Try to fix my sub but got stuck. Advice?

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Hi there,

What a great forum with alot of helpful experts and information. This is my 1st post so please go easy on me.

Recently my Altec Lansing AVS500 subwoofer started giving a hum noise. Quite loud actually. It hums even when i just plug in the power, without anything else connected. Another syntom is, the AC adapter/transformer gets extremely hot. After some googling, I think i have narrowed down the problem. I suspect the subwoofer output transistors have gone bad and hence putting pressure on the transformer. I figure it would be a lot cheaper if i just replace the transistors myself than buying a new set of speakers/sub.

I successfully opened up the sub but i don't know which ones are the transistors :blush:. Any advice is greatly.... greatly appreciated.

Here are some photos of the board.
 

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Now the picture appears finally! It wasn't visible this morning.

Your woofer looks like it has chip amps in it. Chip amps are integrated circuits with many components built in. The black things bolted to the aluminium plate, how many legs do they have? 3 or more?

Are you sure the transformer was not getting very hot before?

Have a look at the capacitors and see if any have burst.
Quite often the larger ones are glued down, so what appears to be stuff leaking out may be glue.

Check the big black one has not broken it's solder joints.

If it's not something simple this probably won't be worth fixing. However it's a good starting point to learn some electronic basics.
 
From your picture, the transistors are in the back, 2 of them clearly visible, fastened to the heatsink with screws? or bolts. It is possible they are not the issue. If the hum is 60Hz (or 50Hz for some countries...) then it may be the rectifier gone bad, leaking in the AC to the driver. If the transformer had gone bad (too) this could have resulted in a dead driver. (120VAC @ 8 Ohms = 1800 W (P=V^2/R)). So if the driver still plays music as well as this hum, it still may be easily fixed.

It may also be the circuitry leaving the rectifier, as OzMikeH said capacitors are used to smooth out the rectified current. If this current is bumpy, then an audible signal may be produced.

It is also possible this is a very difficult fix :(


Good luck!
 
Based on your information, i found the rectifier. It is square and with a hole in the middle but instead of 4, mine has 3 legs. :)

The largest cylinder (black with silver strip) is glued on the circuit board. Is that the transformer?

I have checked the legs of every capacitors to make sure they are not broken, or loosen. So far everything seems ok.

The speaker set has 4 satellite speakers and a subwoofer. They still play music/movie sound fine. It is just when the power is turned off (still plugged in to AC), it gives a hum. I can live with soft, subtle hum noise but this one is a bit loud, even to me. :D
 
Oh boy.:eek:

In the picture, all the small cylinder shaped things to the right are capacitors, as is the large "(black with silver strip)" thing. The thing with 3 pins is a voltage regulator, not a rectifier.

There are two "chipamps" on the PCB; no output transistors. One chipamp is for the sub, and the other is for the satellite speakers.

There is no transformer inside the sub. The power comes the large plastic box that gets plugged into the wall.

This looks exactly like a unit someone threw out at work a few weeks ago. I opened it up to see what was inside, decided is was worthless, and put it back in the trash.:smash:

Sorry I can't really help with the actual issue though.
 
Hi there,
this is the first time I am logging in. I have the same model that gives a very loud humming for about a minute and then quietens down. After this all the speakers work normally. This happens every time the set is switched off for a while. Replacing the condenser in power supply does not solve the issue. Rectifiers are OK. All filter condensers on pcb in the woofer are ok. output IC TDA 2007A (3 of them are OK). I gather that this humming is due to a bad filter condenser in the input stage or a defective feedback condenser that starts working after the set is on for while. Have experienced this in feedback circuits of smps supplies etc. I have got the schematic diagrams of the other Ics ( LM324N, MC 4558CN, TDA 8199). Can anyone give me the circuit diagram of the set. Better if someone has found a solution to it. Thanks
 
Hello everyone I have solved the issue of output IC overheating and possibly shorting in the AVS 500. If you have replaced the power supply condenser and still not solved the problem of humming (more like motor boating) then read on.The output circuit uses 3 ICs TDA 2007A. Two of the ICs are used in normal stereo mode (6+6w) for the surround and satellite speakers. IC 10 is used in bridge configuration (12w) for driving the sub woofer. This circuit uses six electrolytic condensers of value (0.15mf){ C93, C94, C65, C66, C77, C78} for preventing the output going into high frequency oscillations. Unfortunately the engineers in Altec lansing seem to have not realized that these condensers being polarized will not perform as they are supposed to do and the whole circuit goes into unstable high frequency oscillations that bog down the power supply due to over current and hence the humming sound from the sub woofer. It reduces after sometime but the 3 ICs tend to get very hot as the high frequency oscillation (not audible to human ears) persists. Solution is very simple. Replace all six condensers with metal polyester types. (0.1mfd, 50V). Happy listening :)
 
Hello everyone I have solved the issue of output IC overheating and possibly shorting in the AVS 500. If you have replaced the power supply condenser and still not solved the problem of humming (more like motor boating) then read on.The output circuit uses 3 ICs TDA 2007A. Two of the ICs are used in normal stereo mode (6+6w) for the surround and satellite speakers. IC 10 is used in bridge configuration (12w) for driving the sub woofer. This circuit uses six electrolytic condensers of value (0.15mf){ C93, C94, C65, C66, C77, C78} for preventing the output going into high frequency oscillations. Unfortunately the engineers in Altec lansing seem to have not realized that these condensers being polarized will not perform as they are supposed to do and the whole circuit goes into unstable high frequency oscillations that bog down the power supply due to over current and hence the humming sound from the sub woofer. It reduces after sometime but the 3 ICs tend to get very hot as the high frequency oscillation (not audible to human ears) persists. Solution is very simple. Replace all six condensers with metal polyester types. (0.1mfd, 50V). Happy listening :)

Thanks very much for this useful post. My problem was C93. Replaced it with a polyester non-polarised cap (0.1μF).

The 'motorboating' has stopped!
 
I had a similar problem with my Gale Storm 8 Subwoofer just last week. After changing all the caps and re-inserting the TDA7377 amp I discovered the problem wasn't in the two PCBs at all but was in the black box thing you plug in the wall socket. This produces 12v A/C. The full-wave rectifier is mounted in the speaker unit along with the amp chip. I (last resort this) substituted another 12v. A/C plug in (from an old scanner, actually) and the rumble and hum just ... went away! It's a cheap repair! Worth a try.
Michael UK
 
Great suite! I also have Altec Lansing AVS500 the power supply and bottom of sub get very hot. Humming sound dissipates slightly after warm up. Faint static noise from all 5 speakers. I have the tools to replace capacitors, but my electronic knowledge is very limited. So, with more time than money, I'll order a few .1uf metalized polyester capacitors from ebay (China). As a precaution should I replace the power supply condenser (don't have a capacitor tester)? I'll order that condenser if someone would be kind enough to provide the specs. Thanks
 
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