Broken Behringer KX1200 Keyboard Amp Ideas?

[doc_drop]

Hey guys. I'm hoping someone can help me decide what to do about this stupid amp I bought. I wanted to use it as my 2-person-band's PA. It worked great for about a year. But after 4 or 5 gigs, it crapped out on us mid-set. I have done many searches trying to find service info. and/or schematics of the amp circuit, but it turns out Behringer keeps it secret.

I also read a number of reports of the same model doing the same thing for other people. One guy mentioned the "power chip" getting fried repeatedly.

Do you guys think it is worth my time trying to find out what the "power chip" is and replacing it? I am pretty good with 9v guitar pedal circuits, but I am not experienced with high voltage, and I am a little intimidated...

My other thought is to use it as a simple speaker cab. I am perfing up a Noisy Cricket, and I think I will try that out. But, the speakers in the KY1200 are 4 ohm and 8 ohm. I guess my best bet is to just try attaching the speakers individually and together, and see what noise comes out. But, can anyone advise me on hooking up different ohm speakers? I know the Cricket is too weak to hurt them. I just want the best sound I can get...

Thanks, and take my advice, don't buy Behringer amps!

[Arn C.]

You can also try over here>   http://www.ampage.org/

There are some discussion forums for amps there.

Peace!
Arn C.

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Thanks Arn. I'll post the same question over there.

But you guys must have some crazy DIY ideas about how to "think outside of this box" and Frankenstein it, no?...

[jefe]

I don't blame you for not wanting to mess with high voltage, I feel the same way. But give it a shot at ampage, you never know, it could end up being an easy fix.

IF you decide it's not worth repairing, then it would make a good speaker cab for messing with cricket type circuits. Just experiment with the speaker wiring. What kind of speaker(s) does it have? Woofer & tweeter?

[Nasse]

http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Behringer/KX1200/10/1

No knowledge about this but somebody says that it uses two LM3886 chips

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Thanks Jeffe and Nasse. Good info.

The amp actually has 3 speakers. A 150w 4ohm woofer, a 120w 8ohm high-mid speaker and a piezo tweeter.

Time to look up the LM3886 data sheet...

[anchovie]

Once you've located the LM3886 within the amp, use the datasheet to identify the pin for the signal input. Use an audio probe or oscilloscope to see if you're actually getting signal from the preamp, if so then the 3886 is indeed the culprit. However, can it be guaranteed that replacing the power chips will stop the amp from failing again? If this is a fault that has happened to several people, there may well be a problem with the design. The power supply could exceed the maximum ratings of the chips or the ICs themselves might be mounted on an inadequate heatsink.

[doc_drop]

Cool Anchovie. I will do what you said to see if it is the chips, but I agree. If this is systematic, and it appears to be somewhat, I don't know if it is worth fixing it and not knowing when it will next blow. Which sucks if I want to gig with it.

I saw somebody suggested replacing all the electrolytes. I was considering doing that too.

Oh well, I'll open it up and see what the chips are up to, and decide after.

Thanks for the help!

[drewl]

the 3886 is an 11pin self contained amp used in alot of these type amps, I've replaced alot and they're pretty cheap.
Alot of amps use them or this style of output amp and they all blow eventually.
Just take yout time replacing it, make sure to use thermal compound and make sure it's getting enough airflow.
marshall uses little fans to keep their amp output drivers from blowing and so do alot of other mfgs.

[doc_drop]

Drewl, gotcha. I saw the chips are like $.20 on Mouser. I think I'll order a few just to have them around. I also saw there are a lot of amps using the little feller.

Thanks for the installation tip!

[drewl]

$.20??
maybe 2bucks but 20cents?
Anyway, make sure you don't pull any of the circuit board material or traces off the board when removing the old one.
You're better off cutting the legs of the old chip and removing them individually.
As you can tell I broke a trace on a Cube (Ithink) while replacing this chip and luckily found it before I powered the unit up.

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Drewl, you're right. Well, so $14. I better be sure this is the problem!

http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMvkC18yXH9iIq1DBHp2rkNA903n3nuYu%252b8%3d

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Guys,

I posted about this on the ampage.com forum. A guy who is an authorized tech for Behringer advised me to follow this routine to isolate the problem. I thought I would share in case it helps anyone else with amp repair:

Plug a signal into the power amp in jack. ANy sound? If not, check the speaker, does the speaker itself make sound? Touch a 9v battery to its terminals for a moment, a good speaker will pop. Pull the wires off first. If speaker is OK and no sound, then check internal power supply. If that is OK< then I would indeed suspect the power amp IC.

Play into the front of the amp. ANything coming out the preamp out jack? COnnect that to some other amp for a listen.

Got effects send and return jacks? Try plugging a signal into the return and listen out your speaker. Likewise send the FX send signal to some other amp for a listen.

All these things are to isolate the problem to a section of the amp.

In my experience, the most common repairs have been the TL074 IC on the jack board, voltage regulators on the power amp for +/-15v. And th occasional power amp IC.

[grapefruit]

Behringer only supply schematics to authorised techs and they have to sign non disclosure agreements.

The recommendation from Ampge is a good course to follow.
If it does turn out to be the amp IC, if they are LM3886 then they must be using two chips in bridge mode to get the rated power. Often these types of amps use two LM3886 in bridge mode for LF and on LM3886 for HF.

Stew.

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O.K. Just reporting in on the "progress" I made last weekend.

I first looked at the crossover in the rear of the cabinet. It was absolutely clean as a whistle. Beautiful solders with no damage at all.

So, I decided to try to isolate the problem. I first plugged my stereo signal into in 1 & 2. It works fine for a while at low volumes. But when I pump up the volume it starts to crackle and sometimes loses volume all of a sudden by at least 6db. I tried all the channels, with headphones and through the cab. It does the same thing for all channels.

So, I decided to try using either the tape ins and the FX return to input the signal. It works a little better, in that I can push the volume a little harder. But at a certain point (and I'm talking at about 45% on the volume knob) crackles and loss of volume.

I figured it must be a problem with the amp circuit. But when I looked, even though I see the 2 LM3886's etc., but it is all SMT, and it looks like it was built by a robot with a human applying gobs of hot glue after. I just don't feel comfortable trying to replace parts in this kind of computerized technology.

So, on to phase 2, aka what can I use this useless box for. I unplugged the wires running to the crossover. I tried plugging a chord out of my little Crate keyboard amp speaker out, and connecting it to the crossover. It sounded like crap, and I heard something arc and start to make our favorite, I just burned some electronics smell. Luckily after a quick power off that amp is fine.

I next decided to use the Noisy Cricket I just built to power the thing. I pulled the speaker cable off the crossover, and started connecting the speakers to the amp. The 8 ohm mid range sounded pretty good. But it sounded best if I ran the mid range and the 4 ohm woofer in series. I don't know why in terms on impedance, and I hope I won't burn out my Cricket, but it works pretty well. And it gets pretty loud.

So, I have a temporary use for the cab of the amp at least.

But, does anyone know of a kit or project to build a mono amp that could handle this speaker box? Any ideas on how to go about figuring out what amp would work best?

Thanks for all the suggestions!