Behringer XY1200 Keyboard Amp repair?

[frogman84]

Hey guys, i am a complete noob, and have just signed up to the forum since it seemed like there are some knowledgeable people here.

I have a KX1200 that i am trying to fix for a friend - it turned on but had a constant tone, even when turned down at volume.

I took the amp out, and after removing the heatsink i found that one of the LM3886t's was all melted. I removed this from the board, and after reading that the two LM's work in parallel, i turned the amp on to see if it would work. It did, and i was able to play through it fine for about ten minutes. Worked just as an amp should.

I gave it back to my friend to see if it holds up, but have told him to keeep it at lower volume levels for the moment.

I have ordered another LM and will attempt to fit it in the spare spot and see how it goes.

My questions were though: Is it ok to be running the amp at all with only one LM circuit in operation?

And also: when i looked at the LM's they still had a piece of plastic between them and the heatsink. I thought this might have been a factory fault and removed the plastic so that the metal of the LM could touch the metal of the heatsink (with some thermal paste applied also). Am i right in thinking that could be a factory fault or is the plastic strip supposed to be between heatsink and back of LM?

Thanks guys.
Stuart, Australia.

[Enzo]

Hi Stuart, welcome to the forum.

Why not start a new thread for your amp. That way we won;t confuse yours and his. And you will get better response with a fresh thread than starting yours on page 3 of another.

And also: when i looked at the LM's they still had a piece of plastic between them and the heatsink. I thought this might have been a factory fault and removed the plastic so that the metal of the LM could touch the metal of the heatsink (with some thermal paste applied also). Am i right in thinking that could be a factory fault or is the plastic strip supposed to be between heatsink and back of LM?

no no no no Leave the insulator there.

And to be honest with you, I don;t recommend operating the amp with the missing IC. That is OK during testing, but not really the way you want to let an amp be used.

[flashback]

Much like the run of this thread, my KX1200 has serious problems. It seems that in my case the problem leads directly to the crossover after all the various tests I have performed. My question would be, has anybody tried switching to a stouter crossover (like Selenium or Fender) to prevent this from happening again? Using chart formulas, I think the crossover point is around 1.8Khz (10mf capacitor and a .68mh coil). Although the solder points on the capacitor are clean, shiny, and solid, it is possible for the capacitor to just plain dry out and disintegrate if overworked even though it looks fine from the outside, if I am correct.

Any advice, or opinion, would be appreciated.

FLASH

[Enzo]

The only problem I have seen with these amps' crossovers is the parts vibrating off the boards. I don;t think your capacitors are "overworked", nor do I think they are dried out.

I think right square in the middle of the rear panel enclosure is a poor location for this little board, with vibration in mind. And for a company like Behringer who just loves to coat everything in hot melt, I think these components are not very sturdily mounted.

More tie wraps, more glue, move the thing to a corner in there.

[flashback]

Thanks, Enzo. I'll go ahead and re-solder the connections and move it to a corner and see if that helps. That was my first post to this site and I find everyone helpful and understanding. I am a 40 year professional keyboard player, but that does not necessarily qualify me as a technician although I've had most of my keyboards apart at one time or another for both major and minor repairs, but most are physical in nature rather than electronic. Thanks for the help and suggestion.

FLASH

[Enzo]

Welcome to the forum.

DOn;t just resolder everything, it is really important to secure the parts mechanically. I often open those amps up and find the tie wraps that were holding the parts on the crossover board are broken.

[drewl]

I just repaired one yesterday that had the crossover fall apart.
I reflowed the connections, glued and tie wrapped it to the board.
While I had it apart I secured the ribbon bables between boards that were loose and of course cleaned the pots jacks and switches....oh and reflowed any suspect pc board connections as always.

[megazone]

Kx1200 schematics

PM me if schematics for the kx1200