Hi All,
I'm new to the forum trying to fix a Boston Acoustics powered subwoofer that came with the MCS130 Home Theater system. The sub started to develop a humming noise that initially went away, at least temporary, by moving the RCA signal cable, ie, moving it in relation to the power cable.
Now the humming is loud and permanent regardless if the audio signal cable is connected or not. The unit will hum by just being powered on, humming level will vary when the volume know is turned either up or down. Humming noise sounds like a typical 60Hz humming noise. Humming will get louder by increasing the volume.
By reading this post about the "the dreaded Boston acoustic VR2000 sub woofer hum" http://www.diyaudio.com/forums/subwoofers/231605-dreaded-boston-acoustic-vr2000-sub-woofer-hum.html it looks like I'm facing a similar issue so I'm going to replace the power supply main capacitors even though the values from the VR200 and the MCS130 sub are a little different. On the MCS130 the power supply capcitors are:
2 x 4700uF 63v black caps
2 x 1000uF 35v blue caps
Now, there is another blue capacitor (I think) that is connected right at the power supply transformer's connector (see picture). Unfortunately I'm not sure if the markings on the capacitor are complete, ie, somewhat faded so I'm not sure if I have all the info I need and/or if I actually need to replace it. What I can read on the blue capacitor label is "MD .1k 250V" Not sure if the dot "." is actually a dot a not the remains of some other label that faded. I got this unit a remanufactured system, so I guess this capacitor got installed when it the unit was remanufactured since it doesn't look as part of the original circuit design.
The other symptom that I'm not sure is related to main humming problem, probably caused by bad power supply capacitors, is that if I put a jumper between the RCA negative/ground to the board metallic bracket, and even touching the RCA ground with my hand, the humming will be almost gone, it'll still be there but lower than without the jumper and/or my hand touching.
The other symptom this unit has had since I got it, was a very low humming when the unit was active. This unit will "shutdown" or go quiet when there wasn't a signal for about 5 minutes. Could there be another capacitor somewhere that is causing this ??. I guess the schematics will be needed but I couldn't find them. For now all I want is to cure the loud humming so I can actually use the sub again with the Homer Theater system.
So, based on all this, my questions are:
- Should I change this blue capacitor connected directly to the transformer ??, if yes, what will be the right spec ??. Doing a search on "MD .1k 250V" I got to http://www.mouser.com/ds/2/449/mfpoly-205889.pdf but I'm not sure if the 1K is just a 1k uF or some other capacitance.
- Is there a way to actually determine if these capacitors are bad ??, I guess is just cheaper to replace them, but I'm curious if there is a way without having an oscilloscope and just a DMM.
- Polarity of the caps. The black 4700uF capacitors doesn't have a clear "-" or "+" symbol, there is just a white band on one side with somewhat of an horizontal zero "0" symbol, not sure if these is the negative side. On the blue ones there is a black band with somewhat of a rectangle sign, maybe a big "-" symbol. Are these the negative side of these capacitors ??
- Anything else I should be looking for ??
Thanks in advance...
I'm new to the forum trying to fix a Boston Acoustics powered subwoofer that came with the MCS130 Home Theater system. The sub started to develop a humming noise that initially went away, at least temporary, by moving the RCA signal cable, ie, moving it in relation to the power cable.
Now the humming is loud and permanent regardless if the audio signal cable is connected or not. The unit will hum by just being powered on, humming level will vary when the volume know is turned either up or down. Humming noise sounds like a typical 60Hz humming noise. Humming will get louder by increasing the volume.
By reading this post about the "the dreaded Boston acoustic VR2000 sub woofer hum" http://www.diyaudio.com/forums/subwoofers/231605-dreaded-boston-acoustic-vr2000-sub-woofer-hum.html it looks like I'm facing a similar issue so I'm going to replace the power supply main capacitors even though the values from the VR200 and the MCS130 sub are a little different. On the MCS130 the power supply capcitors are:
2 x 4700uF 63v black caps
2 x 1000uF 35v blue caps
Now, there is another blue capacitor (I think) that is connected right at the power supply transformer's connector (see picture). Unfortunately I'm not sure if the markings on the capacitor are complete, ie, somewhat faded so I'm not sure if I have all the info I need and/or if I actually need to replace it. What I can read on the blue capacitor label is "MD .1k 250V" Not sure if the dot "." is actually a dot a not the remains of some other label that faded. I got this unit a remanufactured system, so I guess this capacitor got installed when it the unit was remanufactured since it doesn't look as part of the original circuit design.
The other symptom that I'm not sure is related to main humming problem, probably caused by bad power supply capacitors, is that if I put a jumper between the RCA negative/ground to the board metallic bracket, and even touching the RCA ground with my hand, the humming will be almost gone, it'll still be there but lower than without the jumper and/or my hand touching.
The other symptom this unit has had since I got it, was a very low humming when the unit was active. This unit will "shutdown" or go quiet when there wasn't a signal for about 5 minutes. Could there be another capacitor somewhere that is causing this ??. I guess the schematics will be needed but I couldn't find them. For now all I want is to cure the loud humming so I can actually use the sub again with the Homer Theater system.
So, based on all this, my questions are:
- Should I change this blue capacitor connected directly to the transformer ??, if yes, what will be the right spec ??. Doing a search on "MD .1k 250V" I got to http://www.mouser.com/ds/2/449/mfpoly-205889.pdf but I'm not sure if the 1K is just a 1k uF or some other capacitance.
- Is there a way to actually determine if these capacitors are bad ??, I guess is just cheaper to replace them, but I'm curious if there is a way without having an oscilloscope and just a DMM.
- Polarity of the caps. The black 4700uF capacitors doesn't have a clear "-" or "+" symbol, there is just a white band on one side with somewhat of an horizontal zero "0" symbol, not sure if these is the negative side. On the blue ones there is a black band with somewhat of a rectangle sign, maybe a big "-" symbol. Are these the negative side of these capacitors ??
- Anything else I should be looking for ??
Thanks in advance...
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