Repairing Alesis M1 Active mk2 Monitor Speakers

[vintagevalves]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Joining the blown up M1A620 club. A pair of these were 'gifted' to me not working during Covid shutdowns. One required a C8 replacement and now works fine.

The other has been troublesome. After C8 replacement the power supply seems good with
High rails at 36.8V and low at 17.25. When all hooked up, the woofer pulsed a very loud square wave type sound. Original woofer got toasted by this prior to my ownership.

I removed the LM3886 for the woofer and the tweeter side seems stable without it in, though the gain pot seems intermittent (a separate issue?) Suspected a bad LM3886. They were very hard to get for a couple of years. I replaced with an NTE7179 which appeared to be equivalent, but still did not work. I was finally able to source a real LM3886TF and am doing some checks before putting it in.
Voltages as follow with no chip installed:

1: V+ - 36.8
2: NC
3: Output +1.0vdc
4: V- -36.8v
5: V+ 36.8v
6: Nc
7: Gnd 0
8: Mute -4v
9: Vin +1.0vdc
10: Vin + 0.3vdc
11: NC

The 1vdc on the input and output is concerning. Any suggestions on what to check next? Don't want to blow up this LM3886 that I waited SO long to get my hands on... Thanks!

[biggertv]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yikes! Those Caps sitting right on the Heatsink...they're gonna be Toast. If there's any way to replace and extend leads to fold them over they will last much longer. Those designers will Never Learn!

[xfreakx]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I can't believe I've read through all the 26 pages of this thread.

First, thanks to everyone here, it helped me a lot to fix this problematic power supply. It took some time and 3 orders at the electronics shop but it's working now.

Someone replaced already the notorious C8 in a previous repair and the speakers were working for a while. As I got them, Q1 was cracked from overheating as well as Q4. R11 and R15 were black and Q2 and D5 shorted. C6 and C7 looked bloated so I replaced them as well. Fuse was alright (!).

After replacing this parts the power supply still didn't start, so I replaced U1, Q3, C8 and C35. Still no output. Compared to the other board in the working 2. speaker all the other transistors, diodes and resistors seemed fine.

Since most of the parts are cheap and available, I replaced U3, Q5 and D6 and the power supply started. 36 / 18V on output. Yay!

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

The top of C6 capacitor feels like a firm bubble, meaning I can push it down and it springs back up.

It's probably just the plastic sleeve on top, then.
Still doesn't mean the cap can't be bad.
You can try removing it and seeing if there is any blue-green or brown corrosion near it's leads.
Another test is to take a 19V or 20V laptop power adapter and connect the adapter's (+) output to the cap's (+) lead along with the adapter's (-) output to cap's (-) lead. Basically, let the cap charge to 20V (it should be instant as soon as you connect the cap to the power adapter.) Then disconnect the cap and measure the voltage across it with a multimeter. It should have close to 20V or whatever the adapter outputs. If not, it's probably bad. Also, if you short-circuit the cap's leads with a metal object, you should see and hear a small spark. If no spark at all, cap might be high-ESR. With these high-voltage caps, though, they typically either go open-circuit or loose capacitance... and in rare instances, they may also go short-circuit. But if that was the case, the input fuse or something else would have popped.

The noise is loud enough to hear, especially when turning the unit off. Not sure if you can hear the sustained tone in the video but you can most definitely hear it when it turns off... https://photos.app.goo.gl/HboTvjY7sfEbCBpJ6

Audio signal does *not* pass through.

I can hear a high-pitched noise.

If the audio is not going through, though, that might be something on the amp side. Hard to say, though.

I'm at the point now where I'd like to ask if anyone wants to try and fix it for me? I have two speakers that work so I can get back to rebuilding my studio now.

Well, if you drive by the Northern Virginia (NoVA) or DC area, send me a PM. I'll be here for another month or two. Probably will need to start packing my bench in early April, if not before that. Could still take a look, though.

