Broken subwoofer, any ideas?

[Roze]

Hi,
I'm been struck by a case of a broken subwoofer and given the relatively high price of the subwoofer, it would be nice to be able to fix it.

Symptoms:
Upon starting the subwoofer it "chokes" heavily a couple of times before picking up in frequency ending in a constant VERY loud humming, I'm gussing the end frequency is about 50 Hz cause that is the mains frequency here.
The volume/etc controls on the back have absolutely no effect, the humming wont go away.
Upon connecting a sound source, you can VERY faintly hear the sounds of the music, but the humming is so loud its barely noticable.
Upon disconnecting the sound processing card from the transformer/output stage, the is no humming at all so my guess is that the fault is somewhere in the audio processing.

Controls/input plate

Model plate


Board front 1

Board front 2


Board back 1

Board back 2


Any suggestions are welcome.

Kind regards, me

[tszaboo]

Where are the power elements? This board surely wont deliver 100W. Something is missing from the pictures.

[Roze]

Sadly, I could not bring the power-board with me home. But, as far as I could tell, that board had one Really big Heat-sink mounted thing with lots of pins, 2x TO-220, a bunch of caps and some more. The main transformer supplies -30 V, 30 V and GND.

But as I said, the hum/noise is present regardless of if this board has audio connected or not, and its completely quiet if this board is disconnected, so the fault supposedly is in this one. Maybe in the part supplying the voltage reference for the signal processing. I wont have access to the subwoofer for another week, but I'll take pictures of that board then too.

[envisionelec]

It could be as simple as a faulty OPAmp. Generally, the first one in the signal chain can be damaged by static electricity discharge. If the output is stuck to a rail, it may try to inject a lot of current into another stage and oscillate. You will need to disconnect the loudspeaker and attach a high power resistor during troubleshooting.

P.S. The PCB on that amp was made with EAGLE. I can tell because someone left the >NAME symbol in the silkscreen layer around the potentiometers. And it was auto-routed (poorly).

[Roze]

There are three MC33079DG OP-amps on that board, I guess I could start by changing them, only like $1.5 anyway.
There is a ic-component I cant find any datasheet for, close to the input stage:


Also, to note, Someone seems to have plugged the output from the amplifier into the output of the subwoofer, instead of to mono input. I guess this could have fried one or two things.

[elgonzo]

There is a ic-component I cant find any datasheet for, close to the input stage:

Try searching for "ST 074C", and you will find.... "074C" is the marking for the TL074C (a quad JFET op-amp).

[Roze]

Thank you all.
I shall get to work with replacing all op-amps as soon as the package arrives from my local vendor, good thing both where available so close by.
Will be returning with the results when I have tried it.

[Grapsus]

It could be as simple as a faulty OPAmp.

+1 I had 5.1 system with one channel spitting unpleasant sounds and it was a faulty pre-amplifier op-amp.

Also, you should really make sure that the power supply is clean. As you have only one channel, the humming could be coming from the power rails too.

[Roze]

It's a Mains-connected subwoofer with 3-pin regular power cord, however, the earth/ground wire is unconnected.
And I have tried with cell phone. As stated before, connected/disconnected audio-source has no real effect on the noise. Except those really faint beats greatly overpowered by the noise when audio source is connected.

[Tim F]

My guess - output transistor has failed short circuit, what you are hearing is the 100Hz ripple on the powersupply rail because it's being loaded down short circuit through the speaker voice coil. I would not run it long with the speaker connected as you may blow the speaker.

See here for basic troubleshooting:
https://www.eevblog.com/forum/repair/audio-amplifier-repair/msg625490/#msg625490

[Roze]

But if it where the output transistor, wouldn't it still sound with the pre-processing board taken away?


With low and high power boards disconnected its totally quiet, and all power transistors are on the high power board, if this was short-circuited, the sound "should" be there even with them detached. I'm gonna assume faulty OP's, at least initially.

[techricky]

The control panel does have an auto power function switch though so the signals board is able to power off the main amplifier board. This may be the case when it is disconnected.
It could be done with a relay in the main supply to the output amp or by biasing it off by other means.

[Roze]

I really hope its not any of the power transistors as there are only two TO-220 and one really fat transistor (the only one mounted to a heat sink) with about 10-20 pins, I only know that its dual line TO-package, not really sure about the designation for those kinds of packages.

*holding thumbs for failed op-amps*

[radioFlash]

I really hope its not any of the power transistors as there are only two TO-220 and one really fat transistor (the only one mounted to a heat sink) with about 10-20 pins, I only know that its dual line TO-package, not really sure about the designation for those kinds of packages.

*holding thumbs for failed op-amps*

That thing with many pins mounted on the heatsink is probably a chip amp, something like a LM3886 http://www.ti.com/product/lm3886 . Chip amps are integrated parts which included the high power output transistors. Can you get the part number on it?

[Roze]

I'll try when I get access to it next time. Probably around Wednesday or so. I only took the low level processing board with me.

[Roze]

Now I got the pictures for the output stage:
( http://www.st.com/web/en/resource/technical/document/datasheet/CD00002107.pdf )

[Roze]

There is a ic-component I cant find any datasheet for, close to the input stage:

Try searching for "ST 074C", and you will find.... "074C" is the marking for the TL074C (a quad JFET op-amp).

Which would be the alignment of this one? Normally there is a white line but not on that one, and the text alignment is making me unsure.

[radioFlash]

There is a ic-component I cant find any datasheet for, close to the input stage:

Try searching for "ST 074C", and you will find.... "074C" is the marking for the TL074C (a quad JFET op-amp).

Which would be the alignment of this one? Normally there is a white line but not on that one, and the text alignment is making me unsure.

The edge along the bottom of the text has more of a chamfer, so I think pin 1 is is close to the "S" in ST. Your other photo shows the TDA7294 which is the chip amp. Pins 2 and 3 on the TDA7294 are for the signal input and output is on Pin 14. You should check and see what you get on Pins 2 and 3 with no signal and what you get on pin 14 for the output.

[SoundTech-LG]

Exactly.

With no input, you should have nothing on the output. ESPECIALLY DC. With DC you'll have a hella hum on the output with the woofer across the power rails essentially.