Repairing Alesis M1 Active mk2 Monitor Speakers

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you.

[VikHU]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi all, amazing to see that this thread is still going!!!

My pair of M1 Active 620 are still going strong after repairing them based on the knowledge in this thread back in 2018! But I also have a pair of M1 Active 520 (the original, not the USB) and one of the pair has the LED on but no sound. I've opened it up but it has a totally different power supply based around a toroidal transformer. Had anyone repaired one? Should I start another thread for it as it's a different design?

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Should I start another thread for it as it's a different design?

Yes, better to keep this thread focused on only the Alesis M1 MK2 speakers.

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Per Hansson:


Well, I have ruined another sound card/interface. I just ruined Bluetooth interface #3 (that's in addition to 2 sound cards). So, after all this work, I am sorry to say I am discouraged and I give up. This bothers be terribly, but I don't want to risk causing more sound card/interfaces to be broken. I think that if I do anything more with these speakers, I will learn how to make them Passive. The speakers are in pristine condition, like they were when they came out of the box brand new. Any advice from you is certainly welcome. Thank you many times over for sharing your knowledge and patience.

Skeeter

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well you can just measure and compare what voltages you see on the ports with no input vs the other working set that did not have the issue with the shorted post.
Possibly one of the op-amps is damaged and outputting the regulated +15v on the input channel.

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

No voltage is being issued from the input channel on either unit.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, I have no idea then what the issue can be.

[Takeover]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Greetings all! It's been 9 years since I lasted post on the forum. Where does the time go???

I have had these speakers for quite a few years now and have been very happy with them. About 6 months ago, the right speaker started making some weird buzzing and glitchy noises when idle. It would happen intermittently, but then it started happening more and more. Then one day, the sound just stopped working.

Trying to diagnose the issue, I swapped the 1/4-inch cable and power cable, with known, working cables, but that made no difference. After some Googling, I was led back to this forum! I opened up the back of the speaker to look at the board, but I don't notice any blown or leaky capacitors like what has been mentioned in this thread. Maybe I am not noticing something, so I took some pictures.

Thanks!

[Takeover]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I fixed my right speaker!

After reading through this thread and watching some YouTube videos, I found out that C8 was the culprit in the majority of cases. At first, I thought that wasn't my issue, because I was expecting to see a blown, bulging, or leaky capacitor. I didn't realize it looked normal but was completely dried and out dead from the resistor next to it!

I replaced the Jun Fu 220 uF 35v with a Rubycon 220 uF 50v. I'm no expert in capacitors, so I went with the comments said. I ordered a few from Digi-Key and they came the next day.

It only took me about 20 minutes to install the new capacitor. It was a bit tricky getting it through the holes. The old solder was hard to melt. Also, I bowed one of the legs of the new capacitor. I was worried it was going to snap off, so I didn't push it all the way through. My solder job was also kind of piss poor, but there was no issue! Put it back together, turned it on, and got the blue light with no interference!

Thanks to everyone who contributed to this thread! Now I know how to fix it if the problem happens again (could happen to my left speaker in the future).

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Could've considered my idea of relocating it closer to the 3842 chip, but hey - if it works.. .

[Takeover]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Could've considered my idea of relocating it closer to the 3842 chip, but hey - if it works.. .

Where is 3842? I don't know anything about relocating capacitors, but I am always up to learn more!

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

It's the chip in your photo here: https://www.badcaps.net/forum/attach...3&d=1664498435
In one of Khron's older posts he mentions how to do that mod.
But it is also in his blog here: https://khronscave.blogspot.com/2021...ilitation.html

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi all. Sorry to be resurrecting an old thread, but I've got a pair of Alesis M1 Active MkII speakers which are showing signs of some of the issues mentioned here and I can't seem to solve them. My right speaker is the biggest concern currently - until a few weeks ago it was taking longer and longer to come on after turning on the power switch but now it's refusing to start at all and the blue led flickers without going solid.

Having read through the whole topic it seemed most people were having issues with C8 and C35 so I've replaced both with a 330uf 50V Panasonic and a 1uf 50V Rubycon respectively. Sadly neither seems to have solved the problem - the blue led still flickers feebly and never comes on.

I've looked over the board closely and can't see any signs of damage or obvious culprits. I'm comfortable with a soldering iron but don't have much experience of how to find or diagnose a fault. I've tried checking the resistance across as many components as I can to see if anything seems amiss but other than C34 giving me 0 nothing jumps out and I'm stabbing in the dark really!

U1 on my board is UC3844N - how would I go about checking that? I saw one or two posts mentioning they've had issues with that in the past.

Any help gratefully welcomed!

Joe

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Greetings zinger5!
Maybe you can start with the posts where I helped Skeeter and compare readings and general questions with those posted?
Quite allot of extensive information is provided in those last three pages:
https://www.badcaps.net/forum/showth...71#post1151571

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I did an ohms test UC3844N of the working power supply and the nonworking power supply, using Pin #5 GND for Neg probe, and testing each pin. Below are the results.

