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Altec Lansing Sub of computer speakers

 
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ndeleal
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PostPosted: Wed Aug 19, 2009 7:42 pm    Post subject: Altec Lansing Sub of computer speakers Reply with quote

My wife accidentally shorted the input line which has caused a 60 cycle hum that increases when the bass is adjusted up on the speaker control. The capacitors have tested good, but I am unsure how to test the amplifier circuit. The circuit has three transistors with five legs each factory labeled as UTC2030. Does anyone know how to test them without burning them out, or have any other suggestions as to fix the problem?
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plcnut



Joined: 18 Aug 2009
Posts: 6
Location: Missouri, U.S.A.

PostPosted: Wed Aug 19, 2009 10:15 pm    Post subject: Reply with quote

how did you test the caps? a regular DMM that has a cap. setting doesn't really mean anything.
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Wed Aug 19, 2009 10:30 pm    Post subject: Reply with quote

UTC2030 is not a transistor. It is an audio IC that can be used in various configurations and there is no standard way of testing the device. Just about all you can do is to pull up the data sheet and do a voltage testing which wont tell you much.
What did you mean by "shorting out the input"?...like when hooking it up?
If you encounter a 60Hz hum that increases with volume, it is usually indicative of an open ground loop such as a bad or broken connection/open trace etc in the amplifier input stage.
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ndeleal
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PostPosted: Thu Aug 20, 2009 2:59 pm    Post subject: Reply with quote

The 3.5mm input was disconnected from the computer, but the speaker was powered on. She shorted across the input with a piece of metal, which resulted in the hum. the hum is only present when the bass level is turned up. If the bass is turned all the way down, the master volume can be turned up all the way with no hum and the regular speakers work fine. The caps are 2200 uF, and were removed to be tested with a cap tester. With the caps removed the hum was constant across all outputs, but only on the bass with the caps in place.
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