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Audio Mixer Problem.

Zoomup

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.

  • 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

    Hi Zoom

    OK I looked it up. 9VAC supply so how does it get more than 15V? The spec says the current is 1A, quite a lot, does it have another transformer in it? Look  at the rectified DC, what voltage? It has USB connections, so thats what the 5V is for, but it also weighs a lot, 3.4 kg, so I suspect another transformer with 9V input. If DC was applied to this anything could have happened!

  • 0

    Hi Zoom

    OK I looked it up. 9VAC supply so how does it get more than 15V? The spec says the current is 1A, quite a lot, does it have another transformer in it? Look  at the rectified DC, what voltage? It has USB connections, so thats what the 5V is for, but it also weighs a lot, 3.4 kg, so I suspect another transformer with 9V input. If DC was applied to this anything could have happened!

  • 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

  • 0 in reply to David Stone

    Hello David, Mike and  G.K. My model has no U.S.B. sockets just phono sockets. I guessed at the 4 diodes making a bridge, I may be wrong there. There is nothing burnt or blown up that can be detected by eye. I am awaiting a suitable 9 Volt A.C. power supply before I can test this unit. I will report back. I suppose that I could use an 8 Volt bell transformer to test it. There are two big electrolytic capacitors rated at 35 Volt near to the 4 diodes and some smaller ones, so perhaps these are used to produce the 15 Volts needed.

  • 0

    an 8v bell transformer may be OK - verify the voltage first.

    Note that quite a lot of transformers only generate the rated voltage on 100% load, and open cct or underloaded can be 10-20% more, especially small ones. Verify by meter.

    Mike.

    35V caps and not 12V or 16V ones do suggest rather more than 9V and a bridge. A bit of buzzing out what connects to what with the diodes and earth and so on is probably called for.

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