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Definitely need more info - such as type of amplifier or receiver, how many speakers you are running, whether the subwoofer is powered or not.
As a simple test procedure: Disconnect all speakers and plug in a pair of headphones. Get a strong audio signal? Good, move on. No signal or week signal, you have an amplifier issue before the audio even gets to the speakers.
Next: Disconnect all speakers, turn the volume down, and connect one speaker to the left output channel. Slowly increase the volume. If that works, do the same on the right channel. Everything OK? You're putting out enough gain on both channels. Sometimes one channel goes bad. You can still listen to your sound in mono moder, or drive two speakers from the single working output. Not stereo, but dual mono. Move on.
Connect left and right speakers but not the subwoofer. Repeat the test. Whatever is left is probably the problem. If the subwoofer is powered, check the power supply or incoming voltage. If the other speakers are driven off the subwoofer or powered from there, that's likely the problem.
This type of active speaker by Mackie has a issue with operational amplifier ic's inside, they are just those little black things with 8-pin soldered to the pcb, you may have some of them damaged, because of the erratic operation of the units, let me know if i can help more.
Normally these low end systems components that depend on each other and cannot be interchangeable from other system. So if one of the components (sub woofer or right control satellite speaker) breaks or gets lost the whole system is unusable...
I was in the same situation. Got a sub woofer from VS4121 without original satellite speakers. I acquired satellite speakers from Altec Lansing 251 which used same connectors but the internal wiring was different and incompatible out of the box. Speakers are also glued together so it is impossible to open without damaging it. Not to mention there is no service documentation anywhere..
But if you like electronics you can try to make something based on the information bellow:
Altec Lansing VS4121 connector pinout
If you take the male connector (from right satellite speaker) and look at the pins and number them first row from left to right pins 1-3, second row pins 4-7, the bottom row pins 8-9 and the circle enclosure as 0
0 - Ground
1 - Speaker Right
2 - Treble (seems just like voltage control but may need to be confirmed: Min=0V; Max=voltage from Volume control Pin 9)
3 - Head Set Right
4 - Bass signal (not sure what the exact original design is as I have not opened the speaker) But seems like they use 50K potentiometer and some sort of RC low pass filter configuration. When potentiometer is set to Min it is shorted with ground. When set to Max it is taking signal from Right and Left Head Set (pins 3 and 5) combined through some larger resistors and low pass filter.
5 - Head Set Left
6 - Ground for Head set (grounds 0 and 6 seems to be connected in the subwofer but within the right satelite speaker they seem to be isolated with separate leads to sub)
7 - Power ON/OFF. When OFF or head set jack plugged = 0V; When ON and head set jack unplugged = 5V
8 - 5V Power in
9 - Volume (min=0V; max=5V). It seems they use 50K potentiometer but probably anything >10k should work.
At minimum for standalone sub woofer you will need to implement:
- Power On.Off (switch between pins 8,7)
- Volume control (50k potentiometer (probably anything beteen 10k and 50k will do)between pin 7 and 0 with variable output to pin 9
- Bass control - join two resistors R (I used 7k) and lets call this join point A. Attach other ends of the resistors to pins 3 and 5 respectively. Attach 50k potentiometer (I used 10k) between point A and ground 0. Attach Variable pin of the potentiometer to pin 4 (Bass input signal). Attach capacitor C between Pin 4 and Ground pin 0 (I used Electrolytic 1uF with negative end to ground) . Resistor R and capacitor C should make simple passive low pass filter (Google for formula) that should bypass anf frequency higher than 80-120Hz to ground and feed frequencies lower than 80-120Hz to pin 4.
PS: I managed to hack 251 control satellite speaker to somewhat work with VS 4121 Subwoofer but that is another subject. All electronic parts were salvaged from old broken TV so total cost was $0 but took several days of tinkering for speakers that I could buy used for $10-20 :)
Sounds like you "blew" the left channel. Make sure your connections are good, and all positives and negatives hooked up the way they are meant to be. If it still does not work, take it to be repaired. Call around first, with the AND serial number. Get repair estimates, ask about the "bench charge"
The problem are worsen that before. It is wisely to replace both left and right amplifier
IC, but make ensure that the power supply feed a good amount of voltage to amplifier,
Also a strong good signal. Be sure to measure it.