supply.

With new products with higher power consumption, the PSA adapter was introduced which provides regulated 9VDC (regulated means that provide a constant voltage at any current under the max. capacity 200mA). This kind of adapter is the most common and used in all pedals nowadays.

The Daisy chain Trick: Powering the old CE-2 pedals designed for ACA 12VDC input with either a newer 9V ACA or PSA adapter will not work properly. The voltage drop over the resistor and diode will under-power the pedal making the power-on LED glow faintly. One easy solution is to use a daisy chain cable together with another standard pedal. The link between the two pedals will short the resistor-diode circuit and the pedal will receive full power.

The task of the Input Stage Buffer is to create a high input impedance so as to preserve signal integrity and avoid high-frequency signal loss. It is implemented with a plain Emitter Follower with unity voltage gain (GV=1):

The capacitor C1 forms a high-pass filter filtering the low frequency humming in conjunction with the input resistors R1, R2 and the transistor stage input impedance. It also separates the guitar from any pedal DC potential, protecting the pickups in case of circuit failure.

The analysis of this parameter is similar to the Tube Screamer input stage, if you want to follow all the mathematics check the Tube Screamer Input Impedance Calculation, otherwise just stick to the general formula:

The assumption of using these filters is that applying treble emphasis (pre-emphasis) and a corresponding identical treble reduction (de-emphasis) to the dry signal, the output signal will end up with the same original tonal balance. But applying the same filtering to the delay signal, the output signal will reduce any hiss acquired via the delay path.

For this purpose, two 1st order Shelving Filters are used in the input and output stages, each of these filters will create a zero and a pole, with a rising/falling slope of 6dB/octave or 20dB/decade.

The Shelving architecture is simple first-order filter function characterized by a frequency response with two different gains: GainLOW and GainHIGH between frequencies much higher and much lower than the corner frequencies.

Note: In order to achieve clock noise attenuation properly, inverting shelving filters (pre & post) are mandatory. Blending non-inverting & inverting will give mismatched gain & frequencies. The nature of non-inverting topology provides no attenuation and minimum gain=1. Only inverting topology offers both gain & attenuation, and both low & high inverting shelving filters share exactly same resistors & capacitors in reverse order, yielding matched frequencies and desired opposite gains, and this is why they are used in the Boss CE-2.

If R1 were much smaller than R2, there would be a 20dB/decade roll-off between the two gains. If R2 is not so much less, then the slope never quite makes it to 20dB/decade before it flattens out to the high frequency gain.

Note: The ratio fHIGH/fLOW is equal to the ratio of GainHIGH/GainLOW. It is fixed, so if you want a lot of cut or boost in the shelf, then the frequencies where the shelf starts and ends will be further apart and vice versa.

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