Hi All,
I have a harman kardon TS-7 subwoofer, which I've fixed. The power transistors and drivers were burnt with resistors. I have replaced them and measure the voltage around. The board is the same like JBL SC140 (power supply is adapting to output power from 22V to 45V).
After replacing elements, subwoofer started working fine, but it started oscillates. The voltage +15 and -15V is ok and stable. +22V and -22V as well. The oscillations are started from 100Hz and going up to 250Hz and power transistors are going hot. I have disconnected pre-amp and connect directly the sinus signal to amp board with 0.2V amplitude.
I have clean the limiter board from the glue (common case), but amp enters into oscillation without pre-amp. To check what could be wrong, I've reconnect feedback path from power transistor to 2N5551/2N5401 and it doesn't start oscillate.
The oscillates can be see on attached screen.
When frequency is going up, the system is starting to disconnect the speaker.
What is funny, for the first 2 minutes (when all elements are connected) everything is working fine, no noise and signal is clear, but after these 2 minutes it starting oscillate. Looks like one of the element stop working properly when it warm/hot. I have replaced all capacitors and some transistors. But no luck.
Any ide what could cause that issue?
best regards
M.
I have a harman kardon TS-7 subwoofer, which I've fixed. The power transistors and drivers were burnt with resistors. I have replaced them and measure the voltage around. The board is the same like JBL SC140 (power supply is adapting to output power from 22V to 45V).
After replacing elements, subwoofer started working fine, but it started oscillates. The voltage +15 and -15V is ok and stable. +22V and -22V as well. The oscillations are started from 100Hz and going up to 250Hz and power transistors are going hot. I have disconnected pre-amp and connect directly the sinus signal to amp board with 0.2V amplitude.
I have clean the limiter board from the glue (common case), but amp enters into oscillation without pre-amp. To check what could be wrong, I've reconnect feedback path from power transistor to 2N5551/2N5401 and it doesn't start oscillate.
The oscillates can be see on attached screen.
When frequency is going up, the system is starting to disconnect the speaker.
What is funny, for the first 2 minutes (when all elements are connected) everything is working fine, no noise and signal is clear, but after these 2 minutes it starting oscillate. Looks like one of the element stop working properly when it warm/hot. I have replaced all capacitors and some transistors. But no luck.
Any ide what could cause that issue?
best regards
M.