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Audio Mixer Problem.

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Hello All, I have a faulty Citronic PRO-3 audio mixer. It runs on 9V A.C. The seller claims that he connected  a D.C. power supply

and after that the mixer produced a muffled output. The input has a four diode bridge rectifier at the input for the power, several

capacitors, and two 7915A three lead regulators plus one 78M05A three pin device prior to the main mixing board.

Any ideas where to look first when attempting a repair?

Thanks,

Z.

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  • 0

    First check the voltage output of the voltage regulators, -15 V (right pin when looking at the marking side) to GND (centre pin) on the 7915A, and +5 V (right pin when looking at it from the marking side) to GND (centre pin) on the 78M05A's. That will tell you whether the problem is in the power supply section, or in a mixer OPAMP type circuit.

    How does the 7915 get its voltage input > 15 V, if the AC input is 9 V AC (full-wave rectified is usually < 13.5 V less rectifier forward volt-drop from a 9 V rms output transformer)? Is there a small transformer in there, or a DC-DC converter?

  • 0 in reply to gkenyon

    check it is really  9VAC, if it is the bridge is probably centre slung. Confirm if the '05' reference is actually the same '0v' as that on the 15V regs. I have seen it done where the regs hang sloth-like from the rail, so and the ground of one is the flying rail of the other.

    Or post a photo.

    Are the 4 diodes really a bridge or a multiplying rectifier ?

    Failing that apply 9VAC, and tell us if

    either side of that 8V is the same as audio ground, or if if it is flying, and

    what the voltages on the pins of the regs are, all 9 pins please, relative to audio ground.

    Mike

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  • 0 in reply to gkenyon

    check it is really  9VAC, if it is the bridge is probably centre slung. Confirm if the '05' reference is actually the same '0v' as that on the 15V regs. I have seen it done where the regs hang sloth-like from the rail, so and the ground of one is the flying rail of the other.

    Or post a photo.

    Are the 4 diodes really a bridge or a multiplying rectifier ?

    Failing that apply 9VAC, and tell us if

    either side of that 8V is the same as audio ground, or if if it is flying, and

    what the voltages on the pins of the regs are, all 9 pins please, relative to audio ground.

    Mike

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