Roland JC-120 circuit help

[WFDD]

I have been recently taking a JC-120 amp head and putting it in its own case to cut down the amount of weight I have to carry around all at once. I took the speaker output lines and instead of going straight to the speaker I put jacks in the back of the amp head chassis and when I went to turn it on the 2 fuses (6.0amp inside) blew. I have now hooked everything back up the original way and I now have one external speaker output and one of the main speakers do no work. I think I may have shorted something else out when I had the 1/4in jacks attached to the metal of the chassis. I have looked through the circuit boards and to my eye I see nothing burnt but unless the "blown item" is discolored or actually has a burn mark in it I do not know what to look for. If anyone can help me get this back in working condition it will be much appreciated!

Thanks to all

[Ted]

Are you saying that both speakers no longer work?

The JC120 has two separate power amps, one for each speaker.

You may well have blown one or more output transistors. There are two for each amp. They are the black plastic things bolted to the metalwork with three wires coming from each of them.

This is where it gets tricky. If you dont know how to do a basic diode test on them with a multimeter, you'd be wise to stop right here and find someone who knows what they're doing.

The transistors are probably long obsolete Japanese types. Your first job is to identify them - a transistor labelled Dxyz is acually a 2SDxyz. You can now search the internet for data on the devices. Once you've found data sheets for them, you can usually locate current production devices with equivalent or better specs and fit them. Be aware that they may not have the same package (not necessarily a problem) or pinouts (not a problem if you're aware of it).

[WFDD]

Thanks for that information. One main speaker works and one main speaker is out. Also one external speaker is out and one is working. I will try and locate these not sure how to do the testing you are talking about but I will look up information and try and locate new ones for the future!

[oc disorder]

As Ted said one of the power amps has had its output shorted to ground causing the output transistors to collapse under the strain .Unfortunately the transistors preceding them in the circuit may have also given up like dominoes collapsing in reverse.

The more power is applied to a faulty circuit the more likely other components get damaged.
The good news is you have one to compare it with ie one blown and one working power amp.
Had you used insulated jacks the possibility of the hot wire touching the chassis would have been greatly reduced.

As Ted said you really need to consider if you have the tools.. It should be possible to find the shorted output transistor easily by measuring the "legs" or pins with a multimeter.
These transistors have a live case or metal back which has to be insulated electrically from the chassis while maintaining physical contact via a thin insulating washer either mica or silicon impregnated rubber which helps transfer the heat to the chassis.

There have been several versions of the JC120 with different circuits and components.
Here's a picture I gleaned from the net (thanks by the way to whom it concerns) of one type of 120.
Lets hope you are lucky and its only a matter of replacing one or two o/p transistors.



EDIT: On reflection the damaged transistors are probably open circuit as one side still works.
They wont measure anything. If you know which speaker works follow the wires back to the amplifier board and you should be able to deduce the faulty one.

[WFDD]

Thank you much OC. With the multimeter I am not sure where to have it set or what to look for. Doing some research now to try and figure out what the multimeter will read if its a bad transistor. I went online to try and find a transistor with the same specifications but no luck so far. I like to try to learn and be able to do as much as I can since it is my amp but I might have to take it to someone who knows more about this I am not sure if the new output transistor needs to be EXACTLY the same or if someone else knows what can be different and not harm the amp in the long run!

[WFDD]

UPDATE!! Tested the four output resistors and they all have good strong readings. Not sure where to go from here.. Thanks to all for any and all help!

[Jazz P Bass]

Not to dis you, but if the output transistors are good, stop.
Take it to a repair center.
There is a whole lot going on in this amp.
And a few parts are hard to get.

[Ted]

What he said

[WFDD]

No problem at all thanks again!