BEHRINGER KX1200 Keyboard Amp problems.

[Eduard]

I just had one of my band members dropped off a keyboard amp that makes knocking noise from the speakers or should I say very loud hum when turned on.
With the instrument plugged in or not it still makes very loud noise. What are the possible issues with the Amp. I have read that it could be TL074 or LM3886.
What is what and how can I check them out and verify which one is bad.
Would you please give me some ideas because we are getting ready to play a concert soon and need the amp in working condition.
I have done very little work with electronics but very handy I think I can handle a small repair but need to know what to replace on the amp.

Any ideas???

[Enzo]

Loud hum is usually one of two things. Most times it is DC voltage on the speaker due to a blown power amp. The other reason would be loss of filtration in the power supply.

DC is easy to daignose. Look at the speaker cone when you turn on the power switch. Does the cone move one direction and stay there? If so, that is DC voltage. TURN OFF IMMEDIATELY and do not run it with a speaker attached until we get rid of the DC. It is perfectly OK to run the amp without a speaker. If we have DC, you can verify that with your volt meter. Connect it to the speaker wires - if you see 20-30-40 volts, that is bad.

In this amp there is an LM3886 power amp IC, and if there is DC on the output it almost certainly is bad. LM3886 is the multilegged IC clamped to the heat sink. It has about 10 legs. It is a whole power amp in one IC. Lm3886 comes in two types. One has a metal mounting tab with a screw hole. The other has all plastic body with screw hole. Get the same type as replacement. plastic one is LM3886F.

It is also possible one of your main filter caps has failed or broken free. That will not move the speaker cone, nor put DC on the output, but it will hum loud.

Both those things will still happen even with ALL the controls at zero. If turning the controls all to zero stops the hum, then the preamp has a problem. In my experience, usually a TL074 IC.

[Eduard]

Behringer KX1200 Keyboard Amp.

When I have turned all of the knobs to zero or all of them the way up I still have the same loud hum.
Can you please clarify what did you mean by "main filer caps has failed or brocken free"? Are you talking
about the speaker itself and the mount that holds the cone to the base?

Thank you.
Eduard

[Enzo]

Your speaker obviously works, it makes the loud hum. Did you watch the speaker cone closely to see if it moved or not? This is important, if the cone moves one way and stays, that is potentially damaging to the Speaker.

A filter apacitor is an electrolytic capacitor - a cylidrical part on the circuit board. If I recall correctly what your heat sink looks like, there is a space in the middle of it where a bunch of parts live. Your main filters are on that section of the circuit board.

[Eduard]

After close inspection I have found that one of the lines on the PC board is burned out. So I have reconnected the line by using a piece of wire and solder.
I have checked the connectivity behind the main speaker to make sure there are no loose wires or bad connection or cold solder. Then checked that when plugged
in the speaker still makes loud hum but does not stay pusshed forward (DC/AC problem as suggested in some of the threads.
After running the amp for about a minute or even less I have checked the temperature of the two LM3886T's and one was extremely hot (the one that is located
closer to the instrument plugin board.
The capacitors look normal and not bulged or blown, even though I can guess it is not always the case. But right next to the capacitors (filters) I can see
multiple white boxes (not sure what they are) and they look bulged on of the side and it seems to me that it not to good. If I can read correctly the numbers
of the parts (comparing to the schematics ) are : R10, R11, R12 and other that I can not see due to small space between parts.
I bet you know what they are and can find them on the schematics. My guess would be they are a resistors of some kind...

[Eduard]

For some reason pictures can not be attached.... Anyway, if you can possibly help please do. Just let me know what are your thoughts and what else I would need to check or test and what to replace....

[Eduard]

I guess it works I just needed to figure out how to post. My bet... Next picture is coming...

[Eduard]

Main colling assembly...

[Eduard]

LM3886T - The one on the right side of the Main Coolling Assembly.

[Eduard]

The burned out line taht I have soldered. I guess it a part of connection between IC8 and IC9 nad possibly had high overload and burned out.

Any suggestions would be greatly appreciated.
Thank you.
Eduard

[Enzo]

The white boxes are wirewound power resistors.

I(f you have no DC on the output, that is good, but a 3886 getting very hot is bad. The two 3886 are essentially in parallel, so if they act different then one is likeloy bad. I;d replace the hot one. Look at their outputs, each runs through a 0.1 ohm resistor R11, R12. Is either open? A 3886 could be making DC but not having it get to the speaker because that resistor opened. Worth checking.


Beyond that, you have two main power supply rails. +V anbd -V. Make sure both are up to about the same DC voltage and mostly free of AC voltage. Your filter caps may look good, but that doesn't guarantee they are working well.

[Eduard]

Do I need to check each of the resistors (white boxes under the cooling structure) and each must have .01 OHM.
To my knowledge, I would need to test it with multimeeter for OHM, so in case it resistors do not have have any or more then .01 OHM that they are bad and need to be replaced?
Where I can order the parts from? (LM3886T). Do you have any so I can buy to try to repair the amp?
I am in Buffalo Grove Illinois, maybe you know the place to get them locally or just order it on-line?

[Enzo]

I have no idea what local suppliers you might have, most local shops sell the NTE replacements anyway. I recommend not using NTE parts. If nothing else, they cost three to ten times what real parts cost. I use Mouser mainly, but Digikey is another popular supplier.

The resistors I am looking at are 0.1 ohm, not 0.01 ohm. Most hand meters do not measure such small resistance very accurately, but it doesn't matter. The resistor will either be open (bad) or it will probably be OK. So really the test is to see if they have continuity or not.

[Eduard]

I have ordered LM3886TF from Moser. I will let you know when I get it replaced and what will be the outcome...
Thanks.

[Eduard]

I have replaced both LM3886TF with new and knocking noice didn't come back but amp is still not working. After letting it run for a minute I have turned it off to prevent any other issues . Is there a way maybe I can send the chassy to you so you can help me out in repairs. I am afraid I have done everything I could. Maybe I can try replacing TL074 but I am not sure it will do it. Maybe you will be somewhere in the area of North West suburbs of Chicago (Buffalo grove ) or close so you will have a chance to take a look at this piece......... If you can call me if any options possible to get get fixed... 847-877-8998.