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Most behringer transformers are 18Volts 1-2 amps to my knowledge, specially in the smaller units. If you are not sure start with reconnecting a 12 volt (AC) external supply and see how you go. Behringer uses (indicated) 17Volts transformers but that is neither here nor there. Good Luck.
In spite of the display, does the effects work and do they change when you dial up and press the knob down? If they do, then likely the display is at fault. If the effects are NOT working sound wise, check the +5 volts as the whole effects runs from that, It should be within 10% of the 5 volts.
The internals run on +/-15 Volts and +5 volts. While schematics show the audio and digital circuits they fail to show the power supply. I suspect the supply uses regulators such as the 7815 family so one would want around 18 to 20 volts DC of each polarity.A transformer would likely be about a 28 to 30 VAC centertapped. HOWEVER transformers rarely fail. There should be fuses to protect the transformer. Some Behringer equipment uses a switching supply. Common failures in those is the switching transistor. When testing these units, it is useful to place a 25 watt incandescent lamp in series with the power cord to save fuses and act as a resetable fuse that will send a limited current into the unit for testing.
Check the 5 volts to the FX unit, If the voltage is low, the FX processor will keep reseting generating clicks, In a different product, the 5 volt filter cap was degraded and voltage dipped down to 4.5 volts causing resets.
You should find pwer supply voltages of +/- 15 volts going to most of the op-amps.
You should find 48 volts for the phantom power source.
You should find 5 volts for the FX DSP or any other logic circuits.
You should find a +12 volt.
Here is a manual close to your unit (yours is an expansion of it):
After reviewing the schematic, I don't understand how the Phantom power can REMAIN on with the power switch off... this CANNOT occur except for what energy is left in the 48 volt supply filter cap... This might last for minutes after the fuse went... UNLESS you have devices connected which are backfeeding the 48 volts. (which would be a disaster)
The 48 volts, +/- 12 volts and +/- 15 volts and the +5 volts ALL come from the SAME switching power supply. I suspect one of the filter caps went or the primary switching regulator a TDP245Y chip.
You should ONLY work on the line side of this using an ISOLATION transformer since the line side has lethal voltages.
To do testing, one replaces the fuse and puts a 60 Watt light bulb in series with the hot side of the inout line. The lamp "acts" as a resetable fuse and limits the current for testing. When all is well again, the lamp will remain out with the power flowing through it.
The D1 diode bridge and C7 input rectifier and filter are first things to check. You can do those with an ohmmeter.
I will caution you to ALWAYS power any and all equipment from the SAME power source. Plugging in devices from across the room will set up ground loops and any ground fault will fry equipment.
I repaired a Behringer guitar amp that had a DSP effects in it (BX1200 I believe).
The effects were on a seperate board and the board had its own 5 volt power supply. The filter capacitor for the 5 volt power on the board was UNDERSIZED and it had degraded causing the 5 volts to drop out of regulation, dipping to 4 volts. This caused the DSP effects to fail.
YOUR device MAY use the same DSP OR design.
Check the 5 volt power with an OSCILLOSCOPE to look for dips below the 5 volt operating point that occur at 120 HZ.
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