JC-120 Rear Panel MAIN IN Inputs

[Niles Mills]

Greetings.

I've got an old JC-120 (S/N B980793), the kind with the toggle-switch
for the Vibrato/Chorus selector. Works great. On the back of the amp
are two "MAIN IN" female 1/4" phono-jack inputs. I've spent a few
days digging around the Internet and Usenet News for a schematic and
manual and the closest I can get are the manuals at Roland's website

(1) http://media.rolandus.com/manuals/JC-120_OM.pdf
(2) http://media.rolandus.com/manuals/JC-120E_OM.pdf
(3) http://media.rolandus.com/manuals/JC-120H_OM.pdf

and the following schematics

(4) http://www.blueguitar.org/new/schem/_ss/roland_jazz-chorus1.gif
(5) http://www.blueguitar.org/new/schem/_ss/roland_jazz-chorus2.gif
(6) http://www.freeinfosociety.com/electronics/schematics/audio/rolandjazzchorus.pdf
(7) http://www.freeinfosociety.com/electronics/schematics/audio/Roland-JC120-JC160.pdf

The schematic at (7) has all the same inputs/outputs as my JC-120, so
I'm guessing it's the right one. Ditto for the manual at (1).

The diagram in the manual (1) shows separate devices feeding the two
MAIN IN inputs, so I'm assuming they're not LEFT and RIGHT inputs.
The question I'm trying to answer is whether the MAIN IN inputs are
mono or stereo. IANAEE (I Am Not An Electrical Engineer), but the
MAIN IN layouts at sections D-20 and G-28 of the schematic (7) look
like they're mono inputs.

Can someone who knows more than me kindly take a look at the schematic
and tell me if I'm on the right track (or on crack)?

Thanks in advance.

Niles Mills
niles...@gmail.com

[teemuk...@gmail.com]

Schematics 4, 5 and 6 are same and they are for JC-77. You can find a
second true JC-120/160 schematic from here:

http://www.fortunecity.com/tinpan/humperdinck/223/Rolandjc120.zip

It'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.

[teemuk...@gmail.com]

> Schematics 4, 5 and 6 are same and they are for JC-77. You can find a
> second true JC-120/160 schematic from here:

Oops. A correction to this: It looks like the head model of JC-120/
JC-160 uses similar arrangement of controls, jacks etc. as JC-77. I
believe those schematics are for the head model instead.

[Jim]

That amp has two discrete power amps. Each is a line level in to one of
the sides, and pulls the preamp out of the circuit. So it is stereo on
the power amp side.

[Niles Mills]

A big thanks to Jim and teemukyttala for the quick posts and
solving the problem. Have a great weekend!

Niles Mills
nmi...@gmail.com

[-Phil Clemence]

"Niles Mills" <niles...@gmail.com> wrote in message
news:pan.2007.10.07....@gmail.com...

So .. what are you going to feed it them with?
:-D

[Niles Mills]

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