Hey Guys,
I am a newbie to this forum, and to diy to a lesser extent.
I just purchased a Boston Acoustics digital media theater (was somewhat top of the line computer speakers many years ago) from some jerk on ebay who sent it without the proprietary interconnect cables. Luckily I have some BA stuff here and set it up just fine.
However upon powering it, there is a constant and fairly strong bass humming. The humming stays at a constant level regardless of the music volume. I opened up the box just to see if there was anything obviously and apparently loose or broken inside, but I didn't see anything obvious, and i dont know too much about circuit boards and their components.
Does anyone have the knowledge to diagnose this given the problem I have described?
I am pretty sure it lies within the powered amplification in the subwoofer, but any more than that I just have no idea. Any suggestions would be great.
I am going to post a separate post for this next question, but I am wondering If i can convert this to my home theatre sub, will I have to rip out the old circuitry and amplification and put in a subwoofer amp? or can I do it without power?
I am a newbie to this forum, and to diy to a lesser extent.
I just purchased a Boston Acoustics digital media theater (was somewhat top of the line computer speakers many years ago) from some jerk on ebay who sent it without the proprietary interconnect cables. Luckily I have some BA stuff here and set it up just fine.
However upon powering it, there is a constant and fairly strong bass humming. The humming stays at a constant level regardless of the music volume. I opened up the box just to see if there was anything obviously and apparently loose or broken inside, but I didn't see anything obvious, and i dont know too much about circuit boards and their components.
Does anyone have the knowledge to diagnose this given the problem I have described?
I am pretty sure it lies within the powered amplification in the subwoofer, but any more than that I just have no idea. Any suggestions would be great.
I am going to post a separate post for this next question, but I am wondering If i can convert this to my home theatre sub, will I have to rip out the old circuitry and amplification and put in a subwoofer amp? or can I do it without power?
Humming sub
Hi mate,do you get hum when you disconnect the signal in lead from the sub end? Do you have a Digital Multi-Meter?If a fault lies with the sub itself,put the DMM on volts DC(start at say 20volts)and put the probes on the speaker to check for any DC voltage appearing across the speaker.If you can get to the circuitboard look for 2 big round cylinder shaped componets.These are your Electrolytics for smoothing your powersupply.They could have a number in thousands printed on them,a voltage rating and there orientation by way of minus symbols printed along its body.They're either Blue,Black or grey in colour.
An example
4700uf 64v
Check to see if your getting equal voltages across both of them.
On the board they are connected so that you have
+ .......-+ ..... -
50 volts ov 50 volts
Do you see that plus on one cap is conected straight to the minus on the other?This is your o volts or ground point and all grounds lead to it.This is known as a split rail supply so check them to see if the rails are equal in voltage.Remember that you would reverse the meter leads for the negative rail.Mostly reversing leads apply to Analouge meters.My digital meter shows a negative symbol.
Check these if you can and post result back.And we go on from there.
Cheers Mikee55
Hi mate,do you get hum when you disconnect the signal in lead from the sub end? Do you have a Digital Multi-Meter?If a fault lies with the sub itself,put the DMM on volts DC(start at say 20volts)and put the probes on the speaker to check for any DC voltage appearing across the speaker.If you can get to the circuitboard look for 2 big round cylinder shaped componets.These are your Electrolytics for smoothing your powersupply.They could have a number in thousands printed on them,a voltage rating and there orientation by way of minus symbols printed along its body.They're either Blue,Black or grey in colour.
An example
4700uf 64v
Check to see if your getting equal voltages across both of them.
On the board they are connected so that you have
+ .......-+ ..... -
50 volts ov 50 volts
Do you see that plus on one cap is conected straight to the minus on the other?This is your o volts or ground point and all grounds lead to it.This is known as a split rail supply so check them to see if the rails are equal in voltage.Remember that you would reverse the meter leads for the negative rail.Mostly reversing leads apply to Analouge meters.My digital meter shows a negative symbol.
