M3-8 Speaker Repair - loud 150 Hz buzz when power is applied

[stokesmusic]

I have a pair of M-Audio M3-8 speakers laying around, and one morning when I turned them on, the left speaker started making a saw-tooth-ish 150 Hz noise, which I'll attach to this post. Predictably, M-Audio was of no help, and the cost of repair exceeds the value of the speakers. I also wasn't able to find a schematic for them, which complicates trying to trace down the problem.

I checked the boards for burnt resistors or bad solder joints, and didn't find any obvious issues. The sound occurs without any inputs connected, just power. I checked the speaker with multiple power outlets, and power cables, and all had the same result.

Thank you in advance for any advice you all might have!

Colin
View attachment M3-8 Speaker Noise.wav

 

[Khron]

Dried out power supply capacitor(s)? 150Hz just "coincidentally" happens to be the 3rd harmonic of 50Hz mains frequency...

 

[stokesmusic]

Dried out power supply capacitor(s)? 150Hz just "coincidentally" happens to be the 3rd harmonic of 50Hz mains frequency...

Oh that's interesting. Perhaps recapping the power supply board would solve the problem?

 

[Khron]

Can't hurt. They most definitely won't have used anything but (almost) bottom-of-the-barrel brands.

 

[stokesmusic]

I should be replacing the large capacitors on the board above the toroidal, right (see 2nd to last photo)? In the last photo it's possible to see the hard yellow substance around the base of the caps. I'm not sure yet if that's some type of glue, or if it leaked from the capacitors?

Edit: I found this video, and it looks quite similar to what I see here. It looks like it's glue that I should be able to remove with some heat and patience.


There's so many varying glues on these boards, it's crazy! The white gloopy stuff, super glue on screw connections, and the clear plastic glue around the base of the caps.

 

[Khron]

Count yourself lucky there's none of the "brown glue" (that turns brittle and corrosive with time and heat).

CapXon, "yummy" - indeed, THE bottom-of-the-barrel, guaranteed to fail (sooner rather than later). Yes, replace all the four big black capacitors, both the two snap-ins and the two smaller ones next to them

 

[Tubetec]

Cap-Top brand , by the looks ,never seen those before ,
capXon ,capXoff
Ive measured high quality Japan made caps next to similar values of Chinese origin ,
Every parameter I could measure was orders of magnitude worse in the cheap components even from brand new .

 

[stokesmusic]

Interesting! I wasn't able to find anything about the Cap-Top brand caps online.

I ordered audio grade Nichicon replacements.
-4700uf 80v mfg. # LKG1K472MESCBK
-4700uf 50v mfg. # LKG1H472MESABK

If recapping fixes the non-working speaker, I'll recap the working one as well with matching components. It's a bit pricy for hobby speakers, about €10 each for the 80v caps, and €6 each for the 50v.

 

[Khron]

Well, that's what you get for "audio grade"

"Normal" 105c caps, even from good (japanese) brands, shouldn't have been more than a couple euros a piece.

 

[stokesmusic]

Haha I see! Would you use 'audio grade' components in higher end builds/repairs? Or you don't think they're worth it, in general?

I just realized that the capacitors I ordered are only rated to 85c, not sure how I missed that...

I see some folks on forums saying good quality 85c caps can replace low-quality 105c caps in audio gear. Is that BS? Mouser doesn't have any of the 80v caps at 105c in stock. I see some on ebay, aliexpress, etc., but maybe fakes...

So I would be tempted to try the 85c caps if I could get away with it.

 

[Khron]

If 80v caps are used from the factory, but you can't find any to buy, 100v ones will do just as fine (it's a slightly more widespread / "standard" value).

Just make sure you stay within the physical limitations of the original (mostly diameter, but you might also have height constraints).

Even the most bog-standard low-rated Nichicons i'd trust waaaaay more than any 105c rated "china specials", so i wouldn't worry about that.

 

[dmp]

Look at 100v caps as it's a more standard value, but they will be more expensive due to the size and voltage rating
Audio grade would only make a difference in the signal path as a blocking cap. These are in the power supply and don't pass audio.
Measure the diameter and lead spacing to make an easy fit, especially if ordering a 'snap in' type.
Here is a 105degC in 10mm LS

This is the selection at mouser with 4700uF and 100V

85 degC will be fine in a lot cases as that is a VERY hot temperature. I don't think a powered amp will have internal temps approaching that. Rated hours may be a better criteria is you really want the best of the best. And I doubt a 85 deg cap will be the weakest link in that cheap mass produced SMD circuit board.
AND, new PSU caps may not even fix the problem (I am doubtful it will)

 

[stokesmusic]

Thanks for your response DMP. That makes sense to me now that you say it - of course outside the signal path, 'audio grade' caps shouldn't make a difference. Thank you for the link. I already got the 85c caps, but I will keep this advice in mind for future repairs...I honestly don't feel like paying anymore shipping fees from Mouser for this little repair.

I'm interested that you think the PSU caps might not be the problem. Do you have an idea of what other issue might cause this noisy buzz (without inputting any audio through the signal path)?

 

[stokesmusic]

An update on this project...

I replaced the capacitors with the new Nichicon caps.
-4700uf 80v mfg. # LKG1K472MESCBK
-4700uf 50v mfg. # LKG1H472MESABK

I did a bit of an ugly work-around that I'm sure is generally a bad idea - The original caps were 20mm and 22mm in diameter, which I wasn't able to find, so I used hook-up wire to connect them to the board. I figured that if this created a problem, it would create a different problem, so I could at least see if the old caps were the problem. I also figured that if this repair fails, I can reuse the expensive caps in another project.

I got everything hooked back up, powered it on...and...ugh the same noise. @dmp, you were right.

I know these are cheaply made monitors, but the idea of tossing them bums me out.

What do you guys think. Should I stop wasting my time on these, or is there something else I should try?

 

[Khron]

What are the amplifier chips in these?

 

[stokesmusic]

There are two STMicroelectronics TDA7294 amps, which I think are the main amplifier chips?

There are also 5 STMicroelectronics 074C chips on the Pre-Amp board, and 1 074C on the EQ board as well.

The other ICs are 2 TJM4556 on the pre-amplifier board, 2 L7912CV voltage regulators, a bipolar transistor and two more regulators from International Regulators.