JBL BALBOA 230 SUBWOOFER amplifier oscillating

[dansity]

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:

• 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.

[dansity]

That tan glue around the large capacitors/resistors is known to go conductive with age. This would explain the humming noise. Not sure about the overheating though.

Thank you for your reply. I have sat two days on it and found the culprit. The oscillation was really weird for me so I traced back that Q114 and C112 capacitor was responsible for stabilizing the voltage for the transistors.
I have pulled out most of the caps and transistors by this time and did not found any damaged so I was sceptical but there it was. The cap had ~60% of the capacity and this was enough to push the entire circuit into oscillation.

[freethink]

That tan glue around the large capacitors/resistors is known to go conductive with age. This would explain the humming noise. Not sure about the overheating though.

[dansity]

That tan glue around the large capacitors/resistors is known to go conductive with age. This would explain the humming noise. Not sure about the overheating though.

Thank you for your reply. I have sat two days on it and found the culprit. The oscillation was really weird for me so I traced back that Q114 and C112 capacitor was responsible for stabilizing the voltage for the transistors.
I have pulled out most of the caps and transistors by this time and did not found any damaged so I was sceptical but there it was. The cap had ~60% of the capacity and this was enough to push the entire circuit into oscillation.