Need help with fixing pc subwoofer+speakers please

Each one of the components will need to communicate somehow. Following these lines helps after making sure power is being distributed properly.
Do you have any pictures or a model number? It sounds as though the remote, and both speakers plug directly into the sub and have their own socket for such a thing.

 

You have located the power line already. The input looks to be AC, I am unsure what voltage. This feeds through a fuse and those four diodes that are used to rectify the AC voltage to DC.

To measure the AC side, set your meter to AC and measure where the power from the transformer comes in.
To measure the DC side, set your meter to DC and make note of the 4 diodes near the fuse. They are setup in pairs. You can take your meter and put one probe on the right diode, and the other on the left. Touch the diode's wire that is facing the capacitor.
That capacitor may be charged though, so be cautious and if you have clips for your meter's probes, use them. If clips are out of the questions, try with one hand just in case you short the capacitor, you dont want to have one hand on each probe.

 

OK Can you please snap another picture of the top and the bottom of the board with the flash off... the glare makes it pretty hard to see. Bottom of the remote would also help
https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/

There are two little 8 legged ICs on the board as well. I can make out one of them, but if the picture does not clearly show, can you please tell us the marking on each.
Please also tell us the markings on each of the white pasted items. (Were these components against metal prior to disassembly?)

Next step is to identify some of the more complex items used so we know what pin is supposed to do what. The new pictures will help to follow the traces on the bottom of the board to find out what components the power goes to.

Just to clarify. Are any of the capacitors bulging slightly?

 

Service information would be pretty helpful. Do you have a model number we could Google? It's unlikely, but possible, that there's a schematic or even a service manual available somewhere out there.

Have a good close look at the underside for dry joints. These happen often on single-sided non-plated-through boards, especially on components that get hot, such as the "TFA9842J" power amplifiers, and on connectors, especially on end pins. But check everything carefully. Any large or suspect components, wiggle it slightly and gently from above while watching the leads on the underside to see whether anything is not firmly connected.

The data sheet for the power amp ICs is available at http://www.nxp.com/documents/data_sheet/TFA9842J.pdf and it shows the VCC and GND pins (9 and 5) which should have the DC voltage you measured across the main electrolytic between them. If that voltage is present on both, you can work from there. But service information would really help.

 

Oh, X-210 is the model number! Sorry. Thanks for the photos.

OK, so the output ICs have their 20V DC supply voltage. That's a good start!

I don't see anything wrong with those joints on those pins, but if you're not sure, you could resolder them. This is actually best done when the ICs are mounted back onto their metal heatsink, but you will have to delay reattaching them to their heatsink if that would make it impossible to access the circuit board.

For each pin, with the soldering iron on one side and the solder coming in from the other side, press the soldering iron gently against the bottom of the pin where it meets the circuit board. Once the existing solder melts, feed some new solder into the L-angle between the pin and the board on the opposite side from the soldering iron, so that it flows around the pin. Quickly remove the solder, then the soldering iron, and leave the board without moving or bumping it for at least ten seconds. Then move on to the next pin.

I can't find any service information, but we know the pinouts for those ICs. We need to know how those output ICs are used though. One of them will be used as a stereo amp, for the left and right speakers, and the other will be used as a mono amp, in bridge-tied load configuration (BTL), for the subwoofer.

You can check the DC voltages on some pins of the TFA9842J ICs. They should be about the same on both ICs.

Pins 2, 6, and 8 should all be about half the supply voltage, i.e. about 10V.

Pin 7 should be at least 2V. If it's less than that, the device is being held in mute or standby mode.

You can also check for power supply voltage on the two 4558 ICs. Pins are numbered from 1 to 8 anticlockwise (looking down on the IC from above), starting with the pin anticlockwise of the semicircular indentation, like this:

Pin 8 on both of them should measure at least +12V, probably close to +20V. Pin 4 on both of them should measure 0V.

All of these voltages should be measured relative to the 0V rail of the circuit. This is the large tinned copper area that I've highlighted in blue on this photo. You can connect your multimeter black lead to any component that connects to that copper. I suggest you use one of the four large diodes in the bridge rectifier in the power supply section.

 

Thanks for jumpin' in Kris. You've got more tricks than I do

 

It seems unlikely that both power amp ICs would fail. Pin 7 of each output IC is the mute/standby control input. These inputs on the two output ICs are connected together. Check this voltage; if it's more than 2V lower than the supply voltage on pin 9 (the 20V rail), the output ICs are being held in mute or standby.

This signal appears to come from the wire that connects at the front of the board. I traced it from the output ICs, through a link wire that's obscured by the red/black wires, then through a link wire called JMP4, and through another called JMP1 which is just north-west of the area where the wire enters the board. From there, it appears to go to the fourth pad (counting from the left) of the wire connection. You'll need to find where that wire goes, to figure out why the voltage is wrong. If you can't figure it out, you could just remove one of those wire links and tie those IC inputs to the 20V supply rail.

Edit: BTW you did a great job with those photos! The two sides line up almost perfectly with no adjustment. That has made it very easy for me to follow the signals.

 

Agreed. Wonderful job. I stuck the two pictures together and did a minor tweak. This is why clear pictures helps

 

Good job on the find there.
Caution should be used if you replace the cable, as it does appear to carry audio and using the wrong cable could pick up noise.
I've got three suggestions, we'll see what Kris thinks.
Add an additional wire and run it along side the current cable.
Use Cat6 ethernet cable and replace the whole cable. * (my pick, carefully using the pairs to carry audio to reject noise that may be picked up by using unshielded cable)
Simply add a push button or toggle on the amp to bypass the blue wire.

 

I wouldn't use Ethernet cable; it should be shielded. I'd go with Roberto's suggestion in post #18 - add another wire, either spiralled around the existing cable or held to it with rubber bands or cable ties, to replace the broken one.