Strange buzz after capacitor change on speaker (Solved) - Page 1
Forum section: electronics / repair

author: kebab
author_rank: Contributor
timestamp: July 20, 2020, 03:36:04 pm
content: Hi,I changed faulty capacitors in a studio monitor, they made a loud 115hz buzz when powered on, the problem now after the fix is that when powered on they emit a low 115hz buzz that lasts for a good minute and then the buzz goes away, if turned off for long enough the buzzing is present again for a minute when powered on, you can kinda hear the frequency rise for the whole duration until it reaches a peak and then stops and the speaker works perfectly, it's as if it's buzzing while the capacitors are getting charged? Does anyone have a guess as to why this is happening?Here's a picture of the board:Thanks!

author: Haenk
author_rank: Super Contributor
timestamp: July 20, 2020, 04:45:43 pm
content: All of those caps (electrolytic) look like the bottom of the bucket of the cheapest random chinese brand ("LEAGUER", hmm). I'd just replace them *all* with some branded caps (Nichicon, Chemicon, ELNA, Matsushita etc. pp.) - I assume most of them are way out of spec now.You might also want to check for noise of the regulators, but I still suspect the caps...

author: mikerj
author_rank: Super Contributor
timestamp: July 20, 2020, 05:17:48 pm
content: Possibly the gunk holding the caps down has become conductive.

author: bob91343
author_rank: Super Contributor
timestamp: July 20, 2020, 05:51:00 pm
content: I suggest a parasitic oscillation. Try some bypass capacitors.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 20, 2020, 06:35:41 pm
content: I would also suggest the black gunk has gone conductive, AND the caps bottom right are crap.I have found this stuff is really good at breaking down the conductive gunk.No idea why this should work and most other solvent doesn't.https://uk.farnell.com/servisol/100019000/flux-remover-160-200ml/dp/3821470?st=flux%20cleanerEdit: It is good at breaking up the gunk once it has gone hard and conductive, not much use on the the original squidgy material unfortunately.

author: kebab
author_rank: Contributor
timestamp: July 20, 2020, 07:12:27 pm
content: Thanks for the answers, the goo does indeed look suspicious, there's some small watery bubbles forming on it, I can't find out anything about that black goo and it's hard to scrape off, what can be used to get it off the board?Edit: just saw your post above, I'll give this a try, thanks

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 03:09:03 am
content: So I managed to remove all the goo with some isopropyl alcohol and a small tool to scrape it off and it hasn't solved the issue, none of the capacitors have any visual defects so I'm not sure what to do next, I don't have much experience and almost no equipment, I'm thinking about buying an ESR meter to test all the capacitors, would this be a good way to narrow down the problem?

author: bob91343
author_rank: Super Contributor
timestamp: July 21, 2020, 04:17:30 am
content: Forget about ESR. For most situations it's just a buzz word.I really think you have an oscillation that needs suppressing. A few strategically placed bypass capacitors could help but without decent test gear and the knowledge to use it, you are unlikely to solve it.

author: james_s
author_rank: Super Contributor
timestamp: July 21, 2020, 05:38:08 am
content: The fact that you can't see any visual defects means nothing, a capacitor has to get *really* bad before it starts to bulge or leak.I'd bet the problem is related to temperature, a can of freeze spray or air duster held upside down can help narrow it down. I'd second the suggestion to replace those capacitors with something better.

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 01:12:40 pm
content: I can order new caps since I need to change two in another pair of monitors anyways, only the two Leaguer caps? How about all the other smaller caps on the board, could one of them be culprit? Also care to explain why an ESR meter is a bad idea? According to the reviews it does accurate in-circuit testing, wouldn't that be a handy tool to have instead of just changing all the caps and hoping for the best?

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 02:49:16 pm
content: I'll go ahead and just replace those caps, will report back once I received and switched the caps

author: themadhippy
author_rank: Super Contributor
timestamp: July 21, 2020, 02:57:09 pm
content: totally off the wall thought,but is there a mute on power up circuit thats gone awol

author: Haenk
author_rank: Super Contributor
timestamp: July 21, 2020, 03:36:33 pm
content: Quote from: kebab on July 21, 2020, 01:12:40 pmI suspect the Leaguer, but all the electrolytic caps on the north side are close to amplifications ICs (I assume); basically if the noise is the same on both channels - it's likely coming from the power supply part, i.e. the both large caps (which you replaced, I guess) and the southeast parts with the 78/79 regulators. Btw. those regulators can fail as well, and are pretty cheap, so maybe just order a replacement for those to save shipping costs just in case.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 21, 2020, 03:42:58 pm
content: It's hard to tell in your 2nd photo, but did you get the goo off the board at the other end?Could be a problem up there if not.

