| Vandergraaff27 |
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? |
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| poobah |
Sounds like you have a cap that has gone bad in the power supply.
Find and post a schematic and we'll tell you where to look. Sometimes a noisy cap will get hot. If you have some sort of temperature probe, you could look for bad that way. Read the thread on safety FIRST. |
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| 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 |
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| Vandergraaff27 |
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). |
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| poobah |
Tell ya what,
Just replace all three caps and be done with it.
Or post your model number and all that so someone can come up with a schematic for you. It is almost certainly one of those power supply caps.
;) |
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| 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:) |
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| Vandergraaff27 |
| 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. |
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| cityman |
| 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. |
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| SY |
| Seems more likely to be a grounding issue. That might take some skill to track down... |
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| DigitalJunkie |
| quote: | Originally posted by cityman
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. |
It was fine,and they slowly started humming 18 months ago?
Sounds like the caps drying out to me.
Replace that big 22000uf cap,(I'm assuming it's the filter cap?) |
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| bobjohnson |
| I am wondering if a solution was ever found for this, cause I just found these exact speakers in my closet from my parents old 1997 gateway and they are great speakers to hold me over until i can get better ones. I get the same problem, constant humming. |
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| sceva |
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/D...ame=493-1075-ND and wondered if someone could verify if this is the correct type of cap.
Thanks. |
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| gfiandy |
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 |
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| sceva |
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.... |
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| gfiandy |
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 |
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| sceva |
| You must have been reading my mind, because there are so many components on here that I am not sure which are the diodes. I am uploading the pics here. |
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| sceva |
Here is the last one. Let me know if you need a larger pic and I will post it elsewhere. the forum has a small file size limit.
Thanks. |
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| gfiandy |
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 |
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| vasyachkin |
| so did he rule out a ground loop ? |
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| gfiandy |
If it was a ground loop the hum would go away when the source was disconected. He reports that it continues to hum even after a source is disconnected.
Regards,
Andrew |
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| vasyachkin |
| quote: | Originally posted by gfiandy
If it was a ground loop the hum would go away when the source was disconected. He reports that it continues to hum even after a source is disconnected.
Regards,
Andrew |
actually if i really wanted to be sure i would short the signal and ground on RCA inputs, not just disconnect the cable. |
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| marchel |
"actually if i really wanted to be sure i would short the signal and ground on RCA inputs, not just disconnect the cable."
Thats non sense and waste of time, turning the Volume control al the way down on the sub amp will do.
Your persisting problem most be ground loop , caused by the use of the non proprietary cable. Not impossible to fix though. |
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| AndrewT |
| quote: | Originally posted by marchel
"actually if i really wanted to be sure i would short the signal and ground on RCA inputs, not just disconnect the cable."
Thats non sense and waste of time, turning the Volume control al the way down on the sub amp will do.
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I don't agree.
leaving the source connected but with min volume setting does not achieve the same as removing the source and shorting input to ground. |
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| marchel |
| Of coarse, You have to remove the source connector. The volume potentionmeter at minimum should ground the signal., If it still hums , its ground loop. |
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| sceva |
The rectifier has been removed, I have tested it with an analog multimeter and found the following:
Rectifier: - ~ ~ +
Meter:...........- + reads 9 ohms at x1k
Rectifier: - ~ ~ +
Meter:.......-.....+ reads 8 ohms at x1k
Rectifier: - ~ ~ +
Meter:....-........+ reads 22 ohms at x1k
Rectifier: - ~ ~ +
Meter:....-.....+ reads 8 ohms
Rectifier: - ~ ~ +
Meter:....-..+ reads 8 ohms
all other combinations show no continuity.
I read the link you gave to ...allaboutcircuits... and I didn't quite get it, then I searched around and found this page,
http://www.kpsec.freeuk.com/components/diode.htm#bridge
and the animated picture helped me understand a bit better, although I am still fascinated that it shows open circuit one way and short the other. :-)
I still don't know enough to tell if what I found means it is good or faulty.
Thanks so much for the help with this. I don't know if we will be successful in repairing this, but I have learned a lot so far, and have also developed my soldering and de-soldering skills! |
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| AndrewT |
Hi,
the reverse direction will show as near open circuit on the ohms scale.
The forward direction can show open circuit or a false resistance depending on the voltage applied during the measurement.
If the test voltage is >700mV then current flows and a resistance is indicated. If <500mV then no current flows and open circuit will be indicated. Some meters have a dual test voltage switch to allow this test to be done at <diode voltage. This dual facility is also useful for checking transistors and assembled PCBs.
