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?

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Robert Hancock Saskatoon, SK, Canada
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