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 04:04:39 pm
content: The goo on the top capacitors seemed like it was in good shape and wasn't conductive when I checked so I didn't touch it, I ordered new caps so I'll start with that first and then I'll see, it seems likepeople with the same issuehave solved the buzzing by replacing those caps so fingers crossed

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 04:15:26 pm
content: Quote from: Haenk on July 21, 2020, 03:36:33 pmThose two correct?I'll check on DigiKey before they ship my order

author: Haenk
author_rank: Super Contributor
timestamp: July 21, 2020, 04:21:51 pm
content: Those diodes in the front look like being badly soldered, maybe got very hot (that would explain the bubbly goo) and desoldered themselves? You need to visually inspect the solder joints to be sure those are good (pictures can always mislead...)

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 04:29:00 pm
content: The diodes look fine, it looks worse on the picture with the goo, also can't seem to find the voltage regulators on DigiKey but I'm not exactly sure what to look for

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 04:51:04 pm
content: Are those two what I need?https://www.digikey.com/product-detail/en/stmicroelectronics/L7915CP/497-7295-5-ND/1038324https://www.digikey.com/product-detail/en/stmicroelectronics/L7815CP/497-8353-5-ND/1657318

author: james_s
author_rank: Super Contributor
timestamp: July 21, 2020, 05:32:19 pm
content: Quote from: kebab on July 21, 2020, 01:12:40 pmAn ESR meter is not a bad idea, but it's not infallible. ESR doesn't tell you everything, and sometimes you just can't test a part in-circuit no matter what tool you use. I've found many bad capacitors using my ESR meter, but it has also lied to me and told me a capacitor was fine when it in fact wasn't. It all comes down to understanding the limitations of your tools.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 21, 2020, 05:53:28 pm
content: Those regulators are fine.They are 1.5A rated, many similar regulators are only rated 1A, though there is very little difference in price so why not.Incidentally, when this goo gets REALLY out of control, it not only becomes conductive, it also becomes very corrosive.I have seen many a resistor's metal legs and wire jumpers literally eaten in two.Your's doesn't look to have got to that point yet, but it may still be a good idea to check for corroded joints or vias where the goo was.And also, the conductive glue in these types of powered monitors has a habit of causing the mute circuit to mute intermittently, or to crackle as it partially does so, this can be at the top of the board near the power amp IC's, which is why I think you should remove all that too.A previous poster mentioned the mute circuit, this could be what you are hearing, or even a combination of this and bad caps.And finally, an ESR meter can indeed be very useful - just today I found a bad SMD cap in circuit using one.They are not really such a good indicator of a good cap, though and often can't test in circuit anyway.But, if you put it across a cap in circuit, whatever circuitry is surrounding it, and it DOESN'T show a low ESR, it MUST be bad.Which is what happend to me today. And very useful that was.

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 06:03:11 pm
content: Thank you so much for all that useful info, I'll start with the caps and the regulators and if that fails I'll give those other suggestions a try, even if all this fails I'm still getting a little bit of experience out of this, about the regulators, does it matter if mine says 7815Aand the one from DigiKey saysL7815CP? I'm not exactly sure if those prefixes/suffixes matter, I did find aL7815AVB, just want to make sure I order the right partEdit: I was too slow and DigiKey has shipped my order, so I'll wait and try the caps first, the shipping cost is a bit high just for those regulators

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 21, 2020, 06:23:02 pm
content: The various prefixes and suffixes indicate manufacturer and packaging differences, as well as ratings.The first two you linked from Digikey are plastic encapsulated, so the metal tab is insulated from any accidental shorts or fingers.The metal tab is connected to one of the 3 pins.The last one you posted a link to has the metal tab exposed - so it could be possible to accidentally short out one of the connections if you are very clumsy, or burn your finger on it if you are a child.And, a 7815's tab is connected to ground, a 7915's tab is connected to -V input, so if these touched, it would ruin your day.The only other difference AFAIK is that most of these 7815 / 7915 regulators are either 1A or 1.5A rated as I said before, the datasheets will indicate which of these they are.In your case, any 7815 or 7915 variant will do just fine.Unless you are clumsy or have a child with poky fingers.

author: kebab
author_rank: Contributor
timestamp: July 21, 2020, 06:25:53 pm
content: Awesome, thank you so much for the explanation

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 22, 2020, 06:51:48 pm
content: I probably don't really need to add this, but 7815 and 7915 regulators are also available in smaller packages.https://uk.farnell.com/texas-instruments/lm78l15acz/ic-voltage-regulator-23v-to92/dp/9490310?st=7815This one is still a 7815, but only rated at 140mA, so is probably not suitable for your speaker!Always read the datasheet when fishing around for replacements, its a good way to learn stuff.

author: kebab
author_rank: Contributor
timestamp: July 22, 2020, 10:46:04 pm
content: So changing both the 470uf Leaguer caps fixed the issue! Thanks everyone for the suggestions