I was wondering, though, since all you guys have experience with these speakers, are you audio engineers too?

Well, I can't speak for the others here... but I've never touched or seen these speakers with my own eyes, other than through what's been posted here. I'm definitely a "wannabe" audio engineer . A real one - no. Just a hobbyist tech with an EET degree that's so far been very useful for keeping one of my audio amplifiers covered so it doesn't get dust in when I'm not using it.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

You can try to pinpoint where the noise comes from, if it is from the new inductor try to put the old one back like I mentioned in a previous post.

[jgulyas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

One more thing: did you check C6 that you mentioned in the post quoted above?

Updates...

I was supposed to work some more on this Sunday but She Who Must Be Obeyed decided she wanted to paint the kitchen, so there's that...

The top of C6 capacitor feels like a firm bubble, meaning I can push it down and it springs back up.

The noise is loud enough to hear, especially when turning the unit off. Not sure if you can hear the sustained tone in the video but you can most definitely hear it when it turns off... https://photos.app.goo.gl/HboTvjY7sfEbCBpJ6

Audio signal does *not* pass through.

I'm at the point now where I'd like to ask if anyone wants to try and fix it for me? I have two speakers that work so I can get back to rebuilding my studio now.

I was wondering, though, since all you guys have experience with these speakers, are you audio engineers too?

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Also, in the section with C8 that I've already taken out, C6 is rounded/puffy at the top while C7 is flat. The guy at the pro audio place said he'd replace both of those as well. Both of those have the yellow (now brown) glue on them as well.

About the large capacitors, it might just be the plastic top of C6 that is convex, see if you can easily push it down with your thumb...

I second this. Those caps don't usually go bad... but if they did, just find an old cheap junk ATX PSU (anything that has a red 115/230V selection switch.) Those will usually have similar caps you can use, at least for a test.

One more thing: did you check C6 that you mentioned in the post quoted above?

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well... mild success I guess... the unit powered on and the blue LED is no longer blinking... but now it's emitting simultaneous tones 13.5kHz and 6.3kHz.

Any ideas with that one??

Is the noise loud or faint? Also, is it coming from the speakers or the power supply itself? Any chance you could provide a quick audio recording?

Before anything else, perhaps double-check your setup. In particular, try taking your working speaker set and wire it up the exact same place and cables that you used with this one. Then repeat the test with this one. If the noise is persistently staying with this set, then perhaps we are not done yet with the repair.

Per mentioned a valid point - could indeed be something with the new inductor/toroid making the PSU oscillate and produce extra noise.

It could also be that perhaps the output caps on the PSU are failed or have started to fail.

Since you already have the old inductor, you can try swapping the new one with the old one. (And when you do, definitely make sure to use the dim bulb trick, just in case.) If the noise persists, then I would suggest doing a full recap on the unit with proper high-quality caps. There is also a small chance that perhaps when the PSU failed, it also damaged or partially damaged something in the audio / amp section of the speakers. But I think that would be a rare/unlikely case. On that note, do the speaker play audio OK, aside from the high-pitched noise(s)?

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Not really but it might be worth a test to put back the original inductor instead of the new one you bought.

[jgulyas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well... mild success I guess... the unit powered on and the blue LED is no longer blinking... but now it's emitting simultaneous tones 13.5kHz and 6.3kHz.

Any ideas with that one??

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

*EDIT*
Sorry I'm "echo-ing" Per's post again.
By the time I type up my post and hit submit (and especially if there were other distractions around me), there's always another reply, lol.

Regarding C8 and its legs, I put it over there to get it away from the hot resistor. How do I get it away from the heat of that without having long legs?

You can raise it up to about 1/4" from the board and bend it away from the heatsink or hot resistors... or just place it elsewhere on the board (I believe this was mentioned in several places somewhere in the first few pages of this thread.) Otherwise, placing capacitors like this with very long leads makes them ineffective at high frequencies. The long leads raise the ESR and also act like little antennas, spewing electrical noise to other components and parts of the circuit. So generally, it's not a good practice in most applications.

