need some help repairing a Jamo SW 3015 active sub please

[khalidlib]

hello everyone i am new to this forum, came here as per the advice of some friends, hoping to find some help repairing an active sub woofer i have for more than 10 years now, it is the Jamo SW 3015 active sub woofer with THX dedicated input , it was in storage for about 3 years since i moved to a new place, and i decided it is time to put my home theater back up ,however after bulging it in the sub and hooking it up to the amplifier pre out it did not make any sound , the on standby light came on but no sound with a faint buzzing sound coming from the amplifier plate, i turned it off and back on again and same story, then the on indicator light went off still the buzzing noise is their, like a small running fan blade hitting a small wire, i took the plate off but there was no fan and the noise is coming from the preamp board or the power supply unit , can't tell for sure but if i disconnect the power from the supply to the amplifier unit the buzz stops, all caps look ok and no burn marks anywhere, after asking around it appeared it was the capacitors at the hi voltage mains input , changed one and the buzz went away, i ordered a better quality and i will put them in when they arrive, sadly that did not fix the subwoofer
still no sound and the indicator led would not come on, i left no stone unturned, looked everywhere for schematics and could not find anything as far as my knowledge goes apparently it does not go very far.
i made a crude sketch of the connections and ics on the board for reference and i noticed a few things
1- the 084C ICs on the small board get hot
2- in the small board the continuity test between column A and column B is + except for the signal line there is no continuity
3- it is printed in stencil on the board that Red line in column A is 12v but i could only mesure 6v
4- the led terminal on the control board measure 2.6v when it is floating but when i connect the led to measure 0.6v
so i am asking for help again please


i am hoping someone with some knowledge or had a similar problem and fixed it could help me out no how to proceed to troubleshoot and fix the problem

[Jon Snell]

Disconnect everything and run the power supply.
Check the correct voltages are outputted first, if not fix the problem.
TL084s getting hot is not a good sign, that to me is a power supply issue and I expect the TL084s will be toast.
Are you confident with repairing class D amplifiers and switch mode power supplies to component level ... if not, take it to someone who is.
Sorry to be blunt but I have seen a lot of people get their fingers burnt fiddling.

[khalidlib]

thank you very much for your reply,
i did what you suggested, thinking that the power supply was the issue, and fixed it so it works ok now , i got 12v and 22v~23v , i am not sure about the 22v~23v value if it is the correct voltage i am supposed to get , it don't say on the board and a schematic is not existing , i looked everywhere with no luck but the polarity is correct with matching values on both rails so i would assume i got the right voltage
as for the TL084s i got no problem changing them but i don't want to change them not knowing if they would instantly go bad again, and endup chasing my tail, in other words i need help in guidance of how to know for sure the defective part, changing it is no problem

[Jon Snell]

The TL084s should not run hot. If they do they are usually damaged due to excessive voltage on them but if you are lucky, they may still be OK.
They are cheap to buy so not an issue.
Try them, they either work or they don't.

[khalidlib]

ok let me try that out, on the other hand i assume that the SEC KA7812 is providing the 12v supply to the red line coming out from the amp board to the small board that have the TL084s , i measured the output at the KA7812 it appears that it is not putting out that voltage, and not getting any input, i am not sure how is the KA7812 getting input voilage, i removed the big capacitor, suspecting it may be the faulty component but it was not the case, what do you think?

[Jon Snell]

ok let me try that out, on the other hand i assume that the SEC KA7812 is providing the 12v supply to the red line coming out from the amp board to the small board that have the TL084s , i measured the output at the KA7812 it appears that it is not putting out that voltage, and not getting any input, i am not sure how is the KA7812 getting input voilage, i removed the big capacitor, suspecting it may be the faulty component but it was not the case, what do you think?

The 7812 is a standard 12 volt 1 Amp regulator. There must be at least 14 volts on pin 1, pin 2 is ground and pin 3 is 12 volt output.
There will be a feed resistor between pin 1 and the power supply that has failed. Find out where the supply to pin 1 has gone and it may restore it to working condition again.
Job to tell without a service manual.

[OssiFT]

Can someone tell me how big this capacitor is ?