JC-120 Low Volume

[AtomicMassUnit]

Just starting to work with this one, I'm looking at various schematics to figure out which model JC-120 this is. The amp has very weak output, on both channels, with the volume up, it sounds like it's barely on.

I have a strong signal all the way through the Line out, and also in the channel 2 effects loop send (there is no channel 1 loop).

The effects all work as well.

It's a JC-120U, says made in USA on the back, but the boards all say made in Italy, has the knob for chorus/vibrato instead of the switch. No main in.

***Ok, I read every JC120 thread on here I could find and additional google search results, and it looks like this is a '94 model. I have scoured all the schematics here and online and none of the ones I found match perfectly. Yikes.

[soundguruman]

The (bloody) FETs go sour, there are also a few op amps, and these amps usually need the circuit boards re-soldered.
When you have low volume it can be a crappy FET transistor.
However there is a good chance that Roland will e mail you a schematic, if you call and beg for it. You have to call and wait on hold, while they torture you with their recorded commercials.

[Jazz P Bass]

After checking the amp,make darn sure the speaker jacks are clean.
The one unused jack "passes" the signal.
Insert a shorted 1'4" plug in the unused jack & see if the volume comes back.

[jamesmafyew]

One other thing to check is the supply electrolytics. Sometimes I get JC-120s that make a "whooshing" noise when they're turned off, and this typically accompanies "it's not as loud as it used to be and sounds harsh" -- it's usually that the 3300uF supply caps have gone high-impedance and need replacing.

[soundguruman]

The speaker jacks are not "thru" connections, there is no output transformer, the jacks are simply parallel, no switches, in fact, there are no speaker jacks on a JC 120, come to think of it.
The jacks to check are the two "main in" jacks. These are thru connections that depend on an internal jack switch. A dirty jack can interrupt the preamp out / power amp in connection.
Also, the three foot switch jacks are supposed to ground, when the FS is unplugged. Those switches can be oxidized too.
Other than that, the 4 input jacks are internally switched, clean those switches.
But what we have seen the most of other than oxidized jacks, is that the soldering on the circuit boards needs to be re-flowed.

[g1]

The speaker jacks are not "thru" connections, there is no output transformer, the jacks are simply parallel, no switches, in fact, there are no speaker jacks on a JC 120, come to think of it.

Please examine the schematic that YOU provided here: http://music-electronics-forum.com/a...c120-jc160.pdf
CLEARLY, there are extension speaker jacks which provide the speakers ground when nothing is plugged into them.
Any dirt/oxide build up in the ext. speaker jacks will interfere with correct operation.

[AtomicMassUnit]

Called them, they said schematics are not available to the public, but I'm having a friend who's a Roland dealer call them to try and get it. It is a JC-120U.

Thanks for the replies above. I will check out those speaker jacks and FETs asap. I'm pretty confident that if I get a schematic I can just shoot a signal through it and see where it gets lost with my scope, but without the schem I'm a little lost!

[J M Fahey]

Don´t waste time on those FETs.
If you have strong good signal up to and including line out, you've already gone through all the board, end to end, since the line out is Power Amp output derived.
This includes

The (bloody) FETs .................. also a few op amps,

Since you have strong "hot" speaker signal but low speaker volume, I would check the speaker side that goes towards ground ... including the extension speaker jack if available.
Good luck.

[AtomicMassUnit]

Big thanks to teemuk for getting me the right schematic for this guy. I attached it below.

I have signal all the way through R99 on power amp Right and R98 on power amp Left. At R86 on power amp right, and R91 on power amp left, the signal is dramatically lower.

that's making me suspect Q23 (R) and Q16 (L), but what are the chances they'd both be bad?

[melvin]

I know this is probably silly, but if a stereo combo amp has the speakers wired out-of-phase, there can be a remarkable attenuation in loudness.

[J M Fahey]

Dear A M U , your attachment disappeared

[AtomicMassUnit]

Big thanks to teemuk for getting me the right schematic for this guy. I attached it below.

I have signal all the way through R99 on power amp Right and R98 on power amp Left. At R86 on power amp right, and R91 on power amp left, the signal is dramatically lower.

that's making me suspect Q23 (R) and Q16 (L), but what are the chances they'd both be bad?

Tried again on the attachment. Here's a link to the full file if that doesn't work.

http://www.schematicsforfree.com/arc...d%20JC-120.zip

It's the JC-120UT schematic in the zip file.

[AtomicMassUnit]

On Q17, I'm reading 15 ohms from the center to one of the sides (G-S if I'm reading it right). Facing the flat part, it's center to right lug reading shorted.

[52 Bill]

Your posts point to the two mute fets at the input of both power amps. While it's not likely that they both have gone bad, they are both controlled by the same voltage source. That's where I'd start looking for a missing gate voltage.

[AtomicMassUnit]

Your posts point to the two mute fets at the input of both power amps. While it's not likely that they both have gone bad, they are both controlled by the same voltage source. That's where I'd start looking for a missing gate voltage.

I ordered a replacement for the shorted transistor. What would happen if I turned the amp on with that part pulled out?

Also, the replacement is an NTE part, I've read some bad things... do I need to change Q26 so they match or does that matter in this case?