Someone needs to show me what you mean about adding a light for testing. All I can see in my head is a standard household lightbulb attached to this thing somewhere.

This one was posted by Toasty a long time ago, but right to the point:


And same thing with more details:
https://www.badcaps.net/forum/showpo...4&postcount=70

Jackson capacitor??

Jackson, Jackcon... whatever. These are all goofy China brands. Look at the two photos you posted of C8. The replacement cap has "Jackcon" written on it, which is its brand.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Regarding C8 and its legs, I put it over there to get it away from the hot resistor. How do I get it away from the heat of that without having long legs?

Reply in this quote:

Heat is not a problem in the set in general, only the stupid location of C8 is a problem.
Instead of a fan I would do what Khron recommends in some of his posts: basically swap physical locations with C8 & C14.
Apparently this is what Alesis did themselves in a later revision (F).
However Khron's mod is even better, because C14 will be too far away from the UC3844N to be useful in that physical location.
So he just solders it to the bottom of the chip instead, see these two posts:
https://www.badcaps.net/forum/showpo...0&postcount=27
https://www.badcaps.net/forum/showpo...&postcount=384

Someone needs to show me what you mean about adding a light for testing. All I can see in my head is a standard household lightbulb attached to this thing somewhere.

Here is an example: https://www.badcaps.net/forum/showpo...4&postcount=70

Jackson capacitor??

That's the manufacturer of capacitor C8 you put in there, a really "meh" brand (not even likely to outlive the original that was in there)...
There are only a few consistently reliable capacitor manufacturers in general: Rubycon, Nichicon, Sanyo (Suncon), United Chemi-Con (UCC), Panasonic
P.S: You should change C35 too.

[jgulyas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

+1

Also, if the PSU does end up working, it would be a good idea to replace that Jackcon capacitor with something better for the long run. These aren't known for their reliability... and really that applies to just about every non-Japanese brand. Should have gotten some Rubycon, Panasonic, Nichicon, or United Chemicon caps while purchasing the inductor from Mouser. But for testing, it should be OK. I have a few of these Jackcons in my "test parts" bin as well... and they are OK sitting there. Definitely won't last in a stressful or hot PSU, though.

Regarding C8 and its legs, I put it over there to get it away from the hot resistor. How do I get it away from the heat of that without having long legs?

Someone needs to show me what you mean about adding a light for testing. All I can see in my head is a standard household lightbulb attached to this thing somewhere.

Jackson capacitor??

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well it wont win any beauty contests and I would not run it long term with the cap on those long legs!
But it should be enough for testing, I do recommend to have an incandescent light bulb in series with the live or neutral to limit the current if there are any shorts.
Did you measure the diodes on the multimeter to make sure they are ok when you had them out?

+1

Also, if the PSU does end up working, it would be a good idea to replace that Jackcon capacitor with something better for the long run. These aren't known for their reliability... and really that applies to just about every non-Japanese brand. Should have gotten some Rubycon, Panasonic, Nichicon, or United Chemicon caps while purchasing the inductor from Mouser. But for testing, it should be OK. I have a few of these Jackcons in my "test parts" bin as well... and they are OK sitting there. Definitely won't last in a stressful or hot PSU, though.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well it wont win any beauty contests and I would not run it long term with the cap on those long legs!
But it should be enough for testing, I do recommend to have an incandescent light bulb in series with the live or neutral to limit the current if there are any shorts.
Did you measure the diodes on the multimeter to make sure they are ok when you had them out?

[jgulyas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes exactly, just before you test the unit take a good photo of your work so we can confirm no mistakes have been made.

Welp... it sure ain't pretty... It really felt like the holes were too big for the solder to pool at the bottom of the lead on C8.

I hope L4 and D14 look alright?

John

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes exactly, just before you test the unit take a good photo of your work so we can confirm no mistakes have been made.

[jgulyas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, your board is toast in the literal meaning.