Check these if you can and post result back.And we go on from there.
Cheers Mikee55
Hey Guys,
Thanks for the replies. On the board, I found 3 caps. 2 of them are small and next to each other, and if i read correctly, are 220uF 25v.
The third one is much larger, like the size of a D battery, and says 22000uF, 25v.
I powered the sub so it was humming, and used a multimeter first on the connection to the speaker, which gave almost nothing as a reading when the multimeter was set at 2.5v. I tried higher settings and nothing as well.
I didnt see any place to check the voltage on the circuitboard, so I placed the probes on the circuitboard on the opposite side of the caps. I did this for both the smaller ones and got no reading, the bigger one was somewhat inaccessable so I havent gotten the chance to try it.
Thanks a lot for your help so far.
I did not mention this last time in thoughts to simplify the problem, and I still dont think it is a factor, but the sub has an external switch/volume control that connects to the sub through a proprietary cable. So, when I plug the ac adaptor into the sub, the light comes on, but I believe it is not actually receiving the power until the volume switch is turned and clicks on. Humming does not occur until the volume/power is clicked on, and no matter what volume you turn it to, humming stays the same.
The volume control is for the satellites which plug directly into the subwoofer as does the external input. As I said before, everything works fine, sub sounds good, sats sound good, and everything else seems to work fine. When music is loud enough, pretty much drowns out the humming(which stays at a constant level).
Thanks for the replies. On the board, I found 3 caps. 2 of them are small and next to each other, and if i read correctly, are 220uF 25v.
The third one is much larger, like the size of a D battery, and says 22000uF, 25v.
I powered the sub so it was humming, and used a multimeter first on the connection to the speaker, which gave almost nothing as a reading when the multimeter was set at 2.5v. I tried higher settings and nothing as well.
I didnt see any place to check the voltage on the circuitboard, so I placed the probes on the circuitboard on the opposite side of the caps. I did this for both the smaller ones and got no reading, the bigger one was somewhat inaccessable so I havent gotten the chance to try it.
Thanks a lot for your help so far.
I did not mention this last time in thoughts to simplify the problem, and I still dont think it is a factor, but the sub has an external switch/volume control that connects to the sub through a proprietary cable. So, when I plug the ac adaptor into the sub, the light comes on, but I believe it is not actually receiving the power until the volume switch is turned and clicks on. Humming does not occur until the volume/power is clicked on, and no matter what volume you turn it to, humming stays the same.
The volume control is for the satellites which plug directly into the subwoofer as does the external input. As I said before, everything works fine, sub sounds good, sats sound good, and everything else seems to work fine. When music is loud enough, pretty much drowns out the humming(which stays at a constant level).
DON'T DO THAT !
You can't just change components at expect it to work,unless you know why it became faulty in the first place.If there was no dc across the speaker terminals then a good sign that its not a major problem.Is there a possibility of an earth connection going stray.Try to establish,with a continuity check and the amp sub turned off.Also check that no signal wire is near a mains line or vice versa.I've had a power wire with 240v in it fall near a circuit and hum was induced straight away.If you ain't too fussed about the hum,just carry on using it a just check it ain't cooking.Until a schematic is availible,not much can be done.
All the best.mikee55
You can't just change components at expect it to work,unless you know why it became faulty in the first place.If there was no dc across the speaker terminals then a good sign that its not a major problem.Is there a possibility of an earth connection going stray.Try to establish,with a continuity check and the amp sub turned off.Also check that no signal wire is near a mains line or vice versa.I've had a power wire with 240v in it fall near a circuit and hum was induced straight away.If you ain't too fussed about the hum,just carry on using it a just check it ain't cooking.Until a schematic is availible,not much can be done.
