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Solid State Amplifiers => Amplifier Discussion => Topic started by: teemuk on July 26, 2011, 01:33:27 PM

Title: The ultimate JC-120 thread
Post by: teemuk on July 26, 2011, 01:33:27 PM
...or hopefully something becoming one.

As some of you might know "the JC-120" amplifier is actually a collection of very different kinds of product revisions, all revolving around a design concept of basically featuring

- dual channels, clean + effect
- 2 x 60W stereo output
- stereo chorus or tremolo effect
- reverb and distortion
- uniform cosmetic styling

Everything else but these basic features have been evolving quite a lot since the amplifier was introduced in 1975. That also includes how these basic features were accomplished circuit wise. Since not much is written about this, but the topic seems to come around now and then, I hope we could combine a thread with some info of the different circuit revisions and their features, what S/N's cover those revisions, and hopefully, some user input of how those different revisions seem to have performed...

Title: Re: The ultimate JC-120 thread
Post by: teemuk on July 26, 2011, 01:34:52 PM
Here's a small and simplified list of differences I've found so far:

3rd Edition (1979) - applicable to
JC-120  S/N  380100 - 415099

- Gain stages are discrete, combinations of BJTs and FETs
- Distortion circuit is an overdriven FET+BJT stage permanently in circuit
- Features a FET-differential -based "pre-limiter" preventing power amp hard clipping
- Discrete BBD oscillator driver
- Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel.
- Line out tap at speaker output

JC-120   S/N   420600 – 471649
JC-160   S/N   440100 – 470549

- Pretty much similar to revision described above
- Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel. Also includes a trimmer for chorus adjust in rear panel.


JC-120   S/N   481650 – 502499
JC-160   S/N   480550 – 500849

- Gain stages are TA-7122AP, low noise integrated preamp ICs
- Distortion circuit is a switched on cascade of two BJT+FET based gain stages
- FET-differential pre-limiter preventing power amp hard clipping
- Current feedback power amplifier (meaning, the output frequency response is not linear but instead varies in interaction to speaker's impedance, similarly to many tube amps)
- Discrete BBD oscillator driver
- Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel. Trimmer for chorus adjust in rear panel.
- Line out tap at speaker output
- LDR-based muter for the chorus/tremolo

JC-120   S/N   512500 – 779499
JC-160   S/N   510850 – 282918

- Pretty much similar to one described above

4th Edition (1979) – applicable to
JC-120                 S/N   789500 – 248099 or 950100 – 283149 (USA versions)
JC-120A and R&P    S/N   810600 - 001099
JC-160                 S/N   782921 - 244509

- Gain stages are discrete, FET -based
- Pre-limiter circuit omitted, will be omitted from following versions
- Distortion circuit is an overdriven FET stage switched into the circuit
- “Main In” external inputs
- Current feedback used in the power amplifier
- BBD now with driver, pre driver made out of digital logic and discrete circuits
- Separate LF oscillators for vibrato and tremolo. Normal vibrato controls in front panel, chorus adjust trimmer in rear panel.
- Line out tap at speaker output


5th Edition (ca. 1982) – applicable to
JC-120   S/N   258100 and up or 293150 and up (USA versions)
JC-160   S/N   254510 and up

- Gain stages are discrete, FETs or combinations of FETs and BJTs
- Current feedback from PA is omitted and no longer featured in following versions
- Includes a bright switch
- Distortion circuit is a switched in FET gain stage with diode clippers in feedback loop
- “Main In” external inputs
- BBD + BBD driver setup
- LF oscillators for chorus and tremolo now combined to a single circuit. Chorus mode has fixed settings for vibrato speed & depth
- Line out tap at speaker output


JC-120H - the head unit (1984) – applicable to
100/117V      70126811
220/240V      70126814

- Gain stages are OpAmp –based
- “Single channel”
- Distortion circuit is a switched on combination of two parallel shunting diode clippers + switched in gain control in the preceding stage
- Revised reverb circuit with BJT drivers and current feedback
- “Hi-Treble” potentiometer control
- “Sub In”, external mono input
- “Normal/Mixed” and “Effect” line outs preceding power amp
- Four speaker out jacks (this is a head)
- BBD and LFO setup similar to 5th edition of the circuit


