Roland JC-120

[scratchcoat]

Hi,
Hoping someone can help me trouble shoot my jc-120. It just was given to me by a friend but only one side works. I switched the speaker terminals to the non-working speaker and the speaker works. Does this mean a channel or output is blown? Any help would be appreciated, I always wanted one of these amps for the chorus/vibrato. 

[Roly]

Hi scratchcoat, welcome.


First we diagnose, then we make conclusions.

Take a known good lead and plug it into Ch-2 Effects Loop "Send" and "Return L(Mono)" and see if the missing channel comes to life.  Post results.

[scratchcoat]

Hey Roly,
Thanks for posting. I won't get a chance to test amp till later. But am stoked at the process.
Just so I can wrap my brain around the concept. If the channel comes to life is it a preamp problem?
If the channel does not play it's a power amp problem? Thanks for your patience, I always wanted to
be able to rummage around in an amp!!!

[Roly]

I'm assuming that the dead channel is Channel-2, the one with the Fx loop.

Sockets in the Fx loop are fitted with contacts to pass the signal from the preamp to the main amp when the Fx loop isn't being used, and these are a frequent cause of loss of output.  Bridging Fx Send to Fx Return with a lead bypasses these contacts and the amp/channel coming to life points right at them for a clean (or socket replacement).

I actually had one of these in for repair one time that had been on a sea voyage and the sockets on the rear were in such a corroded condition they all had to be replaced.

If bridging Fx Send and Return doesn't change anything then we try the crossover test, Fx Out to another amp - is the preamp working?

You should be able to just plug a guitar into Fx Return and see if anything comes out the power amp.

So;

* please confirm that the dead channel is #2?

* Does bridging Fx Send and Return with a good lead produce sound?  Yes -> clean sockets.

* No, does plugging a guitar into Fx return produce sound?  No -> diagnose power amp fault.

* Yes, does taking a signal from Fx Send to another amp produce sound?  No -> diagnose preamp fault.

Post results.


The first point of "rummaging around" in an amp is to identify where the power mains come in and is fused and switched - and not touch any of it.

[scratchcoat]

Hey Roly,

I went to test the amp today but was thwarted. My amp(s) are mid to late 70's versions. The back panel has a line out and 2 mains in but no effects loop.

Channel 2 does work, just not the vibrato/chorus or the speaker.

Thanks for the detailed explanation of the diagnosis tree. I'm sure I will be able to put that to good use. Particularly the part about not rummaging around!

Scratchcoat

[Enzo]

There are two power amps, one for each speaker.   Are not the two main in jacks actually inputs to each power amp?  Plug a guitar into one of those and then the other to see if either power amp can be driven to make sound out the speaker.

[scratchcoat]

I got a chance to test my 2 jc-120's today. I plugged my guitar directly into the mains out. Amp number 1 played out of both channels, both speakers worked. In the other jc-120 only one mains out played, the other side did not work. I appreciate the help. I'm looking forward to your responses.
Scratchcoat.

[Roly]

While injecting a signal into the "Main In" of channel that isn't working, take a jack plug and short it with a clip lead o.n.o. and plug it into each of the "Ext.Sp" sockets in turn.  Any joy?


{Third parties please note that this only applies to this particular amp - it is normally not a good idea to plug a short into an external speaker socket.}



(herumph - first and "out of memory" error, now a "double post" error.
Now it's "already a file by that name" but it's not showing my post
Fatal error: Out of memory (allocated 65536000) (tried to allocate 12900 bytes) in /homepages/10/d273646277/htdocs/ssguitar.com/Sources/Subs-Graphics.php on line 412)


Okay, I give up.  The circuit is here;
http://music-electronics-forum.com/attachments/27023d1389850077-roland-jc120-jc160-jpg.jpg

[scratchcoat]

Hey Roly.
I'm hoping injecting a signal can be accomplished by taking a stereo headphone output and running a guitar cable into the mains. Shorting out a jack plug by o.n.o. ? Unless that is a Yoko Ono I have no idea what that refers to(lol). Can you explain that process? Thanks
Scratchcoat

[Roly]

Yep, that should work.  Just about any way of getting a signal in there without having to twang a guitar at the same time is helpful, cassette deck, radio, MP3 pod, etc.

