Try to fix my sub but got stuck. Advice? - diyAudio
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Old 4th August 2007, 02:57 AM   #1
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Question Try to fix my sub but got stuck. Advice?

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 . Any advice is greatly.... greatly appreciated.

Here are some photos of the board.
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Old 4th August 2007, 03:52 AM   #2
OzMikeH is offline OzMikeH  Australia
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Pictures??
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Old 4th August 2007, 04:38 AM   #3
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Sorta sounds like maybe the rectifiers have poofed.
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Old 4th August 2007, 05:30 AM   #4
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The power transistors are mounted to heat sinks, usually with with thermal grease between them.

The rectifier should be near the power transformer if their is one. It will probably be square with a hole in the middle and four leads coming out of it.
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Old 4th August 2007, 05:37 AM   #5
Foxx510 is offline Foxx510  Australia
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I would have thought that if your output transistors were blown you would very likely have a blown speaker also. I'd be looking in the power supply area.
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Old 4th August 2007, 08:41 AM   #6
OzMikeH is offline OzMikeH  Australia
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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.
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Old 4th August 2007, 03:26 PM   #7
Petriej is offline Petriej  United States
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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!
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Old 5th August 2007, 03:44 PM   #8
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I really appreciate you all taking time to comment. The information is extremely helpful.

Here's a photo from different angle. Are those output transistors to the right of the heatsink?
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Old 5th August 2007, 03:44 PM   #9
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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.
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Old 5th August 2007, 04:39 PM   #10
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Oh boy.

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.

Sorry I can't really help with the actual issue though.
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