JC-120 UT / JT (2nd Edition - these are the newest JC-120 amps)

- Gain stages are discrete, FETs or combinations of FETs and BJTs. FX loop circuitry also uses OpAmps
- Distortion circuit is a switched in FET gain stage with diode clippers in feedback loop, very similar to circuit of "5th ed". but some component values are different
- Modern FX loop with loop level selector and all other goodies
- BBD and LFO setup similar to 5th edition of the circuit
- Revised reverb circuit, similar to JC-120H model
- Output –tapped “Line out” feature omitted
Title: Re: The ultimate JC-120 thread
Post by: teemuk on July 26, 2011, 02:03:18 PM
Some pics that display cosmetic differences of JC-120's of different pedigrees:

From top to bottom

- Very old version of the amp: Notice the chrome corners and the overall green-ish colour scheme.

- Somewhat newer version (approximately 1982 and onward): Notice the addition of "bright switch" feature, plastic corners and a grey-ish colour scheme. Jacks are now insulated type with plastic cover (instead of old plain metal jacks) and the potentiometers knobs are different too (they have varied slightly throughout the series' history). The Roland logo in the grille is also bigger.

- Even newer one: Bright-switches are now push-button -type, the Vib/Off/Chorus -toggle has been converted to a rotary selector -type. Speakers are different and no longer have metal dustcaps. At some point of the revision history the metal "lever" -type rocker mains switch converts to modern plastic rocker switch.

(http://img687.imageshack.us/img687/120/3jcs.jpg)

Samples of rear panels differences.
(http://s13.postimage.org/rzxxyeiut/JC_120_rear_panels_00.jpg)

From top to bottom:
1. Very early version. Note the lack of rear cabinet panel and that the labels were silkscreened to the painted chassis. Controls from left to right: Mains/polarity switch, convenience AC outlet, Chorus/Vib F/S jack, Reverb F/S jack, Distortion F/S jack, two EXT speaker out jacks, line out jack (mono).

2. Newer version. A rear panel with label stickers is added (this was probably a cheaper solution than silkscreening the chassis). The control/jack arrangement is the same except that a fuse holder replaces the mains switch and that the convenience AC outlet is removed. A briefly earlier version (ca. 1976 and not depicted) still had both fuse holder and the convenience outlet. A Chorus trimmer potentiometer is also added to the far right.

3. Late 1970’s version. Controls from left to right: Mains fuse holder, chorus adjustment trimmer, two EXT speaker output jacks, line output jack, chorus, reverb and distortion footswitch jacks, two mains in jacks.

4. Very late 1970’s / early 1980’s version: Chorus trimmer is omitted, otherwise the panel arrangement is identical.

5. Bottom: UT/JT modern JC-120’s: Two line out jacks, FX loop stereo return jacks, FX loop send jack and loop level switches, three footswitch jacks for chorus/vib, reverb and distortion.

The head model, JC-120H
(http://s17.postimage.org/erizik8bh/JC_120_H.jpg)
This is actually the model of which's schematics are most commonly referenced when people request for a JC-120 schematic. An important point is that the schematic (often passed on with nickname "1984") doesn't match ANY of the JC-120 combo designs by a long shot. First and foremost, the gain stages are based in opamps only, unlike in JC-120 combos. Unique features incommon to combo versions also include:

- The amp is single-channel design
- "Hi-Treble" control, which is basically a bright switch replaced by a potentiometer
- Effect line out, which is basically a mono line output for the wet chorus effect only
- A more accurate labelling of the revised chorus effect: Manual/Off/Fixed
- Sub In jack, a line in jack that basically bypasses most of the preamp, excluding chorus
- Line out jack, a line out that mixes both channels (wet and dry) to a single mono output. The difference to certain combos that had line out jacks is the take off point. In combos the take off point is at the speaker output, in the head version the takeoff poinr is before power amp.

