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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Monsoon MH-505 speaker problems
I'm trying to fix a set of Monsoon MH-505 computer speakers. These have
a subwoofer and 5 satellite speakers. The problem is that when you power it up the subwoofer's cone pulls all the way in and you get a fairly loud hum from it. The other speakers do seem to work, albeit not very loudly. The speaker output for the sub seems stuck down at -14 volts with no audio input. It's driven by a Philips TDA8510 amp chip (there are two of these in the unit). The inputs for the sub channel on that chip (pins 15 and 17) are sitting up at around 5 volts while the inputs on the other channels on that chip and on the other TDA8510 chip are all sitting at around 2.1 volts, which it seems likely has something to do with it. That input seems arranged differently in the circuit - the other inputs have a cap in series with them before the amp chip but this one doesn't, as far as I can see. It goes through a cable to the other side of the board, through a resistor (measures about 150 ohms), looks like it's bypassed to ground by a cap, and then into pin 14 of another chip, made by ST and labeled 074C 90X124. Not sure exactly what that it is, but seems like it may be an LM124 quad op-amp. There are two of these chips - assuming they're LM124s, all of the input and output pins are sitting at around 5 volts. I think there's something wrong on the input of the TDA8510, but I can't really see what would cause the voltage to be any different than it is.. it's the kind of situation where I can't see how it worked in the first place! Anyone have any suggestions? -- Robert Hancock Saskatoon, SK, Canada To email, remove "nospam" from Home Page: http://www.roberthancock.com/ |
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Robert Hancock wrote:
I'm trying to fix a set of Monsoon MH-505 computer speakers. These have a subwoofer and 5 satellite speakers. The problem is that when you power it up the subwoofer's cone pulls all the way in and you get a fairly loud hum from it. The other speakers do seem to work, albeit not very loudly. The speaker output for the sub seems stuck down at -14 volts with no audio input. It's driven by a Philips TDA8510 amp chip (there are two of these in the unit). The inputs for the sub channel on that chip (pins 15 and 17) are sitting up at around 5 volts while the inputs on the other channels on that chip and on the other TDA8510 chip are all sitting at around 2.1 volts, which it seems likely has something to do with it. That input seems arranged differently in the circuit - the other inputs have a cap in series with them before the amp chip but this one doesn't, as far as I can see. It goes through a cable to the other side of the board, through a resistor (measures about 150 ohms), looks like it's bypassed to ground by a cap, and then into pin 14 of another chip, made by ST and labeled 074C 90X124. Not sure exactly what that it is, but seems like it may be an LM124 quad op-amp. There are two of these chips - assuming they're LM124s, all of the input and output pins are sitting at around 5 volts. I think there's something wrong on the input of the TDA8510, but I can't really see what would cause the voltage to be any different than it is.. it's the kind of situation where I can't see how it worked in the first place! Anyone have any suggestions? Figured it out - what I thought was an (SMT) resistor in series between the op-amp and the TDA8510 chip was actually supposed to be a capacitor, except it was leaky. Replacing that cap with a new one appears to have fixed it. -- Robert Hancock Saskatoon, SK, Canada To email, remove "nospam" from Home Page: http://www.roberthancock.com/ |
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