Klipsh promedia 2.1 subwoofer amp broken

[Dethoff]

A few years ago I purchased the Klipsh Promedia 2.1 speaker system for use with my computer. All worked great until two days ago when I noticed a loud hum emanating from the sub woofer.

I did some trouble shooting which determined the following:

1. The humming is noticed as soon as the power is turned on. It does not change in volume as the volume control for the sats or sub are increased/ decreased.

2. I have tried plugging it into other outlets, which did not help.

3. The hum is there even if the audio input cable is disconnected or connected to another source (tried an mp3 player)

4. The sats seem to work fine.

5. The sub does not seem to work correctly. It seems to put out too low a volume and the sound from it seems distorted.

6. After taking the sub apart, I verified that the hum comes from the speaker (as opposed to an electonic part). The four largest radial electrolytic capacitors on the one board have a brown "gook" around them which I think is an indicator that they have failed and leaked. They are both rated at 4700 uf. One is 63 v, the other 35 v.

Does it make sense that it would be the caps? I would try my hand at replacing them, but after searching radioshack and the net, I can't seem to find capacitors of the same value.

Any advice would be appreciated.

Thanks,

John

 

[Netlist]

Try resoldering the caps.

/Hugo

 

[jaycee]

I'd say either the caps have failed or the (probably pathetic bridged car-amp chip) amplifier has failed. The brown gook is probably adhesive that holds the capacitors to the board for mechanical stability. If they had failed, the tops would be domed or ruptured.

Take some pictures

 

[Dethoff]

I disassembled the electronics so I could get at the boards in question. Upon closer inspection, I do NOT think the "gook" is leaked capacitor electrolyte, but rather adhesive placed there. The caps are not deformed in the sense of bulges or cracks.

With the board out, I can now see another potential problem. The circuit board itself is a brown color, like butterscotch. But, at one place, the board has a really dark brown charred appearance, which I interpret to be a sign of extreme heat exposure. In the vicinity of this discoloration are two resistors which seem cracked and disintegrating.

I will try to take some pictures and post them.

Thanks

 

[Dethoff]

here is a picture of the caps in question

 

[Dethoff]

Here is a picture of the heat damaged area and resistors in question.

 

[Netlist]

Ouch...it looks like there's a little more work to do.
I'd start with measuring the resistors (if still possible) and replace those. A plain resoldering of the board, especially the places that suffered from heat, could solve the problem.
If not, jaycee's tip on the chip might be correct.

/Hugo

 

[Dethoff]

I tried checking the resistors with a multimeter, but I am unable to read the color codes due to the charring. I do not have acess to a schematic, but am inquiring about the resistors values on the Klipsch forum.

If it is just a matter of checking the resistors value and repalacing them, I would be up to the task. Beyond that, I might be over my head.

Thanks

 

[jaycee]

What's on the TO-220 packaged transistors at the edge of the board?

To be honest, the whole board is badly discoloured. You probably have more parts bad there.

Is that a transformer at the top left of the picture?

 

[TheMG]

I think there's more wrong than just the resistors. Resistors don't end up looking like that over time even if they run pretty hot.

If I had to guess, I'd say one or both of those transistors or another part of the amplifier failed and shorted out internally, which would cause the resistors to burn up like that.

There's also a chance that the amplifier failure may have cause damage to the subwoofer driver too if there isn't any DC protection and the amplifier became railed after failure.

 

[Dethoff]

I would like to thank all who offered suggestions and advice. It is obvious that this is not a simple problem to solve and is beyond my capabilities to troubleshoot.

I called Klipsch today. They offered to repair mine for $40 but I would have to pay shipping (approx $20). The turn around time would have been 6 weeks. They would guarantee the work for 3 months.

For $78 (including shipping) they will send me a new unit which carries a year warrantee. They said they could ship it out today.

I chose the new unit rather than wait for the repair.


Thanks again,

John

 

[jaycee]

To be honest, a wise choice. I can almost certainly bet the subwoofer's voice coil would have been toast anyway.