Rear panel arrangement from left to right: Fuse holder, two speaker out jacks (left), two speaker out jacks (right), Sub In jack, three footswitch jacks for chorus, reverb and distortion, effect line out jack, normal/mixed line out jack.
Title: Re: The ultimate JC-120 thread
Post by: teemuk on July 26, 2011, 02:21:49 PM
Another important difference are the speakers. I'm not too familiar with this topic, barely I know that speakers used in Roland JC-120's (and JC-series amps in general) have also changed throughout the years, a variable also having quite big impact on the overall tone of the amp.

The early amps feature Roland Corporation "Musical Speaker" drivers, made in Japan. These have a distinctive metal dustcap and either a black&blue or black&white label.

R&P speakers were developed in collaboration with Pioneer in the late 1970's, they basically appear in circa 1979 amp revisions. These drivers featured some fancy innovation but I'm not entirely sure what it was; basically they seemed to have a wider and HiFi'er response than generic Roland speakers of the time and the new technology was marketed keenly. The Roland amplifiers featuring these speakers were marketed as a unique top notch line, either indicated by "A" or "R&P" in the model number. Some amps even featured R&P insignia but I'm not sure if JC-series ever had that. These speakers again had metal dustcaps.

In the early 1980's Roland started to manufacture some of the speakers in California, USA. These Roland AD "Audio Development" Heavy-Duty Transducers seem to be featured in the newer JC-120's. These speakers also seem to lack the metal dust cap.
Title: Re: The ultimate JC-120 thread
Post by: J M Fahey on July 27, 2011, 10:22:38 AM
Impressive.
Thanks a lot. :tu: :tu: :tu:
Title: Re: The ultimate JC-120 thread
Post by: joecool85 on July 27, 2011, 04:35:50 PM
I agree, excellent information as always, Teemu!   :tu:
Title: Re: The ultimate JC-120 thread
Post by: Kaz Kylheku on July 29, 2011, 12:04:25 AM
Interesting stuff!

I started looking for some schematics because of this.

The vintage ones from the 1970's are harder to find. Here is a time saver for ya: google for the file name "Roland JC-120.zip".

;)


Title: Re: The ultimate JC-120 thread
Post by: J M Fahey on July 31, 2011, 11:02:36 AM
Hi kaz, thanks a lot for your posting/linking.
Very interesting.  :tu:

Hi Teemu: thanks for pointing to the early soft limiter.
It's very similar to somthing I had used as part of a tube emulation circuit, but this one has its own distinctive flavor.
It *is* a long tailed phase inverter.
Mine was the full monty , including the "magic" 82K and 100K "plate resistors" with both "plate signals" (which are balanced or at least symmetrical and out of phase) sampled and summed by an instrumentation type  differential input Op Amp circuit.
Somewhat complex for what I usually do, remember I'm a Minimalist at heart, but I always start "following the rules" and only after I get what I want, start chopping off the deadwood.
Of course, that "differential summing" is performed by the power tubes and OT in a classic tube amp.
It did sound good, adding some "tube flavor", distorting quite different to any classic SS circuit, showing that the base of a sound does not lie in "glass vs. sand" or "solid vs. vacuum" magic but on what the circuit *actually* does.
Later I found that I retained 90% of the sound by using the signal from just one "plate", much simplifying the circuit.
After that, I used a volume pot, which was equivalent to a PPIMV , which, as we all know, i9s not the perfect solution but noticeably better than an "end of the preamp" MV, which easily gets buzzy.
I see Roland took the same approach, but adding (very clever):
1) they do not use the Master Volume but with the preset "starve" the 2nd FET so It *just* provides the necessary signal to drive the Power amp.
I'd *LOVE* to read the service notes explaining its adjustment.
Specially if they show scoped waveforms.
2) the FET does not provide a "fixed" clipping level as a couple diodes or similar but somewhat tracks the +/- power rails, it's fed unregulated voltage derived from them.
I guess it must provide some "sag" at high power levels (when said rails voltage falls under load).
Not much, I guess, but anyway a step in the right direction.
They provide a simpler and much stronger similar effect in the Blues Cube.
The Sag switch kills the voltage rail feeding the last clipping FET.
Anyway, and as always, hats off to Roland (design geniuses) and to you for providing such useful info.  :tu:
Title: Re: The ultimate JC-120 thread
Post by: teemuk on July 31, 2011, 03:30:06 PM
Quote
I'd *LOVE* to read the service notes explaining its adjustment.
Specially if they show scoped waveforms.
There's not much to see, they tell you to adjust the trimmer to achieve results shown in few hand-drawn waveforms.
This [see attachment] is actually from the service manual of SB-100, a Jazz Chorus series look-alike but a bass amp. Pretty much all the Roland amps featured that FET circuit during that particular era.
Title: Re: The ultimate JC-120 thread
Post by: J M Fahey on August 01, 2011, 01:10:57 AM
Thanks a lot.
Even though not explained, those waveforms tell me a lot.
I guess Vr5 still is the trimmer on the 2nd FET drain, of course.
Can you post SB100's schematic (or at least the limiter and power amp) so I am sure about it?
Thanks again.
Title: Re: The ultimate JC-120 thread
Post by: teemuk on August 01, 2011, 03:39:41 AM
VR5 is indeed the trimmer for the FET circuit.

