KRK 10S Subwoofer: RED LED

[higgsboson]

Greetings fellow DIY enthusiasts! I find myself knee-deep in a quest to revive my trusty KRK 10S subwoofer, and I'm reaching out to the community for some collective wisdom. Here's the lowdown on my predicament:

1. The Red LED Dilemma:

The indicator LED on the back stubbornly glows red.
No sound output, not even a whisper of clicks or pops.

2. DIY Detective Work:

Visual inspection of the circuit board revealed a bulging capacitor with a 470uF/16V spec.
Decided to replace it, along with its capacitor neighbor sharing the same spec. Seemed like the logical thing to do.

3. Post-Capacitor Swap:

Swapped the capacitors, but the red LED still proudly shines with no change in symptoms.

4. Schematics Surprise:

Got my hands on the schematics from the service manual, and they match the circuit board exactly. Same IDs and everything.
Schematics state that the capacitors in question should be 100uF/35V. Confusion sets in.

5. Factory Spec Confusion:

Original soldering suggests the capacitors were installed at the factory with the 470uF/16V spec.
Scratched my head wondering why the variance in specs.

6. Black Gunk Dilemma:

Aware of the notorious "black gunk/glue" phenomenon in this brand, I carefully scraped away some dried-up sections.
Visual inspection shows the components beneath are in good shape and no conductivity (hey! amateur here!) was detected under the black glue.
RED led still on.

7. Power Checks:

Confirmed +15V and -15V are reaching the AMP PCB, so the power supply seems intact, should I test the +47 and -47? Should I have special precautions there?.

8. Power Checks (continuation):

Confirmed +47V (≃50V) and -47V (≃50V) are reaching the AMP PCB, so the power supply seemed intact. However this time I left it connected for longer and I saw a bit of smoke coming from the AMP board, and then everything turned off.

9. Fuse check after the power checks:

Since the entire circuit shut off, I chcked the fuse and now its blown (it wasn't before).

10. Next Steps:
Seeking advice on the next steps. What could be causing the persistent red LED?
Despite being an electronics amateur, I'm diving into the schematics, but it's proving to be a challenge.

Conclusion:

And there you have it—the current state of my DIY venture with the KRK 10S subwoofer. Now, I'm turning to you, the experienced DIY community, for insights, suggestions, or perhaps a fresh perspective on what my next move should be.

Any thoughts are greatly appreciated as we work together to bring this subwoofer back to its booming glory!

Schematics:
 KRK10S_Schematic.pdf (5538.98 kB - downloaded 204 times.)

[higgsboson]

Attachments

[Vicus]

I bet there is a large DC offset at the output. You can measure it on L1.

[higgsboson]

I bet there is a large DC offset at the output. You can measure it on L1.

Thanks! How can I measure that? What exactly is this L1?

The problem right now is that everytime I turn it on with the AMP board connected to the power supply, the fuse blows after I hear a slight heating up noise on the amp board.

The fuse does not blow if the AMP pcb is disconnected

[beatman]

It looks like output transistor short.Check the Q442 to Q446 all 6 output devices with digital multimeter (DMM) in diode mode.If there is short component check and the big white and surround  small resistors with DMM in the ΩΜ mode with one leg unsoldered .Check the small To-92 transistors and diodes with DMM  in diode mode.

[higgsboson]

It looks like output transistor short.Check the Q442 to Q446 all 6 output devices with digital multimeter (DMM) in diode mode.If there is short component check and the big white and surround  small resistors with DMM in the ΩΜ mode with one leg unsoldered .Check the small To-92 transistors and diodes with DMM  in diode mode.

Hello! These are the measurements I got from Q441 to Q446:

Digital Multimeter used: Uni-t UT33D

Output Transistors (DMM in diode mode)

Type	Transistor	Base probe	Collector	Emitter
NPN	Q441	black	484	499
NPN	Q441	red	479	495
NPN	Q442	black	002	002
NPN	Q442	red	002	002
NPN	Q445	black	035	035
NPN	Q445	red	035	035
PNP	Q443	black	034	035
PNP	Q443	red	034	035
PNP	Q444	black	479	495
PNP	Q444	red	482	498
PNP	Q446	black	002	002
PNP	Q446	red	002	002

I'm not sure what readings I'm supposed to get since I'm a newbie on electronics. I indicated which probe I've put on Base.

I have to admit I'm a bit confused why I see almost exactly the same values when I switch the base probe (depending on NPN and PNP).

I tested the transistors with them still connected to the amp board (with the amp board completely disconnected from everything else of course).

[beatman]

Q442-Q446 looks suspicious.Just google search how to measure NPN-PNP transistors unsolder and check. found the specs from the output devices in internet and compare.Carefully not damage the pads or board.

[higgsboson]

Q442-Q446 looks suspicious.Just google search how to measure NPN-PNP transistors unsolder and check. found the specs from the output devices in internet and compare.Carefully not damage the pads or board.

I will unsolder them and report back.

