Roland KC-350 Shorted out

[smith5414]

Hello, I recently had an issue with a Roland KC-350 Keyboard amplifier and was wondering if anyone has had a similar problem or can shed any light on it.

I was running a Tascam interface through the Aux in XLR jacks on the amp so that i could play along with a part of a song, and when i turned it on the amp shorted out with no sound coming from it and no lit LED although i could hear a very faint noise from the transformer I think, and I am assuming it had something to do with the interface because I had used it the night before. When I opened up the top of the amp the fuse on the PS1 board near the power chord had blown and an area on the pre-board has had several components fried, resistors and transistors that are near the CN2 connector on the preboard, which connects to WP24 on the PS1 board which is tied into the aux in, stereo in and out and subwoofer jacks. I also opened up the front to look at the transformer, which also had fuses which look shorted but everything else visually looks okay. I replaced the fuse on the PS1 but with have to go buy the other ones along with some of the components.

Has anyone else had a similar problem with possibly power surge from aux jacks?, I'm not positive that's what it is but it's my best novice guess. Also, if I replace the fuses and shorted components on the board do you think it will work properly again, are there some more precise ways anyone can give me to troubleshoot the problem, problem areas? I attached the schematic as well, which I found on another post in this forum if anyone has the time to look, if not no worries.

Any help anyone can give me is greatly appreciated, I apologize for my lack of more knowledge, but I know this place always has good answers.
Thank you again!

[mozwell]

The aux in jacks on a KC350 are RCA, nor XLR according to the service manual.

If you are not comfortable working with mains voltage & the high voltages inside amps, then refer this to a technician.

I suspect the problems you have are not related to the interface you are using.

First we need to replace the failed components & hte fuse, do you have designators for these, eg R183, D44, C288.
I woudl then check all the pwoer supplies are ok & unplug the power amp & preamp while doing this, so we can isolate the problem you have.


Unplug CN12 and WP25 / Wp26 from the PS1 board. This will remove the power supply to the power amp & pre amp.
Measure between pin 3 & pin 2 of WP25, and between pin 2 & pin 1 of Wp25, do you get +/-54V
Measure between pin 1 & pin 4 of CN12, and between pin 4 & pin 7 of CN12, do you get +/- 15V

All ok ?
Then reinstall CN12, and recheck the +/- 15V supplies.
Still ok ?
Connect a signal to one of the inputs, check you also get the signal on the line out's, sub woofer out
Connect a signal to line in, do you still get the signal on line outs etc.

Next step is to muting signals, measure between pin 4 & pin 8 of CN12, and between pin 4 & pin 9 of CN12, what do you get.
We need to check the muting signals are ok, or you will get no sound from the speaker.
If you short muting fet Q29 on PS1 board, does this fix the problem.

next measure the ohms across C-E of hte pwoer transistors & driver transistors of the pwoer amp. If all is ok, we can start checking the pwoer amp.
You can disconnect the speakers until we have made sure the rest of the amp is ok.

[Jazz P Bass]

Where did you plug the interface in?