But it's not "done for" type of "toast".

Just keep drilling / grinding to get rid of as much charred PCB as possible. Yes, there will be a big hole in the board afterwards, but that's not an issue. You will just have to use some jumper wire to connect components to their proper traces and/or other components... and probably glue some of them down afterwards to make sure nothing moves away.

I've seen boards much worse on here get repaired. Namely, this Delta DPS-340CB ATX power supply:
https://www.badcaps.net/forum/showthread.php?t=9660

Here are the pictures direct-linked if you don't feel like reading through that thread:
https://www.badcaps.net/forum/attach...6&d=1273364750
https://www.badcaps.net/forum/attach...8&d=1273364750
https://www.badcaps.net/forum/attach...0&d=1273364750
https://www.badcaps.net/forum/attach...7&d=1273978870
https://www.badcaps.net/forum/attach...9&d=1274834154
https://www.badcaps.net/forum/attach...2&d=1275187900
https://www.badcaps.net/forum/attach...4&d=1275187900

^In case of the above ATX PSU, everell did replace the missing/burned PCB parts... but for that one, it was probably necessary to attach all of the components. Your board has just holes for 2-3 parts that were affected. So no need to fill-in the hole with anything. Just solder the components the way they should be as if the PCB was there, and then connect the floating ends/leads with jumper/copper wire to other traces where they need to connect to.

Alright. I understand about running jumpers to connect the component to its proper trace... It looks like in this instance I can just solder one of each components' legs to those little bare sections (outlined in red) on the edge of the hole?

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, before I get into testing diodes and such, is the board toast? As you can see, there’s nothing remaining of the two holes for L4 and D14.

Well, your board is toast in the literal meaning.

But it's not "done for" type of "toast".

Just keep drilling / grinding to get rid of as much charred PCB as possible. Yes, there will be a big hole in the board afterwards, but that's not an issue. You will just have to use some jumper wire to connect components to their proper traces and/or other components... and probably glue some of them down afterwards to make sure nothing moves away.

I've seen boards much worse on here get repaired. Namely, this Delta DPS-340CB ATX power supply:
https://www.badcaps.net/forum/showthread.php?t=9660

Here are the pictures direct-linked if you don't feel like reading through that thread:
https://www.badcaps.net/forum/attach...6&d=1273364750
https://www.badcaps.net/forum/attach...8&d=1273364750
https://www.badcaps.net/forum/attach...0&d=1273364750
https://www.badcaps.net/forum/attach...7&d=1273978870
https://www.badcaps.net/forum/attach...9&d=1274834154
https://www.badcaps.net/forum/attach...2&d=1275187900
https://www.badcaps.net/forum/attach...4&d=1275187900

^In case of the above ATX PSU, everell did replace the missing/burned PCB parts... but for that one, it was probably necessary to attach all of the components. Your board has just holes for 2-3 parts that were affected. So no need to fill-in the hole with anything. Just solder the components the way they should be as if the PCB was there, and then connect the floating ends/leads with jumper/copper wire to other traces where they need to connect to.

[jgulyas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

No worries, we aren't going anywhere.


Well, like Per noted in post #479, you won't be able to measure an inductor's inductance with a multimeter... but that's OK. The multimeter is for checking all of the other components - namely diodes, resistors, and transistors on the board.

For starters, it's probably best that you check your multimeter's functions first. Connect black probe to "COM" jack and red probe to "V/Hz/Ohms/%/NCV...", then select the Ohms / continuity / diode check function (4th position after OFF on the rotary switch), and select resistance test mode (probably might have to press "Func." button to get to it, if it isn't the function selected already. After this short the probes together and note what resistance you get - this is your "short-circuit" resistance. Depending on multimeter and probes quality, this can be anywhere from 0.1 to 2-3 Ohms (sometimes even more, like those cheapo multimeters from HF.)

After this test, also check the voltage of 9V or 1.5V battery, just to make sure the voltage function on the MM is working too.

