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 85 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?