{you freak me out every time you say "mains" because my first thought is 240VAC power mains, not main-A and main-B power amps}

I never thought Yoko was much good for anything, and she certainly wouldn't help here.

O.n.o. "or near offer".  This can be anything that does the job, plug and clip lead, plug with a bit of wire soldered across it, guitar lead with something metal shorting the other end...

The "Ext.Spk" sockets are in series with the internal loudspeakers and have bridging contacts, so (like the typical Fx Send/Return) if these contacts happened to go open circuit due to a bit of oxide - presto, no sound.  By putting a plug in and shorting it you bypass the possibly faulty contacts, and if you get life then the sockets contacts need a clean, or the socket needs replacing.

[J M Fahey]

taking a stereo headphone output and running a guitar cable into the mains.

and while you are at it, please sit in this comfortable chair   :



unless you prefer this:

[scratchcoat]

Hey Roly,
I finally got a chance to sit with the jc-120's. I inserted a guitar cable into the main out jacks and to my amazement, it worked. Both channels are now working. However, it seems to have developed a new problem(I don't think these amps I got from my friends closed studio got much love). It is making a loud hum(B nat. if my guitar is in tune) that gets louder when I turn the volume up. It turns into a full blown motorboating sound when it gets past half volume. It does not happen when I plug into channel 2 and turn up channel 2. But the general low hum remains.
I'm hoping to get a look at the other jc-120 this week. I appreciate all your help.
Scratchcoat

[Roly]

{Fifty bucks!     For a furschlugger guitar power lead?  I ain't gonna pay no fifty bucks - I can make my own!   8| }

...the jc-120's...

...look at the other jc-120...

Are they breeding at your place?   :lmao:

Before things get terminally confusing a) we need to deal with one amp at a time, and b) since they are the same we need a way of identifying which amp we are talking about.

I inserted a guitar cable into the main out jacks and to my amazement, it worked. Both channels are now working.

Can I assume that you actually mean that you bridged Pre Out and Main In with a guitar lead?

That's good. :dbtu:   As I said, many of these sockets also have switch contacts that are used internally and that with time and lack of use the contacts can oxidise and lose contact.  Sometime, as it seems here, simply the act of plugging a lead in will exercise the contacts and can cause them to scrub a bit and come good (for a while at least).

The indication here is that the bridging contact(s) on Pre Out or Main In are oxidised.  The first treatment is to give them a good shot of DeOxit, and work a plug in and out a few times to cause the contacts to scrub while they are wet with DeOxit (fresh methylated spirits or rubbing alcohol are acceptable stopgaps if you can't get DeOxit locally - we generally avoid generic "contact cleaner" like CRC because they also often contain light oils which we don't want in the sockets).

If this appears to actually fix the problem, i.e. you can plug and un-plug and the results are consistent, then you can call it fixed, but if there is still any dodgy operation you can try re-cleaning, but we generally try to cut to the chase by replacing the socket.

Now if you've got one dubious socket it's a good bet all the others are in a similar condition, so you need to give all of them the treatment and a good testing, and if they are not perfect, 100% operation, then you can try a second treatment, but I'd be thinking about renewal.

"Motorboating" is a specific term we use for very low frequency instability or oscillation, a "putt-putt-putt" down around tremolo frequency, 1-10 pulses per second.

This may be a whole new fault you have exposed, but it may also be caused by one of the other socket contacts being dirty, so before we go chasing the motorboating you should attend to cleaning or replacing the sockets and we'll see if that clears up the motorboating as well.  If not we'll chase that as a separate fault, okay?

Input sockets also use one of these contacts to short the input when nothing is plugged in, and if these contacts fail they leave the input wide open to pick up hum and other signals within the amp that might cause low frequency instability.

So pick an ID for each amp, "1 & 2", "A & B", "Fred & Griselda", and keep your reportage of symptoms of each clearly defined and separate (or we are going to get into headspins of confusion).  I'd suggest that we concentrate on one first and get it going, then we can move on to the second when the first one is healthy again.