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Alesis M1 Active 520 amplifier low frequency pulsing sound

hi, does anyone have a copy of the circuit diagram of an Alesis M1 Active 520 amplified speaker I could have please? One of my Alesis M1 Active 520s makes a loud low frequency pulsing sound with the blue light changing to red on top of the speaker.

It's not a switch mode psu as used in the model 620 but a conventional one witha  mains transformer, bridge rectifier, two reservoir capacitors, two 3 terminal 78xx/79xx regulators to derive the +/- supplies for the amps and preamps. 

Examining the inside pcb, there is what looks like brown cement by the base of the two 6800uF 25volt reservoir caps and underneath the pcb where the power amps fix to the heat sink. Not sure if the main caps popped but I don't want to switch it on to test without isolating the regulators and downstream circuitry.

I note there is no fuse in the circuit to protect the electronics so am reluctant to apply power again without knowing what's at risk.

I'm a qualified electronics engineer and would appreciate any help. Please don't ask me to change Capacitor C4 as this model doesn't use that switch mode psu.

Andorian picture

Did you ever found the schematic or the cause of the problem ? I have one here with the same symptom (and it seems we are not the only one)