All the best.mikee55
The sub is part of the Boston Acoustics "Digital Media Theatre" there isnt a model number or anything like some of the other boston acoustics speakers. Digital Media Theatre is the name. They dont make or sell them anymore, and I havent been able to find any documentation on the web anywhere.
subwoofer hum
I have the same speaker system and the same problem. The diference is that i bought them new with a gateway pc in 1999. They worked fine and sounded great but started to hum about 18 months ago. My youngest daughter now uses this pc and as she likes her music loud and this drowns out the hum it's not a big problem. However if there is a repair possible i would like to know as i would like to continue to use them.
I have the same speaker system and the same problem. The diference is that i bought them new with a gateway pc in 1999. They worked fine and sounded great but started to hum about 18 months ago. My youngest daughter now uses this pc and as she likes her music loud and this drowns out the hum it's not a big problem. However if there is a repair possible i would like to know as i would like to continue to use them.
I have the same problem, except I have the humming on the sub woofer and the two satellite speakers. The humming happens whenever they are turned on, even with the volume completely down. It doesn't matter if the source input is connected or not, or if I connect/disconnect the satellite speakers.
I have taken the sub apart to see the circuit boards and see no visual signs of damage. I see the same caps as noted above, except I have a third cap the same size as the smaller two mentioned above.
Since a couple of people have suggested replacing the large cap, I am willing to try it. I have found one at http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=493-1075-ND and wondered if someone could verify if this is the correct type of cap.
Thanks.
I have taken the sub apart to see the circuit boards and see no visual signs of damage. I see the same caps as noted above, except I have a third cap the same size as the smaller two mentioned above.
Since a couple of people have suggested replacing the large cap, I am willing to try it. I have found one at http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=493-1075-ND and wondered if someone could verify if this is the correct type of cap.
Thanks.
Hi,
The cap you have selected looks like it will do the job. However as it is not a long life part and is rated at 1000hrs at 85C it will most likely fail in about the same time as the origonal one. I would try to find a cap with the same value and voltage but rated at 105degrees C and with a longer life. You should be able to find 2000 hour parts fairly easly. I normally use 6000 hour parts in this sort of location but they are a bit harder to come by.
If you have a multimeter you should be able to measure the ripple on this capacitor by setting the meter to the AC volts setting.
I would think that 2V ripple would be about what you would expect to get in this sort of power supply.
If it is much more than this especially greater than 5V then there are two possibilities.
The cap may have failed, this is the most likely problem if the humming came on gradually.
If the humming started one day at a fairly high level or comes and goes in an on off sort of way then it is more likely that one of the diodes in the bridge rectifier has failed. You can check this by going round the bridge with a meter on the diode setting. However it needs to be removed from the circuit to make this test as the transformer windings will mess up the measurements if it is still connected.
If the diode has failed it will most likely show open circuit in both directions. It should show short circuit in one direction and open circuit in the other. i.e when you swap the red and black meter leads round.
The diodes may all be in one package with four pins rather than individual parts. It is still possible for just one of them to fail.
Hope this helps,
Regards,
Andrew
The cap you have selected looks like it will do the job. However as it is not a long life part and is rated at 1000hrs at 85C it will most likely fail in about the same time as the origonal one. I would try to find a cap with the same value and voltage but rated at 105degrees C and with a longer life. You should be able to find 2000 hour parts fairly easly. I normally use 6000 hour parts in this sort of location but they are a bit harder to come by.
If you have a multimeter you should be able to measure the ripple on this capacitor by setting the meter to the AC volts setting.
I would think that 2V ripple would be about what you would expect to get in this sort of power supply.
If it is much more than this especially greater than 5V then there are two possibilities.
The cap may have failed, this is the most likely problem if the humming came on gradually.
If the humming started one day at a fairly high level or comes and goes in an on off sort of way then it is more likely that one of the diodes in the bridge rectifier has failed. You can check this by going round the bridge with a meter on the diode setting. However it needs to be removed from the circuit to make this test as the transformer windings will mess up the measurements if it is still connected.
If the diode has failed it will most likely show open circuit in both directions. It should show short circuit in one direction and open circuit in the other. i.e when you swap the red and black meter leads round.