Sorry, but the complete service manual is too large to attach. You can download the entire file from eletrotanya.com, though.

http://elektrotanya.com/roland_sb-100.pdf/download.html
Title: Re: The ultimate JC-120 thread
Post by: J M Fahey on August 01, 2011, 07:11:52 AM
Thanks a lot, it was very helpful. :tu:
I don't understand why they dropped it, surely it's not a cost decision, we are talking less than a dollar and sure it improved the final product.
Maybe they realized the average JC120 user *never ever* uses it at full power,  being typically a Jazz/session/Tango/etc. player, even a Rocker but searching a very clean sound, who has "something else" for his distortions.
I live in Buenos Aires and have seen Tango guitar players use them; in small shows they even plug a microphone in the spare channel and use it as a small PA system.
Doubt they use it over 20/30W if anything.
Title: Re: The ultimate JC-120 thread
Post by: JHow on August 08, 2011, 07:29:45 PM
Personally, I was always fascinated by the "rivets".  I wasn't sure if it was supposed to look like fancy upholstery or kind of medieval theme, but distinctive, nonetheless.
Title: Re: The ultimate JC-120 thread
Post by: teemuk on August 15, 2011, 08:40:52 AM
I like that detail too, and at at least to me it sort of gives an impression of "function-defines-form" -type of design; an "armor-clad" amp that's ready to take some serious bumbs on the road like it's nothing. Personally, I have a thing for amps that look more or less like sturdy roadcases.
Title: Re: The ultimate JC-120 thread
Post by: armstrom on August 15, 2011, 08:04:53 PM
What makes the chorus and vibrato effects stereo? (other than the obvious, that they have two channels!) Are the left and right channels out of phase when using the chorus effect? If so, how many degrees out of phase? Do both channels change pitch in unison when using the vibrato effect or does the effect use the two channels in a different way somehow? I understand the way the effects are implemented might vary greatly between the different versions of the amp.
-Matt
Title: Re: The ultimate JC-120 thread
Post by: petey twofinger on August 15, 2011, 10:36:36 PM
i have one i got in the mid 90's , the reverb stopped working a few years back .  i really had a tough time getting a decent distortion tone using pedals . its interesting in a lot of the boss stuff they always show that amp in the instruction manuals . i have the boss gt6 and it has an output setting for the jc120, although i have never tried that unit on that amp . adrian belew and rob fetters used those amps for a long time , they both got some great tones out of em , you name it ! i saw them live many times and they always really pushed their amps . i seem to remember rob having a proco rat . something about the built in distortion on it i didnt care for , with a usa strat . not sure but i think kurt cobain used this amp a  little , and i know metalica did maybe still does for clean tones (why do i know that?) . i think andy sommers was a fan too . adrian belew used the vibrato/chorus to great effect while feeding back with that amp . they really do have a very distinctive tone . they are pretty solid too , i know mine took a serious beating , i did loose a coupla those rivot thingies , and then the reverb went out , i am guessing it got dropped . going to have to look into that , i am sure it may be an easy repair .
 i used a roland gr-50 guitar synth while i was playing bass in a few cover bands . i would use fretless / acoustic bass patches , or octave down fat bass going thru an eq pedal with no mids and highs to a 250 ys combo eqed low . then the straight guitar sound went thru some reverb or a backwards box and then to the 120 . it was quite a sound ,but i would always switch off to a real bass for most of the night (tracking) . quite a few people would be scratching their heads , "where that bass coming from" ? well , a few anyway .