Your 22r reading indicates that your meter is applying a test voltage >1.5V when on ohms scale.
I think the open circuit readings show that your rectifiers are OK.
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| gfiandy |
Hi, Yes I agree looks like the rectifier is OK. I am now running out of ideas.
The bridge animation is good and I can see how it would help.
So to recap.
It didn't hum out of the box.
In the box it hums a bit.
When you connect a source it hums much worse.
If you disconect the source again the hum does not go away.
I guess to be sure we should eliminate the ground loop issue.
Take a peice of wire and short the input to the unit ie from the outside of the RCA connector to the inside of the RCA connector. Does the hum go away. (It is safe to short the input as it effectively puts nothing on the input - do not short the output it could damage the unit)
I don't know this product:- If it has a volume control, does the volume control affect the level of the hum?
Regards,
Andrew |
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| sceva |
Here is a link with some more details about the system:
http://support.gateway.com/s/SOUND/...7002204nv.shtml
There were a couple of revisions, and not exactly sure which one I have, but the main difference is some have a digital input in addition to the analog input, which mine has. I am using the analog (mini stereo plug) input.
I put the rectifier back in, and re-did the soldering on the large cap I installed earlier, as I wanted to make sure that was making good connection. I then attached the 2 speakers (the volume control is on one of the speakers). There is also a control cable from the speaker with the volume, surround mode, and subwoofer controls. I attached it, turned on the volume, and there was a hum, but quieter than before. I then plugged in the source, played something thru the computer, and it sounded pretty good. With music playing I can't hear the hum.
I then turned it off, attached the sub speaker, turned it back on, and the sub hummed very loudly. None of the humming is affected by the volume control. It starts humming when it is turned on now, and seems to always hum. Attaching/removing the source doesn't affect it.
There are 3 other caps, each 25v, 220uf, and one other 10v, 220uf on the boards.
I tried checking the input for ground loop, and since it is a stereo plug, I shorted all the combinations of the 3 wires and it didn't affect anything.
So now I have barely noticeable humming on the satellite speakers, and loud humming on the sub speaker when it is connected. The sub noise is at least as loud as it was before I started working on it. |
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| gfiandy |
Hi,
As its not affected by the grounding or volume control. It still sounds like there is a problem with the PSU.
Possibilities - the bridge works off load and fails as soon as any power is draw through it (I have seen this before)
There is another bridge somewhere and this has failed. I don't think this is likely as I suspect there is only one rail comming from the PSU.
I suggest measuring the AC and the DC voltage on each of the caps including the one you replaced. I.e test it with the meter in AC then in DC. The AC should be low and the DC high.
Let us know what this is and it should help to work out where to go from here.
Regards,
Andrew |
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| sceva |
| quote: | Originally posted by gfiandy
Possibilities - the bridge works off load and fails as soon as any power is draw through it (I have seen this before)
There is another bridge somewhere and this has failed. I don't think this is likely as I suspect there is only one rail coming from the PSU.
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I will check and see if I can determine if there is another one...| quote: | I suggest measuring the AC and the DC voltage on each of the caps including the one you replaced. I.e test it with the meter in AC then in DC. The AC should be low and the DC high.
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Do I do this with the unit powered? |
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| xplod1236 |
| quote: | Originally posted by gfiandy
I suggest measuring the AC and the DC voltage on each of the caps including the one you replaced. I.e test it with the meter in AC then in DC. The AC should be low and the DC high. |
| quote: | Originally posted by sceva
Do I do this with the unit powered? |
Yes. Otherwise, there would be no voltage at all on the caps. |
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| sceva |
The 25v, 22000uf cap shows 37v ac, 18v dc
the two 25v, 220uf caps side by side on the same board as the cap above show nothing at all.
Another 25v 220uf cap shows 37v ac, 18v dc.
The last cap, a 10v 220uf cap shows 10v ac, 5v dc.
I checked the power transformer, and it is putting out about 14v ac.
To my untrained eye, I don't recognize another rectifier. |
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| gfiandy |
Now I am confused.
Either your meter is showing an erronious measurement or there is something really bad going on.
That cap won't last for long with 37Vac as its only made for 25V.
It would certainly explain the hum. But I don't understand where it can be comming from if the AC supply is only 14V, my guess is that your meter is picking up DC and AC for some reason. What sort of meter do you have?
At this point, I would replace the bridge to see if it fixes the problem, its the only thing I can think of that would cause this level of ripple.