If all good, time to jump in and start checking components.

Let's start with diode D14, since that looks very close to where all of the "action" happened on the board.

Select diode test on your multimeter (little diode symbol should appear on the display), put red probe on the anode of D14 and black probe on the cathode of D14 (side with the white stripe). You should get about 560-700 mV reading on your screen (or 0.560 to 0.700V, depending on how your MM displays results.) If you get much lower than these values, note what they are and post them here, though you can do the same even if you do get readings in the above range, just as a 2nd check. Also note that this check on diode D14 tests transistor (MOSFET) Q2 across D-S junction. So if you get a really low reading across D14, take it out of the circuit and measure it again. If it measures OK out of circuit, Q2 could be bad.

Next, check resistor R15. It's a 0.22 Ohm resistor, so you should see more or less under 1 Ohms of resistance on your multimeter (again, depending on how low it can measure) when set on the resistance test setting. If R15 is open or showing very high resistance, then transistor Q1 is more than likely bad too.

If either D14 or R15 were bad above, remove them out of the circuit, then test Q1 and Q2. These are MOSFETs. With the MOSFET's front facing you (the side with the "IRF840" part number written on it), the left-most pin is the Gate, center is Drain, and right-most is Source. Use multimeter on resistance test and check resistance between Source-Drain, Source-Gate, and Drain-Gate. For Q1, you should get open (infinite resistance / 0L) for all tests, or very high resistance in the MegaOhm range. For Q2, same, except Source-Gate - should get about 10 KOhms resistance there. If you get any other values, definitely post them here. If you get low resistance readings (in the Ohms range), one or both of these is likely bad and you'd have to remove it/them to test out of circuit.

So let's start with those for now. And if you're comfortable with those and know what you're doing, you can skip ahead and also test Q3 & Q4 (these would be BJTs, so slightly different testing methodology.) I wrote how at the end of post #461:
https://www.badcaps.net/forum/showpo...&postcount=461


Interesting.
Well, the wire turns on the new one appear to be of the same thickness, so that's a good sign. The smaller core, we will have to see about. Generally smaller core = less surface area to dissipate heat. But if the new smaller core heats up less to begin with, then it should be fine. It's going to be a bit of an experiment, like I said... hence why keep the old core until the speakers have been proven to work reliably and without anything overheating.

You don't necessarily need to glue the new inductor... but given how badly the trace and hole for it are eaten away on the PCB, it may not be a bad idea.

Hot glue should work OK, unless the inductor/PCB/surround electronics get really hot (over 60C), at which point the hot glue may become a little soft and not hold as well. On a side note, now you have a thermocouple probe to check temperatures with your new MM, so you can find out eventually.

I've also used regular household silicone/caulk, but that's not always recommended due to certain types/brands producing more acetic acid when they cure than others... and acetic acid is a mild corrosive. Still, even "the worst" silicones would be a million times better than the tan glue. The stuff I've used is the cheapest brand/type from Home Depot and doesn't seem to have corroded anything so far.

If you want to be fancy / proper, use electronics-grade RTV.

Other glues - read the fine print. Can't be anything (too) corrosive. Also usually not a good idea to use any solvent-based glues, particularly on inductors or anything with coated magnet wire, as that could strip/dissolve away the insulating coating and possibly short out turns.


Dremel with the finest attachment you have. Maybe a drill with a very small drill bit too. Any charred PCB has to go. When you get to clean... or at least overheated but not charred guts, you can stop then. And yes, that could certainly be one of the hardest parts of this fix. Just take your time with it slowly.


On electrolytic capacitors, the negative side is marked with a vertical stripe. So the positive lead is the other side.


Great! More patients coming for this thread.
Hopefully that one won't have any issues (yet). But, see comment below.



Absolutely!
While at it, replace C8 and C35. It's only a matter of time before they fail too.

Well, before I get into testing diodes and such, is the board toast? As you can see, there's nothing remaining of the two holes for L4 and D14.