not sure how the stereo works on it , but i do seem to remember when it was engaged , a distinct sensation of movement , either way they were very deep effects , even on the lower settings . i would just love to know if its possible to get a nice distortion or hard rock tone with that amp . i mean i know its possible , but i never even got close to it .  :(

srry to ramble ! i really do dig that amp . i saw this roland digital amp that looked just like a 120 , it had the 13 pin connector for the gk2 hex pu , it had some cosm and stuff , really cool lookin , used at GC , kinda cheep too , like a vg-8 / jc120 hybrid .  :o
Title: Re: The ultimate JC-120 thread
Post by: phatt on August 16, 2011, 09:10:08 AM
What makes the chorus and vibrato effects stereo? (other than the obvious, that they have two channels!) Are the left and right channels out of phase when using the chorus effect? If so, how many degrees out of phase? Do both channels change pitch in unison when using the vibrato effect or does the effect use the two channels in a different way somehow? I understand the way the effects are implemented might vary greatly between the different versions of the amp.
-Matt

Hi Matt,
           
Tremolo and Vibrato; 
On most Amps these two words often mean the same thing, Vibrato where the *Output Level* is Vibrated creating a vibe effect.

True Vibrato is not *Pitch related* it is just the level output is turned up and down via a low frequency oscillator circuit (LFO)


True Tremolo is just the fast Chorus, Pitch shifting effect,,
So yeah,, it can get confusing. ???


Chorus effect is often just the dry signal through one speaker and the delayed chorus through the other. Some Amps may have true stereo chorus where one side goes up in pitch while the other side goes down while also mixing the original signal through both sides,, You are more likely to find that in later digital effects units but a couple of really old Valve rigs did have some kind of true tremolo effect.
Sorry names escape me tonight.

Teemu probably knows more,  8)

Phil.
Title: Re: The ultimate JC-120 thread
Post by: armstrom on August 16, 2011, 09:35:18 AM
           
Tremolo and Vibrato; 
On most Amps these two words often mean the same thing, Vibrato where the *Output Level* is Vibrated creating a vibe effect.

True Vibrato is not *Pitch related* it is just the level output is turned up and down via a low frequency oscillator circuit (LFO)

True Tremolo is just the fast Chorus, Pitch shifting effect,,
So yeah,, it can get confusing. ???
Thanks for the reply!

I think you have the terms backwards. Vibrato is periodic variation in frequency, tremolo is periodic variation in amplitude.
http://en.wikipedia.org/wiki/Vibrato

I believe guitar and amp manufacturers have created much of the confusion between these two terms. A "tremolo bar" (or whammy bar, if you must) is really a vibrato bar since it changes the frequency of the signal. The "Vibrato" effect found on most Fender amps is really a tremolo effect since it simply varies the output volume.

Now, the question remains... Is the vibrato on the JC120 real vibrato, or did they use it in the Fender sense to mean tremolo (I've never heard a good clip of the vibrato effect being used on a JC120... if I could find one it would be instantly obvious which effect they really use). As for the chorus, I'm aware of the different ways to achieve a stereo chorus, my question was which one (or ones, if it varied between revisions) were used by Roland in the JC series of amps.

-Matt
Title: Re: The ultimate JC-120 thread
Post by: teemuk on August 16, 2011, 09:54:26 AM
The effect's (whether its chorus or vibrato) mechanics itself lie in delaying the signal by very short time periods, furthermore the lenght of these time periods constantly being modulated by a low-frequency oscillator. This "wet" delayed signal is then mixed in with the untreated "dry" signal and the phase shifts will do their thing.

I guess that makes the effect a true vibrato since it's not based on plain amplitude modulation.

