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Disconnect the wires (they will usually be push-connectors, make sure you note which goes where) and if you have a multimeter, meter across the terminals - if the speaker is OK you should get about 3 ohms or 6 ohms depending on if it's a 4 or 8-ohm speaker.
If you don't have a meter, touch a PP3 battery across the terminals - you may need a screwdriver to bridge the gap if they're far apart. If it makes a pop and the cone moves, the speaker is fine. Don't keep the battery connected for any longer than you need to.
If it's the speaker and you can get a direct replacement you can change it yourself. If it's the power amp, it may be beyond repair even by most techs, other than by simply replacing the whole power amp module - Class D power sections are notoriously difficult to troubleshoot and many are not designed to be easily worked on either.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
If the speaker is blown I would be wary of simply changing it without testing the amplifier.
I've seen plenty of amps where the amp has latched up and blown the speaker, in which case changing the speaker will simply result in another blown speaker.
Equally the amp may have expired, and you will still have a non functioning amp but will have spent a load on a speaker (replacement 12" drivers for powered speakers are surprisingly expense when compared with the cost of the unit).
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
It will be more difficult with a full-range cab though - you will need a crossover as well.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Problem is i can't rely on it as it wasn't working last time...
Any idea as to what could cause an intermittent problem like this, i'm a bit baffled...
You're right, you can't rely on it.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
If the horn didn't cut out then you can ignore everything primary of the transformer and the switching and pulse width circuit as any failure there would kill both amps.
Bad joint on the actual power amp that feeds the woofer or a bad spade connector would be my best guess. Have a good visual inspection ... I've seen MOV legs and thermistors legs break to the vibration in active speakers ... it's good form to gunk em but some don't bother
Were you driving the speaker hard when it cut out, it's possible thermal protection has cut in when amp got too hot OR thermal protection is faulty causing amp to turn off
Not an easy thing to diagnose online and sometimes not an easy thing to fix on the bench if it's the first one you've seen. Class D amps are complex in operation and repairs (in my experience) generally require sketching bits of the circuit out and working from there if there's nothing obvious
Any crimped or push contact connector is still the prime suspect.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein