Roland KC 350 crackling

[zedexseven]

Hi everyone.  I bought a used Roland KC-300 (Edit this is a Roland KC-350) keyboard amp a while ago and it worked great for a while.  One day I was plugging in a patch cable while the amp was on and it blew something.  The volume was barely audible and it was crackling from both the tweeter and the main speaker.  It was like a really bad distortion and really low volume.

I ended up taking it to an amp repair shop and they fixed it for 100$.  I don' think they really had to change any major parts but they didn't really explain to me what they fixed.

So lesson learned no to plug in anything while the amp is on.  But I was having some people over to jam and one of them did the same thing and now I am faced with the same issue.

I don't want to take it back to get repaired again for 100$, I only paid 150$ for it.  This time I would like to take a crack at fixing it myself first but I don't know where to start.  I removed the 12 inch speaker and had a look for any exploding or leaking caps but didn't see any.

If it helps, it has this issue only when driving the speakers, when I hook up headphones the sound is clear.

Any help would be greatly appreciated!  :tu:

[Enzo]

The problem I see here is this:  plugging something in while the amp is on should not damage any amp.  It is done all the time, and there is nothing about the signal inputs that can damage something internally.   And especially if the headphones work.  A clean headphones signal means the preamp parts are working, and the preamp is where we plug things in.

But let's find out where the problem is.  I don't have the KC-300 schematic, do you?   I am looking at the KC-500 and KC350 hoping it is similar.

Play through the amp, it sounds crappy.  PLug a cord from send to return on the stereo link, or FX loop.  SOund better?

Inside, are there any internal fuses that have opened?

[DrGonz78]

Here's some info that I have found so far...

http://www.justanswer.com/electronic-musical-instruments/6jbbv-burned-resistors-circuit-board-roland.html

On this site the poster uploaded the KC-300 schematic... Now I am posting that image here... Does your pcb board say PWB = G2927124??

Yeah you should not be having problems just plugging in something to the amp while it is on. However, what are WE plugging in EXACTLY? Please explain how this latest problem might be related to the previous problem... That is to say again.. What are we plugging in to the amp while it is on?

Also, pls look at this schematic I have attached and figure out if it really is the KC300 schematic. It might be, but you never know. I post it anyhow with the link the website I found it on. I hope this might help.

[zedexseven]

Hey guys, thanks for the quick response.  I am terribly sorry this is in fact a "KC-350" model (I just edited/fixed the original message).

I am going to check the things you have suggested and will report back soon, thanks!

Also I checked the fuses last night when taking it apart and I noticed none of the fuses are open like I would expect to see with a blown fuse.  However 1 of the 3 fuses has a sort of bubble on the wire inside the fuse.  It almost looks like a piece of solder.   Could this indicate a blown fuse?  I can take a pic if that helps.

[Enzo]

Take the fuse out of its clip and measure it with an ohm meter.  It will either be good or be open.

[zedexseven]

Ok I was able to find and test 3 fuses and all appeared to be healthy.

I also tried playing through the amp while I had a patch cable plugged in the stereo send connected to return and it didn't improve the sound any.  I don't think the 350 has an FX loop from what I can tell.

Any suggestions on what to check next?

[Enzo]

They don't call it that, but that is what the send and return are.

Plug a signal into that return, how does that sound?

[zedexseven]

I just plugged my mp3 player into the return and it has the same issue.   So sounds like this is in the poweramp stage somewhere?

[Enzo]

Kinda.  There is a bunch of stuff between that jack and the PA.

My first hunch might be to check for mutes stuck on.  Q12 at the PA input, and Q23 on the PS1 board output.  The mute control signals come from the power supply.

[J M Fahey]

For a *quick* check;

1) lift the left end of C88 (meaning the left end **in the schematic**; on the actual PCB it may point anywhere) and touch it with your finger, how does that hum sound?

2) grab a piece of guitar cable; solder one end to the floating end of C88 and the shield to a nearby ground; attach a suitable plug to the other end and plug it into a keyboard or CD/MP3 player.
How does it sound?
We are bypassing all mutes, jacks, preamp, and going *straight* into the Power Amp.
Sound?

[zedexseven]

Ok thanks Enzo and J.M.   I don't know much about schematics but I am going to do some reading up on how I apply this to the circuit boards and will have a look at these once I figure it out

[vanbillsing]

****SOLUTION*****

Hey, I know I'm necroposting on this topic but since there was no usable solution on this thread I wanted to post my fix for the exact same issue. This thread was the first relevant google result I could find when I went to fix my amp today.

It turns out that a resistor on the main board had completely fallen off and was in the bottom of the amp. It was a 5W / 10 ohm resistor that had fallen off the 'R101' placement spot on the circuit board.
(see photo titled 'Circled.jpg' below)
It looks like the two white rectangular resistors to the left of my big red circle.

I soldered the resistor back into place by removing all the knobs and knob nuts, the board screws and flipped the board over. No need to unplug any wires... well... except the main power from the wall outlet of course.

I have always had problems with that amp since I purchased it. Surprisingly the amp will work without that resistor but it sounds like mud and has always crackled and sometimes cut out on me at loud volumes.

After I fixed it tonight, I took it out to the garage and plugged my synth in and cranked it up. The damn thing jumps around like a Mexican jumping bean now and sound better than it ever has.

I really hope this helps anyone else that runs into the same issue in the future.

Cheers!!!
-BillyB 8|

[phatt]

If the schematic given (300) is the same, then R101 is the Hi freq circuit that drives the tweeter. You have now simply reconnected the horn and the sound will be brighter. :tu:
Yes a worry when things just fall off inside your amp,, not something that makes you feel confident about touring with such equipment.

I don't think it is related to the original post but thanks for the input as every little bit of info helps us all. :tu:
Phil.

[DrGonz78]

Yeah that is a KC-300 in the picture. The thread was confusing as it switched from kc300 to kc350 as the OP later made a clarification. One thing to note is when I was working on a KC-300 the filter caps were soldered in very badly. I am not sure if someone else had worked on the amp and simply re-flowed solder on the main filter caps. Either way this amp hangs vertically with the caps sticking outward and to me that makes them prone to stressing. All the vibrations in the amp over time might lead the caps to become weak solder joints. Perhaps that is what happened to the R101? Either way I would possibly apply some glue to those large wire wound resistors and even around the filter caps.

[Roly]

Thanks for taking the trouble to share your experience - every bit helps.

I suspect he had a dirty contact in his headphone socket, but there ya go.

a resistor on the main board had completely fallen off and was in the bottom of the amp.

Some are easier than others.  Had a Fender twin come in with a reverb complaint.  Pulled out the tank and there was half a china plate jammed under it.  Greek wedding I guess.

The components they know are going to fall out of the board are marked on the circuit with