In JC-120's (and as far as I know in every JC-series stereo amp) the effect (whether it's the vibrato or the chorus) created that way is only added to one of the channels, the other channel  always remains totally uneffected. I guess you could call that "stereo", seems to be at least what Roland is doing. I guess you can also call it "true stereo" when the effect is actually added on both channels. Yeh, kinda confusing...

The way how the effect is mixed in that "stereo" configuration does not (in my experience) vary from one JC-120 to another but the circuitry driving the bucket brigade delay chip (the heart of the time-based effects like chorus and vibrato) does: In the early JC-120 versions there were two separate low frequency oscillators to modulate the variable delay circuit; one generating a sawtooth wave, another one generating a sine wave. Sine wave was switched in for the "vibrato" effect, sawtooth for the "chorus", giving each a unique tone and vibe.

In about the 5th edition of that amp Roland, however, simply started to use a single sawtooth oscillator for both of those effects and just made the oscillator's frequency ("Speed") and its output signal's magnitude "fixed" for one effect, user adjustable for the other. (They had been this way all along but the important thing is that they started to use just a single LFO waveshape for both effects). Now both modes actually generated the very same output, though one of them was user adjustable. You could also dial the "chorus" mode to produce the very same effect as "vibrato" mode.

To reflect this, Roland changed labelling in JC-120H (and some other JC-series amps) to simply state: "Chorus (Manual-Off-Fixed)" instead of the older "Vib-Off-Chorus", which was getting to be somewhat misleading. However, I think people actually thought Roland had removed the "Vibrato" feature (and some could with good reason argue that they actually did) so the older style of labelling was kept unchanged in all the JC-120 combos.
Title: Re: The ultimate JC-120 thread
Post by: joecool85 on August 16, 2011, 10:29:20 AM
       
I think you have the terms backwards. Vibrato is periodic variation in frequency, tremolo is periodic variation in amplitude.

I agree, I played cello for 8 years and vibrato is when you rotate/wiggle your finger on the fingerboard and it moves the pitch quickly up and down ever so slightly, almost a "warble" sound.
Title: Re: The ultimate JC-120 thread
Post by: armstrom on August 16, 2011, 01:16:08 PM
Ok, so there's nothing special going on.. If you were to put the two speakers from a JC120 in two different rooms you would only hear the effect in one of the speakers and not the other... interesting. Doesn't sound like something I want to emulate. I think I'll just stick to my plan of using a mono chorus effect that later gets split to two LM3886 power amps driving individual speakers. It's easier that way :)
Title: Re: The ultimate JC-120 thread
Post by: Kaz Kylheku on August 17, 2011, 12:55:26 AM
Personally, I was always fascinated by the "rivets".  I wasn't sure if it was supposed to look like fancy upholstery or kind of medieval theme, but distinctive, nonetheless.

Their job was to be ... riveting. As you can see, it worked.

Title: Re: The ultimate JC-120 thread
Post by: phatt on August 19, 2011, 06:41:20 AM
           
Tremolo and Vibrato; 
On most Amps these two words often mean the same thing, Vibrato where the *Output Level* is Vibrated creating a vibe effect.

True Vibrato is not *Pitch related* it is just the level output is turned up and down via a low frequency oscillator circuit (LFO)

True Tremolo is just the fast Chorus, Pitch shifting effect,,
So yeah,, it can get confusing. ???
Thanks for the reply!

I think you have the terms backwards. Vibrato is periodic variation in frequency, tremolo is periodic variation in amplitude.
http://en.wikipedia.org/wiki/Vibrato

I believe guitar and amp manufacturers have created much of the confusion between these two terms. A "tremolo bar" (or whammy bar, if you must) is really a vibrato bar since it changes the frequency of the signal. The "Vibrato" effect found on most Fender amps is really a tremolo effect since it simply varies the output volume.

Now, the question remains... Is the vibrato on the JC120 real vibrato, or did they use it in the Fender sense to mean tremolo (I've never heard a good clip of the vibrato effect being used on a JC120... if I could find one it would be instantly obvious which effect they really use). As for the chorus, I'm aware of the different ways to achieve a stereo chorus, my question was which one (or ones, if it varied between revisions) were used by Roland in the JC series of amps.