The diodes may all be in one package with four pins rather than individual parts. It is still possible for just one of them to fail.
Hope this helps,
Regards,
Andrew
Thanks for the information!
I had ordered a replacement cap before I got your post... I replaced it, and when I powered up the speakers there was no hum! Then I plugged in the source/input (analog stereo from computer) and I got some hum (but much better than before) with the music thru the speakers. I disconnected the source and the hum ceased. I then connected the sub speaker and had the same results. I then put everything back together into the box, and I get a low hum all the time now, even when the source is unplugged...
After messing around a bit more, turning it off/on, unplugging the sub, etc. it sometimes (but rarely) comes on without the hum, but whenever I plug in the source it hums, and it is difficult to get the hum to stop even after unplugging the source. I have tried 2 different sources - computer and TV output and same results from each....
I had ordered a replacement cap before I got your post... I replaced it, and when I powered up the speakers there was no hum! Then I plugged in the source/input (analog stereo from computer) and I got some hum (but much better than before) with the music thru the speakers. I disconnected the source and the hum ceased. I then connected the sub speaker and had the same results. I then put everything back together into the box, and I get a low hum all the time now, even when the source is unplugged...
After messing around a bit more, turning it off/on, unplugging the sub, etc. it sometimes (but rarely) comes on without the hum, but whenever I plug in the source it hums, and it is difficult to get the hum to stop even after unplugging the source. I have tried 2 different sources - computer and TV output and same results from each....
Hi,
My guess is one of the diodes in the bridge has gone so it is half wave rectifying. This put alot of strain on the smoothing cap which then failed. You have fixed the smoothing cap but it may still be half wave rectifiying, so with no load it is smooth enough that it doesn't ripple enough to cause hum.
As mentioned in my last post you can check this by checking the diodes with a multimeter. If you can't identify them then post a photo of the PCB from the top and bottom and I will see if I can.
Regards,
Andrew
My guess is one of the diodes in the bridge has gone so it is half wave rectifying. This put alot of strain on the smoothing cap which then failed. You have fixed the smoothing cap but it may still be half wave rectifiying, so with no load it is smooth enough that it doesn't ripple enough to cause hum.
As mentioned in my last post you can check this by checking the diodes with a multimeter. If you can't identify them then post a photo of the PCB from the top and bottom and I will see if I can.
Regards,
Andrew
Hi,
In the first picture you posted, your thumb is just shown in the top left of the shot. You thumb is actually touching the bridge rectifier. it is the black rectangle with one corner cut off and four legs on the bottom.
You need to take this out and then check the diodes with a multi meter.
This link will give you a bit of background on how rectifiers work. Unfortunately it assumes you have an oscilloscope to check the wave forms, which I am assuming you don't
http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
This link gives you the data sheet for the part I suspect is used. You should be able to check this by reading the text off the side of the part. The data sheet should give you enough information to check the bridge with a meter. Basically check each of the diodes is performing as expected open circuit one diretion short circuit the other.
http://www.vishay.com/docs/88614/gbu4a.pdf
Your part was probably not made by vishay and is almost certainly a copy.
Hope this helps.
Regards,
Andrew
In the first picture you posted, your thumb is just shown in the top left of the shot. You thumb is actually touching the bridge rectifier. it is the black rectangle with one corner cut off and four legs on the bottom.
You need to take this out and then check the diodes with a multi meter.
This link will give you a bit of background on how rectifiers work. Unfortunately it assumes you have an oscilloscope to check the wave forms, which I am assuming you don't
http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
This link gives you the data sheet for the part I suspect is used. You should be able to check this by reading the text off the side of the part. The data sheet should give you enough information to check the bridge with a meter. Basically check each of the diodes is performing as expected open circuit one diretion short circuit the other.
http://www.vishay.com/docs/88614/gbu4a.pdf
Your part was probably not made by vishay and is almost certainly a copy.
Hope this helps.
Regards,
Andrew
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