Regards,
Andrew |
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| sceva |
I was using a 20 year old analog meter. I will try a different one tonight and see what the results are.
How does the power go from 14v ac at the power supply to 25 or 37v at the caps? And then only 10v at the small cap?
Also, the power transformer shows the secondary should be 12v, but the meter was reading 14v. (I will check that again with a different meter when I get time.)
Thanks. |
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| gfiandy |
Hi, The 14V read by your meter will be 14V RMS. So you get 14 x 1.41 for the peak value which is 19.74V.
So the AC signal is 19.47 x 2 peak to peak 39.48V.
It is normal for a transformers output to be a bit higher than listed on it when it is off load as it sags on load. Also the mains is often a bit higher than the nominal value and this would push the output of the transformer up as well.
10V on the smaller cap is probably because there is a regulator on the supply to reduce the voltage for these circuits.
Regards,
Andrew |
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| sceva |
I just tested everything with a digital meter and found slightly different readings than before:
The 25v, 22000uf cap shows 34.1v ac, 16v dc
the two 25v, 220uf caps side by side on the same board as the cap above show .1 or .2v ac and .3v dc on one, and 0.0v ac and .1v dc.
Another 25v 220uf cap shows 34.1v ac, 16v dc.
The last cap, a 10v 220uf cap shows 9.9v ac, 4.9v dc.
What do you think about the two caps that show so little voltage?
I had to do some more study on the rms, peak, etc. So I understand a bit more of the theory, and that my meters do not take true rms readings, but not sure if that means the 34v ac is high or not.
These parts are relatively inexpensive, so I don't mind replacing the bridge and/or caps if you still think that is the way to go.
btw, I checked the voltage on the bridge under load and got 15.9v dc on the outside pins and 13.4v ac on the inside pins.
Thanks. |
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| gfiandy |
Hi,
I don't really know what is going on. But if it was my peice of kit I would replace the bridge at this point. There should not be that much AC on the caps and the only thing I can think of to cause it is a faulty bridge, but I could easly be wrong.
Don't worry about the caps with no voltage on them they may be part of the Audio circuit which would only have voltage across it when there is a signal present and even then it might not be much.
Regards,
Andrew |
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| svenyonson |
I have the same speakers, with the same problem.
If you look at the back of the subwoofer, you will see the input labeled:
12v in AC/DC.
I simply plugged in a power supply from an old laptop ( 12vDC, 2.0amp ).
Presto. Hum gone. |
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| jdvernarr |
Using DC power was a great idea. Has the fix remained effective? Is the hum completely gone?
I am asking since I recently purchased a Boston Acoustics Digital Media Theater on ebay and it also has the hum problem. (Hums at a noticeable, constant volume level as soon as it is turned on without any input signal. Not a problem at high volume levels but not OK at low volume levels.)
I couldn't find a suitable 12V DC power supply laying around the house so I am planning to buy one that I found on line at the below link.
http://www.skycraftsurplus.com/inde...ROD&ProdID=1183
I also have the analog version of this system for about 9 years(which looks identical minus the digital input in the back) and it is very quiet. No hum at all.
Thanks for the help,
John V. |
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| svenyonson |
Hi John,
Yes, I am listening to music right now! Absolutely no hum. Makes me wonder why they even bothered to design the unit to use AC input when DC supplies are so commonly available and this would have saved them the engineering effort to build an internal rectifier circuit - which they apparently designed very badly.
So, go buy the DC power supply. I didn't check your link yet, but make sure the DC supply you buy has adequate power ( amperage ). I forget the requirements ( look at your existing ac power supply ), but mine is 2.0 amps which I am sure is enough.
Steve
P.S. The symptoms you describe match the problem described in this thread - there is a hum present even with no input signal to the amp. |
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| jdvernarr |
Hi Steve,
My 12V DC adapter arrived today. With the new 12V DC adapter, my hum problem went completely away, just like you told me. Yeah!!!! A very happy ending since I use mine to listen mostly to DVD movies which have alot of quiet moments.
I ended up buying the adapter here :
http://www.12vadapters.com/adapter/...2v/3-5-amp.html for $17 through ebay stores.
The original 12V AC adapter was 3.0Amp so I went up a notch to a 3.5Amp to play it extra safe. The 12 V DC connector plug fit perfectly into the back of the subwoofer.
Thanks greatly for your help Steve.
John V.
P.S. I hope this post helps others that are annoyed with their humming Boston Acoustic Digital Media Theater. |
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