Electronics > Repair

JBL CS300.1 repair.


I got a JBL CS300.1 audio amplifier for repair, it's main problem is that there's no audio at the output and three indicator LED's (in series) flash. First time taking it apart I found R31 10 ohm resistor burnt down, I replaced it with a working one from an old CFL but the fault was still there. The power supply is in specifications, I tested all the larger transistors and capacitors for faults finding nothing.
The PDF of the device didn't give much clue either, even the PCB drawings are a little bit different: http://www.jbl.com/resources/Brands/jbl/Products/ProductRelatedDocuments/en-US/ServiceManual/CS300p1%20sm.pdf
Any ideas where to look for?

The LED is flashing because the power supply isn't working, it senses the negative leg.  I didn't see where the 10 ohm was, didn't look that hard.  Something is wrong with the inverter section.

I think I found the result of the problem - Q12, Q15 and Q19 mosfets are not driven while Q11, Q13 and Q18 mosfets are turned on with gate pin voltage almost same as source pin and getting warm 30-31C without heatsink, 0,5 and 0,8 volts at the output of inverter. R31 is tied to ground at the power supply area.


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