Electronics > Repair
KRK 10S Subwoofer: RED LED
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 :)
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