Sub Issue : Sub Low End Drops off and stays at a consistent low volume

[NellTones]

Been having a weird issue with my KRK 10 S subwoofer. Hopefully someone might be able to give some insight on what’s going on.

So when I got my sub it worked great, low end sounded amazing but the issue I’m having is the low end drops off and stays at a consistently low volume even when I turn the volume knob up high. If I turn my foot switch off and then on again the low end hits properly for a second then drops off again to the consistent low volume. If I turn the volume knob down and up again, again hits properly for a second then drops off again to the low consistent volume. No matter how high I turn my volume knob up the sub stays at its low consistent volume.

Thinking it could possibly be a component issue in the circuitry that may have gone bad?

Thanks again! Can’t find anything on the internet about this issue and I’m just baffled so Anything helps!

 

[crutschow]

It does sound like something in the circuit is bad.
May be difficult to troubleshoot.
Do you have any info on the circuit so can can check the supply voltages?

 

[Lightium]

Looks like it's a class D amplifier. Can you post pics of the board top and bottom? Not too big, don't want to scroll forever.

 

[NellTones]

Here’s some photos also noticed this almost burnt looking resistor (last picture)

 

[NellTones]

Looks like it's a class D amplifier. Can you post pics of the board top and bottom? Not too big, don't want to scroll forever.

Posted some photos and noticed this almost “Burnt” looking resistor on one of the boards (last picture)

 

[picbits]

My sub does this sometimes, I've got a faulty/dirty volume control pot and sometimes a few cycles gets it up and running again.

 

[Nigel Goodwin]

Yes, R476 looks badly burnt-up, but without a schematic it's impossible to know what value it is, and what it does?.

 

[NellTones]

Yes, R476 looks badly burnt-up, but without a schematic it's impossible to know what value it is, and what it does?.

Unfortunately I’m having a very difficult time finding a service manual or schematic on this sub but I did find a picture of the resistor in question, just a little difficult to make out the color bands

 

[Lightium]

It looks like a 1/8th watt resistor.

 

[Lightium]

Is this the subwoofer? https://www.crutchfield.com/S-H1iahepremm/p_848KRK10S/KRK-10s.html

 

[NellTones]

Is this the subwoofer? https://www.crutchfield.com/S-H1iahepremm/p_848KRK10S/KRK-10s.html

Yes that would be it

 

[Lightium]

See here you may be able to order the part or board. For replacement part quotes call 1-800-4GIBSON Option 1

 

[Nigel Goodwin]

It looks like a 1/8th watt resistor.

That's entirely unhelpful

 

[NellTones]

turns out it’s a 300R resistor

 

[Nigel Goodwin]

turns out it’s a 300R resistor

Do you know what it does in the circuit?, if a resistor burns up it's usually because something caused it to, and just replacing the resistor tends to mean it just burns up again.

 

[NellTones]

Do you know what it does in the circuit?, if a resistor burns up it's usually because something caused it to, and just replacing the resistor tends to mean it just burns up again.

It runs in between from the CN401B pin 6 to the 300R resistor to -47V
Not quite sure what it’s doing, I’m not to knowledgeable it terms of circuit functions I just know how to replace parts and solder

 

[NellTones]

Do you know what it does in the circuit?, if a resistor burns up it's usually because something caused it to, and just replacing the resistor tends to mean it just burns up again.

CN401B is a DIP Pitch 2.54MM 7Pin

 

[NellTones]

Do you know what it does in the circuit?, if a resistor burns up it's usually because something caused it to, and just replacing the resistor tends to mean it just burns up again.

Also CN401B connects to CN401A which pin 6 on CN401A runs to -vcc
Hope this helps?

 

[Nigel Goodwin]

Also CN401B connects to CN401A which pin 6 on CN401A runs to -vcc
Hope this helps?

I'm afraid not, we really need a schematic to see what's going on.

However, there is a schematic for a KRK RP10S, though there's no resistor of that designation. BUT, there are a couple of 270 ohm resistors, R338 and R339, which feed +vcc and -vcc to a pair of comparators (U3-A/B) which feed a muting circuit using FET Q6. Presumably this is to do with anti-thump?, as the amplifier turns ON and turns OFF.

So that resistor having burnt-up (I presume you've tried measuring it with your meter?, and it's O/C?) would mean the -vcc feed to the comparator is missing, so presumably causing it to stay muted?.

There's nothing in that circuit though that could cause R339 (the -vcc feed in the RP10S) to burn up, as it just feeds to a 47K resistor, so would need a short to ground from the output of the burnt-up resistor to damage it.

But that's just guessing, based on the RP10S schematic.

 

[Lightium]

You'll have to pull the component out and see if it's burned on all sides.