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The ultimate JC-120 thread

Started by teemuk, July 26, 2011, 01:33:27 PM

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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)


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?


Quote from: 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?

It depends what is wrong.  It could be as simple as a loose wire or bad solder.  Or it could be something more major.
Life is what you make it.
Still rockin' the Dean Markley K-20X


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.


The really old one's have metal type corner pieces on the cabinet.

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.
"A person who never made a mistake never tried anything new." -Albert Einstein


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.


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?


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.


Ok, thanks for the info!


Quote from: 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.

Samples of rear panels differences.

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.