Working Power Supply
Pin 5 to 7 = 28 Ohms
Pin 5 to 1 = 37 Ohms
Pin 5 to 2 = 46 Ohms
Pin 5 to 3 = 47.5 Ohms
Pin 5 to 4 = 35 Ohms
Pin 5 to 6 = 31 Ohms
Pin 5 to 8 = 35 Ohms

Non-Working Power Supply
Pin 5 to 7 = 28 Ohms
Pin 5 to 1 = 37 Ohms
Pin 5 to 2 = 46 Ohms
Pin 5 to 3 = 47.5 Ohms
Pin 5 to 4 = 35 Ohms
Pin 5 to 6 = 31 Ohms
Pin 5 to 8 = 35 Ohms

The measurements are remarkably consistent!



I next tested the MOSFETs on the PCB, with Gate + Drain touched by the Pos probe of the Ohmmeter, and the Source touched by the Neg probe. Both Q1 & Q2 still measured 18.5 ohms.

Finally, I removed both MOSFETs from the PCB, connecting 12V to the Gate, Pos probe of the Ohmmeter to Drain, and Neg probe of Ohmmeter to Source. Both MOSFETs measured between .7 and .8 Ohms.

Hi Per Hansson - thanks for your assistance. I've just tested the UC3844N in the same way Skeeter did (neg probe on pin 7) and I've got some wildly different results! Do these indicate anything to you either with regards to that chip itself or others?

Pin 5 to 7 = starts at 10 Ohms, slowly rising reaches 70 Ohms after a minute. Kept rising!
Pin 5 to 1 = 34.5 Ohms
Pin 5 to 2 = 10.9 Ohms
Pin 5 to 3 = 0.9 Ohms
Pin 5 to 4 = 8.9 Ohms
Pin 5 to 6 = starts at 110 Ohms, rising steadily to 170 Ohms after 1 minute. Still rising
Pin 5 to 8 = 2.8 Ohms

Thanks,
Joe

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yea if those readings are correct I'd say that Mosfets Q1 has shorted from drain to gate and destroyed the UC3844N.
What you can do is compare readings with your working speaker like Skeeter did:
Because such ohms readings will largely depend on your specific multimeter.
For example the increasing ohm value you see is just the meter charging capacitors that are in circuit and that fools the meter...
You can also start the speaker and measure voltage from pin 7 to pin 5 like discussed in post #400.

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thanks - I'll disassemble the working speaker to see if that gives different values. My voltage across pin 7 to pin 5 seems in range- it's fluctuating between 11.1V and 14.8V every second or so, briefly stopping at 13V on the way. Is that fluctuation normal while the circuit is live?

Assuming the measurements on the working speaker confirm the theory, would I need to replace MOSFET Q1 as well as the UC3844N on this one?

Thanks again - it's really very much appreciated!!
Joe

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thanks - I'll disassemble the working speaker to see if that gives different values. My voltage across pin 7 to pin 5 seems in range- it's fluctuating between 11.1V and 14.8V every second or so, briefly stopping at 13V on the way. Is that fluctuation normal while the circuit is live?

No this is not normal, it reinforces the point that Mosfet Q1 is dead.
You can read about how it is supposed to work in post #408 quoted here:

From your voltage measurements quoted below it sounds like the UC3844 controller does get charged up by these resistors to the "kickstart" voltage where it is suppoosed to start working (oscillating).
However when it tries to hand over control via transformer winding T1B via resistor R5 and diode D5 it fails and the voltage sags: the process then repeats.

Assuming the measurements on the working speaker confirm the theory, would I need to replace MOSFET Q1 as well as the UC3844N on this one?

Thanks again - it's really very much appreciated!!
Joe

Yes but this needs to be verified, usually when a Mosfet has a shorted gate there is allot of destruction, several others have posted about that earlier in this thread where they had to replace allot of components...

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ah, I was afraid you'd say that!!

I've just tested the working power supply from my other speaker and the resistance across the UC3844N pins are identical to the first one, so looks like that's just what my multimeter was reporting. However, the voltage across pin 7 and pin 5 is a nice constant 14.5V rather than the crazy fluctuations on the other board. Now I've tried powering up both boards side by side I can hear a very slight ticking every second or so from the defective one. It's really hard to pin down where the sound is coming from though so I'm not sure it helps narrow down the issue.

So, if we suspect Q1 to be dead on the bad board, how should I go about testing which components it might have taken with it? If I'm lucky might it be something that could fail in isolation, or will it pretty much always take down a few other components with it? I can't see any signs of burning or other damage to the board or any components, but obviously that might not mean a lot!

Thanks,
Joe

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, if resistance readings are the same between units it is likely the unit is mostly fine.
Measure Diode D5 and resistor R5.
The diode preferably with a multimeter that has a diode check function.