I was also testing all the other diodes on the board and I've found that the D420 and the D421 also give a 002 reading (even in reverse direction). Should I unsolder and check those as well?



Another question: Can I test the AMP board without having it connected to the cooler? (Because this R509 connects to the cooler):

[beatman]

The diodes looks all so bad.Unsolder one leg and repeat measure.When you are not sure for short parts don't fire up the amp direct on mains.Istead use a current limiter lamp between mains and device under test  (one or two 40-60 watt) in series to not blow fuses every time.If lamp glows then is still short somewhere.If lamp stays dim after you have found and replace defective parts you are on the right way.

[higgsboson]

The diodes looks all so bad.Unsolder one leg and repeat measure.When you are not sure for short parts don't fire up the amp direct on mains.Istead use a current limiter lamp between mains and device under test  (one or two 40-60 watt) in series to not blow fuses every time.If lamp glows then is still short somewhere.If lamp stays dim after you have found and replace defective parts you are on the right way.

Output Transistors Update: I unsoldered both Q442 and Q446 and the measurements confimed the same thing. Seems those two are shorted. I will order a pair.

Interesting thing: after removing those two transistors from the board, measuring again diodes D420 and the D421 give the expected result (with them still soldered to the board). I've kept them in place. The shorted transistors could be the cause of the erroneous readings I got before?

Current limiter: I grabbed all the parts I need, I'm just missing incandescent lamps (they are incredibly hard to find right now!).


I will make more advancementes on the current limiter, so that when the new ouput transistors arrive I can test the circuit safely.

[higgsboson]

Hello again!

I replaced the faulty transistors (Q442 and Q446), reapplied the thermal glue between the heatsink, insulator micas and output transistors.

I built my current limiter, and tested the subwoofer again, with combinations of 4 bulbs with 40W (tested all combinations).

Result: No blown fuse (tested it only connected throught the current limiter circuit) and we're back to the RED LED on.

Seems we are back to stage 1. What should I do next?

[Nefron_37]

Just had a similar issue here. I removed some gunk near the black relay (K701) on the power amp board.
I also gently tapped on the relay a few times.

Ofterwards I had to power on/off a few times and relay started working again. First yellow led camo on very shortly. After I quess 2 or 3 switches it went on as it usually does.

[erikbrenn]

Hi,

I'm afraid simply replacing Q442,Q446 is not sufficient, and I suspect they are toast again now.  The measured values for Q445 and Q443 are also way too low for PN junctions so they are probably toast as well. So all those 4 output transistors need to be checked/replaced. Their junctions should read above 400

The red led indicates a protection circuit has activated. In your schematic I see 2 protection circuits.


1. A DC protection circuit (DC PRT) monitoring any DC on the output from the power transistors via R703. The AC (music signal) coming via R703 will be filtered through the C702 capacitor, which you replaced. And any DC coming via R703 will turn on Q702, which again turns signal DC PRT low. (Since the capacitor was toast the Q702 might be toast because of high AC on it's base. )

2. An overcurrent protection circuit (PRT) monitoring current through the emitter degeneration resisttors R514,R516, feeding a voltage into base of Q701. In other words, too much current in output stage will pull protection(PRT) signal low.

In normal operation, where there is not overcurrent or DC problem both Q701 and Q702 are off, and the DCPRT and PRT signals are pulled high to 15V via resistor R724. This ensures Q711 turns on and activating the relay. When either DCPRT or PRT signals are pulled low transistors Q709 and Q710 are turned on, which turns OFF Q711 and the relay, AND the LED will turn red from signal on pin3 on CN701A 

However, since the capacitor you replaced played an important role in the DC PRT circuit you might have multiple failures. I would do the following

1. Ensure that the DC PRT circuit is OK by replacing also C703 (not only C702 as you did) AND verify that Q702 and Q714 are ok by measuring their emitter and collector with red probe on base. They should read somewhere above 400

2. Desolder all 4 power transistors Q442,Q445, Q443,Q446 again and check them out of circuit

3.  You *must* also check the voltage amplification transistors driving the 4 output transistors, thus Q441 and Q444 . I would probably also check Q437 and Q439

After you have done the above, and *before* you refit and solder the 4 output transistors, power up the amplifier. There should be no red light and you should hear relay clicking on indicating normal operation.  If there is still a red light indicating a protection fault then also check output protection diodes D420 and D421.

If red light is off, you can now check circuitry before output stage. You can do this with a scope:
Put a 100Hz signal into the RCA input of your sub. The preamp filters will pass this low frequency signal to the power amplifier. You can easily find such a signal on youtube if you don't have a signal gen. 
Using your scope you should now see an amplified signal on the emitters of Q441/Q444 (or on hole where base of Q442 is soldered). Note that it shouldn't matter if your oscilloscope ground is connected to either SGND APGND as they are are connected together via 0 ohm resistors, AND they are isolated from PGND on the transformer.
If you don't have a scope the use a multimeter in AC mode and measure between emitter of Q442 and ground, there should be an output.

If this is ok refit the 4 power transistors and cross your fingers