Electronics > Repair

Active Subwoofer - No Sound

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dimky:
Hi all.

Newbie here. Coming to you for an advise.

I have an active subwoofer from ASW-Loudspeaker, Cantius AS 412 and it has no sound output.
Input values:
1) Subwoofer internally consists of 3 major PCBs: Standby power supply, filter/input PCB and amplifier PCB.
2) Power led is ON.
3) Signal detection works fine in Auto mode, once sound signal is coming in, filter PCB is enabling standby power supply to supply power to amplifier part.
4) Standby power supply has output voltage on both rails +/- 6.4V (however on PCB is written 12). Is +/- 6.4V okay for such applications? This power is supplied to filter PCB.
5) Filter PCB has cable connection supplying +/- power, ground, control and signal to amplifier board. Power on both rails is absent, as well as signal is silent, but control signal becomes active once audio input is detected.

So what is the issue here, too low voltage on standby power so op amps does not work properly on filter PCB thus not enabling power and signal further to amplifier PCB or something is wrong on filter PCB itself?

Some photos added for your reference.

Thanks!

beatman:
Looks like the psu is failed i see there deecon crap of the crap capacitors.Check the caps on the secondary on psu unsolder and measure with lcr meter.If is bad low capacitance or hi esr you have to replace with low esr caps.Check all the output voltages on psu check all diodes BE VERY CAREFULLY THERE IS 380 VOLT ON PRIMARY SIDE IF YOU DON T KNOW WHAT YOU DO AND WHAT IS ALL THE COMPONENTS ARE YOU RISK YOUR LIFE.

antenna:
Assuming it's not a power supply thing......

Any component of significant size should have the solder joints reflowed.  The sound can make the heavier parts move and break solder joints.  Its super common in powered subwoofers using class D amplifiers with big coils and big caps.  The glue that was picked off will need to be replaced for the same reason.  Seen it happen several times, and often, the bad joint cannot be seen just by looking for it.

Another thing I've seen happen (I have a DJ friend with employees that abuse their equipment) is a blown preamp opamp at the input from them cranking the mixer board volume with the sub volume too low.  With care not to become part of the circuit, you can use your body (60Hz antenna) to inject audio at the preamp input and output to narrow down the problem. 

dimky:
Thanks for advises, will check.

I assume that in such applications there should be +/- 12 volts on rails, not 6-ish... Is that assumption correct or that could be normal voltage level in some cases?

benj38:
I would suspect that 6.4V may be a bit low for this application (it is also not a common design choice), but I am not sure. What are the voltage ratings of any polar filter or bypass capacitors on these rails? It may give you a good clue if you should expect 6.4V or 12V.

Also, look at all ICs fed by these rails, and compare the actual voltage at their supply pins to their datasheets. If you are lucky, at least one of these ICs will not allow a very broad supply voltage range, and that will allow you to deduce if the rails are indeed too low.

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