JBL BALBOA 230 SUBWOOFER amplifier oscillating
I'm trying to diagnose this amp but my knowledge run out and need advise.
The transistors on the amplifier are rapidly heating up after power on and the speaker produces a wobbly humming noise. I can also measure a lot of weird things on the board.
Checks I did so far:
there is no schematic available for this exact amp but the JBL Sub10 schematic is 95% the same so i have used that
I thought it was something with the bias circuit but i cannot really find a faulty component. Maybe opamp? or something on the preamp board? I'm not sure.
The transistors on the amplifier are rapidly heating up after power on and the speaker produces a wobbly humming noise. I can also measure a lot of weird things on the board.
Checks I did so far:
- visual inspection for burns, bulged caps and so
- speaker terminal measures 0V during operation
- checked all transistors for short and voltage drop - they all pass
- checked all capacitors nearby transistors as well resistors
- no shorts anywhere
- Scoped the transistor and I can measure a 5Mhz sine wave on the emitter
- when preamp board is not connected no signal and no heat
- TIP35 and 36 Transistor Vce measures 6V
- Cement resistors behind transistors measure 145mV difference between the two
there is no schematic available for this exact amp but the JBL Sub10 schematic is 95% the same so i have used that
I thought it was something with the bias circuit but i cannot really find a faulty component. Maybe opamp? or something on the preamp board? I'm not sure.
Attached Files
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