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Does unit power up? A "dead" keyboard could likely be a defective "wall-wart" power supply BUT don't forget to check the AC outlet as well. If the function lights come on but no audio, I would 1st check with headphones but if that is also dead it may be related to either the slider style volume control on the keyboard, or internal electronic failures.
If it blows a fuse when powered on, it has a dead short somewhere. Old units do this when the capacitors (filters) get old and dry out. These create dead short circuits. I recommend going through the unit and change out the bad ones. They can usually be spotted easily. They look like little cans glued on the board with an "X" on top. They are usually blue, gray, or black. The X on top is a weak spot allowing the top to open or pop up when they go bad.
These are a nice unit... just bought three of them that were dead... I have repaired one so far. I assume you mean the blue LED is on indicating power is on. What I want you to test is if injecting signal on the BACK of the unit jack (power amp in) will create sound. If it won't then better exercise your warranty. The one unit I have completed repair had a bad 15 volt regulator in it... the second that is almost fixed somebody had blasted the input circuits... probably connected to a speaker output... don't do it... these are LINE and MIC level units. If you need to tap off a speaker output, use a DI box with the unit. The third unit I have not received yet. Drummer used the repaired one as monitor last night and was impressed with the power this unit has.
Transformers are VERY reliable. Look for another problem... rectifier, filter caps, fuses, regulators. There is a main fuse and if it is blown, repair the unit before applying full power. To test the unit, put a 40 watt light bulb in series with the hot side of the power cord. The light will act as a resetable fuse and will light when there is a short within the mixer, thus absorbing most of the voltage. Enough current will flow to diagnose the problem.
Most common reason probably would be incorrect or failed power adapter. It appears the unit requires a 18 VAC (not DC) adapter. Dead batteries if that is the power source would be the other cause to suspect.
First thing is to check the power supply... dead battery or power adapter will be enough to start unit but quickly give out and turn off. If unit uses Alkaline batteries, do NOT try to use rechargeables as they run lower voltage and fail quickly, especially as they age.
Oh, oh... got one on the bench now... There is a design defect that causes a short and destruction in the power amp portion.
Do a test: Plug headphones into the main 1/4 inch jacks. The sound won;t be very loud since these are line outputs, not for headphones.
IF you get sound out those, then one or both power amps are dead.
Worst case: if one amp is dead, several parts including three power MOSFET transistors are fried for any of the dead channels.
The failure of the amp can wipe out the main power supply. If you effect windows light up, then the small power supply is operating.
If the big supply is fried, there are two large MOSFET switching transistors that are gone. Often several smaller transistors and dides and resistors and one switching IC goes. MOST of these are surface mounted parts and would be beyond the scope of what you can repair.
I am trying to contact Behringer about the flaw but haven't heard back yet.
The nature of the flaw is this: The is insufficient clearance on an internal circuit layer from the edge that fits into a slot in the power amp heatsink. Any humidity that enters can cause the insulation to breakdown and arc to the heatsink causing a chain reaction.
I repaired one of these already before finding the cause, but this one had a smoking gun! I just yesterday milled out the slot deeper and wider and will put insulating tape to prevent further arcing. The arcing that had occured burned away the internal layer and the connections. I will replace thoose with hardwire.
Repair of this problem is time consuming to weed out all the blown parts and to safely bring the unit back to life.
The current unit I am working on took out ONLY the power amp. A 30 milliohm resistor went up in smoke making a real mess. All three power MOSFETS are gonzo and a zener as well... don't know what else I will find. The previous unit was worse as the power supply was wiped as well.
Test the power adapter to make sure that there is power at the plug (volt ohm meter). Try wiggling the plug while it is plugged into the unit with the power switch on. If it works intermittently it means that the solder joint for the power adapter jack has failed on the circuit board. If that isn't the problem it probably is the power switch that needs to be tested next. IF you aren't confident with disassembling the unit far enough to do the tests you will need to take it in for servicing. Let me know how you make out.
Make sure the three position slide switch is set correctly for the stereo usage, that is NOT the bridged OR the Mon mode.
MAKE SURE the pan pots are set to middle as they control the spit of the audio to the two channels.
This is a class "D" amplifier (switching) and there really isn't any fuse for a single channel.
MAKE SURE your speakon connections are good by swapping your speaker cables.
I repaired my unit and had to do that without schematics... Behringer will not give them out. Mine had a severe arc-over on one side power amp taking out about ten transistors and diodes and one swithcing regulator chip. Had to cut out the "cancer" of the burned board.
This unit uses a complementary output stage that is made up of one transistor on one side and a pair in parallel on the other. This drives the inductor/capacitor filter... I think the switching frequency is between 100KHz and 1 MHz as I recall. All those transistors were gonzo. It also fried the main power switching transistors in the power supply, several smaller transistors, diodes, and the switching regulator chip.
Also two 10 ohm power resistors were fried. They soft start this unit by charging the main caps via two resistors that limit the inrush current.
A small switching supply has to come up first before the main supply which sends the DC to the amp section.
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