-Matt

Thanks for the correction Matt. :-[

Trust me to get it back to front.
Must make a note to myself to check my brain is on the right way before I type :duh
Phil.
Title: Re: The ultimate JC-120 thread
Post by: phatt on August 19, 2011, 07:00:01 AM
Ok, so there's nothing special going on.. If you were to put the two speakers from a JC120 in two different rooms you would only hear the effect in one of the speakers and not the other... interesting. Doesn't sound like something I want to emulate. I think I'll just stick to my plan of using a mono chorus effect that later gets split to two LM3886 power amps driving individual speakers. It's easier that way :)

Yep I'd agree with that, no need to make things needlessly complex for little benefit.

My Nobels Sound Studio 1 (SST1, a little red headphone box of tricks) had the chorus ONLY on one channel and dry on the other.
Turn off the *dry signal* and all you got was an up/down warble that was not very musical.
IME, Live stereo efx is overkill and hard to define in a live mix and you may end up worse off but in recording it can be very useful.
Phil.
Title: Re: The ultimate JC-120 thread
Post by: madstereoman on August 09, 2012, 09:06:43 AM
Hi:
Does anyone know how to remove the front grille from the JC-120?
Thanks
Title: Re: The ultimate JC-120 thread
Post by: vin97 on November 07, 2016, 11:11:22 AM
Is there any way of knowing whether it is the old Chorus/Vibrato circuit or the new one apart from getting a guitar and testing it (even then I'm not really sure how you would know)?
There are a lot of '81 and '82 models for sale online that may or may not have the old circuit and I don't know for sure.
Title: Re: The ultimate JC-120 thread
Post by: J M Fahey on November 08, 2016, 11:42:25 AM
Only by opening it and looking, way too many different versions out thyere.

MAYBE a very dedicated collector knows just by looking at the extension cord or smelling it, but us mere mortals need to look and compare with known schematics.
Title: Re: The ultimate JC-120 thread
Post by: vin97 on November 08, 2016, 08:03:32 PM
Ah damn.
That makes getting one of the old models online very difficult.
Title: Re: The ultimate JC-120 thread
Post by: J M Fahey on November 11, 2016, 08:48:37 AM
Maybe Teemuk can chime in, he knows *a lot*  about SS amps (you might say he wrote the book ;) ), including JC120 , so maybe he can tell you to look at knob type or exact name on the label or, preferrably, a range of serial numbers.
Title: Re: The ultimate JC-120 thread
Post by: vin97 on November 12, 2016, 01:36:28 PM
Ok, slightly different question: What is the maximum delay time for the chorus and could it be increased by simply adding more memory?
Title: Re: The ultimate JC-120 thread
Post by: phatt on November 13, 2016, 08:12:55 AM
No memory slots for those chips,, :lmao: you need another chip,, which may not work with that circuit. :'(
Easier to use a chorus pedal. 8)
Phil.
Title: Re: The ultimate JC-120 thread
Post by: vin97 on January 14, 2017, 12:32:15 PM
I've spotted an JC-120 that may be one of the older models but the seller doesn't have the label with the serial anymore.
He says that the vibrato effect doesn't work but the chorus does.
How difficult/expensive is it to repair the vibrato?
Title: Re: The ultimate JC-120 thread
Post by: joecool85 on January 14, 2017, 07:23:02 PM
I've spotted an JC-120 that may be one of the older models but the seller doesn't have the label with the serial anymore.
He says that the vibrato effect doesn't work but the chorus does.
How difficult/expensive is it to repair the vibrato?

It depends what is wrong.  It could be as simple as a loose wire or bad solder.  Or it could be something more major.
Title: Re: The ultimate JC-120 thread
Post by: vin97 on January 16, 2017, 06:50:22 PM
Ok but I can assume that it is one of the old versions since the chorus works but the vibrato doesn't?

Apart from that it has a bright switch and plastic corners.
Title: Re: The ultimate JC-120 thread
Post by: DrGonz78 on January 17, 2017, 03:33:16 PM
The really old one's have metal type corner pieces on the cabinet.
(https://i.ytimg.com/vi/3K3UxHt5avQ/maxresdefault.jpg)

When operating in chorus mode the depth and speed pots have no effect. In vibrato mode then the depth and speed are used. The first thing I would try in this situation would be cleaning those pots just to rule them out as a possible culprits. Not saying that is what is wrong, but cleaning those pots can't hurt. After that point troubleshooting the circuit will get way more involved.
Title: Re: The ultimate JC-120 thread
Post by: smackoj on January 23, 2017, 03:59:18 PM
you should be able to date this amp with the numbers/letter code on the power transformer and also stamped on the back of the pots. One of the numbers (don't recall exactly which but sometimes the first number right of the dash mark -) will give you the last number of the year. You have to use other methods e.g. material of corners to figure out which decade. example  A35-6973 would be either 1976 or '86 or 96' or '06 and use other indicators to decide which of these decades. If you can read the speaker numbers they can also tell the date of mfg. of the speakers which is usually within a yr or two of the mfg. date of the amp.
Title: Re: The ultimate JC-120 thread
Post by: vin97 on June 07, 2017, 06:16:51 PM
Finally got hold of an old ('76) JC-120 for a reasonable price, which will arrive in a few days.
The seller said the reverb doesn't work. Could I replace the original one by a longer Accutronics, like the 9DB2C1B?
Also, where can I find schematics for this early model?
Title: Re: The ultimate JC-120 thread
Post by: phatt on June 08, 2017, 09:25:31 AM
Yes you can swap tanks if the Drive and PU transducers have same specs.
Be aware a tank swap won't magically fix an average or weak reverb circuit.
The tank must be matched to the circuit  and the circuit itself is often the weak link NOT the tank.
Phil.
Title: Re: The ultimate JC-120 thread
Post by: vin97 on June 09, 2017, 11:33:45 AM
Ok, thanks for the info!
Title: Re: The ultimate JC-120 thread
Post by: JB074 on December 17, 2017, 01:05:05 AM
Some pics that display cosmetic differences of JC-120's of different pedigrees:

From top to bottom

- Very old version of the amp: Notice the chrome corners and the overall green-ish colour scheme.

- Somewhat newer version (approximately 1982 and onward): Notice the addition of "bright switch" feature, plastic corners and a grey-ish colour scheme. Jacks are now insulated type with plastic cover (instead of old plain metal jacks) and the potentiometers knobs are different too (they have varied slightly throughout the series' history). The Roland logo in the grille is also bigger.

- Even newer one: Bright-switches are now push-button -type, the Vib/Off/Chorus -toggle has been converted to a rotary selector -type. Speakers are different and no longer have metal dustcaps. At some point of the revision history the metal "lever" -type rocker mains switch converts to modern plastic rocker switch.

(http://img687.imageshack.us/img687/120/3jcs.jpg)

Samples of rear panels differences.
(http://s13.postimage.org/rzxxyeiut/JC_120_rear_panels_00.jpg)

From top to bottom:
1. Very early version. Note the lack of rear cabinet panel and that the labels were silkscreened to the painted chassis. Controls from left to right: Mains/polarity switch, convenience AC outlet, Chorus/Vib F/S jack, Reverb F/S jack, Distortion F/S jack, two EXT speaker out jacks, line out jack (mono).

2. Newer version. A rear panel with label stickers is added (this was probably a cheaper solution than silkscreening the chassis). The control/jack arrangement is the same except that a fuse holder replaces the mains switch and that the convenience AC outlet is removed. A briefly earlier version (ca. 1976 and not depicted) still had both fuse holder and the convenience outlet. A Chorus trimmer potentiometer is also added to the far right.

3. Late 1970’s version. Controls from left to right: Mains fuse holder, chorus adjustment trimmer, two EXT speaker output jacks, line output jack, chorus, reverb and distortion footswitch jacks, two mains in jacks.



Here are some images I took of my "early-ish" JC-120. Despite everyone's best attempt to describe, This is obviously an early cabinet, with a pre-bright switch circuit. Inside however, is a later TA7122 driven preamp, and an 052-190A effects board. The two channel boards are disparate. I am aware someone has been inside before. - I came to this thread via my newbie post, just adding these pics FYI. Important for anyone troubleshooting these to be aware of the board revisions and serial number.