Repairing Alesis M1 Active mk2 Monitor Speakers

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I should say that currently all of those tests were conducted with the components in circuit, is that sufficient at this stage?

Yes.
If you/we see something that we think looks odd, then that's when you might have to remove something to test it. Once done checking everything and if nothing pops up, then it might be time to start pulling components and checking out of circuit.

Finally, in order to reach the rear of the board I'll need to take it off the housing but I presume I'll need to bridge the jumpers that run to the switch to run the board that way? Any recommendations on a clean way to do that or would a test lead with alligator clips on either end be the way to go?

Yup, alligator clips will work for that. Or even better, as Per suggested, connect the series bulb there.

Yes this is correct, lamp in series with the live wire.
But another perfect spot for the lamp is across that missing switch (J3 on the schematic) you mention...
That said since you are already operating the set directly from mains and it has not exploded it is technically not necessary.
It is more when testing a set after having found some blown components and replacing them...
That said it is a very good protection against accidental probe slips...

Which is why I still do it myself, even though I've never had any accidents with that before. However, I did have improper wiring accidents, and the bulb did save me there are few times. Hence why I still think it's worth doing.

The only thing I've noticed on the rear of the board is a little corrosion (?) between pins 1 & 2 of the UC3844N. That's the close up picture of the rear of the board attached. Might that be causing a short or be an indication of the IC having sustained damage/worn out?

Looks like leftover flux from the factory soldering... but then it's also odd that this would be the only spot to have it while the rest of the board looks pretty clean.

Try cleaning it with some IPA and see if that changes anything. Probably not, but you never know.

I'm aiming to test the voltages across those secondary caps and, if needed, the primary ones momaka mentioned tomorrow, but let me know if you spot anything else on the pictures or need any others of specific areas of the board?

R27 (100 Ohms) on the secondary side looks like it ran a bit hot. Probably OK, but check it anyways.

R3 and R4 (both 47 KOhms) also look like they ran hot before. Worth checking those too. But again, make sure the primary caps are discharged (less than 1-2V) before doing that. Otherwise, you may damage your meter.

Finally, the new cap you soldered for C8, while not a problem for the time being (for testing), you may want to re-solder it as close to the PCB as possible once you fix the PSU. Vibrations from the speakers can cause joints to crack over time. (Hence why some components are glued down.)

Speaking of glue, I do see a bit of the problematic tan glue on the primary side of your PSU. But it doesn't seem to have turned dark brown so it's unlikely to be the cause of the issue. Otherwise, this glue is quite often a problem in electronics, particularly in places where things run hot. The heat bakes it into a dark brown color and then it picks up moisture from the air and becomes slightly conductive (in the high-KOhm to low MOhms range, which isn't a lot but could still disrupt some circuits.) So at this point, you can just leave it as-is. But once the PSU is fixed, you may want to remove it from components leads and exposed metal parts.

Oh, one other thing I keep forgetting to mention - my "good" speaker was the one I was originally more concerned about as it had been taking longer and longer to come on after I flicked the power switch. It now takes around 90 seconds to power up and for the blue led to come on. As they were bought as a pair I assume any age related degradation of components might be similar across both speakers, but does that sound like a capacitor on it's way out which might need replacing?

Yes, the slow start-up definitely does sound like a capacitor issue.
As electrolytic capacitors heat up, their electrolyte becomes more reactive, making the capacitors' ESR become lower.

A quick way to test a device that you suspect has dead or dying caps is to blow some hot air on the PCB with a hair dryer until everything gets fairly warm (but not so hot that you can't touch it.) If it works normally right away afterwards, there is a high chance capacitors are the issue. Of course, in some cases, it could also be bad/cold solder joints too. However, these are usually a little less responsive to heat changes and more to vibration / movement of the device.

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Sorry for the delay - I didn't have much time today but I've managed to take some photos of the components on the board in the hope that might give you some clues at to what the issue might be. The only thing I've noticed on the rear of the board is a little corrosion (?) between pins 1 & 2 of the UC3844N. That's the close up picture of the rear of the board attached. Might that be causing a short or be an indication of the IC having sustained damage/worn out?

I'm aiming to test the voltages across those secondary caps and, if needed, the primary ones momaka mentioned tomorrow, but let me know if you spot anything else on the pictures or need any others of specific areas of the board?

Oh, one other thing I keep forgetting to mention - my "good" speaker was the one I was originally more concerned about as it had been taking longer and longer to come on after I flicked the power switch. It now takes around 90 seconds to power up and for the blue led to come on. As they were bought as a pair I assume any age related degradation of components might be similar across both speakers, but does that sound like a capacitor on it's way out which might need replacing? Or might something else cause that long wait to power up after the switch is thrown? I've not yet touched that one since the other one stopped working but thought I should mention it in case it offers insight into the issue with the other one.

Thanks, Joe

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, so here are the results of the first tests you both recommended.
I should say that currently all of those tests were conducted with the components in circuit, is that sufficient at this stage?

Yes this all sounds fine, can you please take some good detailed high resolution pictures of the board for us to look at?
Perhaps we will notice something you missed because at this stage most things really seem fine...
You could just measure each and every component comparing them one-by-one on both sets and see if you find any differences...

Now I ready to start testing the voltage across C18, C20, C22 and C24 but I wanted to sense check your message about the light bulb in series with the live. Am I correct in assuming I would simply wire that bulb in series along the live wire from the wall outlet to the IEC kettle plug input on the speaker? I've got some spare 3-core so I presume making an extension with a bulb holder along it would do the trick for this?
Finally, in order to reach the rear of the board I'll need to take it off the housing but I presume I'll need to bridge the jumpers that run to the switch to run the board that way? Any recommendations on a clean way to do that or would a test lead with alligator clips on either end be the way to go?

Thanks again,
Joe

Yes this is correct, lamp in series with the live wire.
But another perfect spot for the lamp is across that missing switch (J3 on the schematic) you mention...
That said since you are already operating the set directly from mains and it has not exploded it is technically not necessary.
It is more when testing a set after having found some blown components and replacing them...
That said it is a very good protection against accidental probe slips...

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, so here are the results of the first tests you both recommended - firstly R15 measured 0.2 Ohms on my MM so I think that is in range? R5 gave me a solid 10 Ohms and the diode D5 read 0.653 in diode mode.

As all of those sounded like they were probably in the right ballpark I moved on to testing D10-D13 - all of them read 0.468/0.469 on diode mode. Caps C17-C24 I tested in resistance metering mode and all gave the same figures - started at 0.2 Ohms and slowly climbed as the caps charged.

I should say that currently all of those tests were conducted with the components in circuit, is that sufficient at this stage?

Now I ready to start testing the voltage across C18, C20, C22 and C24 but I wanted to sense check your message about the light bulb in series with the live. Am I correct in assuming I would simply wire that bulb in series along the live wire from the wall outlet to the IEC kettle plug input on the speaker? I've got some spare 3-core so I presume making an extension with a bulb holder along it would do the trick for this?
Finally, in order to reach the rear of the board I'll need to take it off the housing but I presume I'll need to bridge the jumpers that run to the switch to run the board that way? Any recommendations on a clean way to do that or would a test lead with alligator clips on either end be the way to go?

Thanks again,
Joe

[zinger5]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Wow, thank you both so much for your time and help! I'll work my way through those tests in the morning and will come back with an update on how it goes.

Thanks again,
Joe

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Been a while since I last looked at the schematics on these...

But with Q1 being the main switching MOSFET... if you have fluctuating voltage between pin 5 (primary ground) and pin 7 (UC3844 VCC voltage), then that means the UC3844 is likely OK and trying to start up. It also suggests Q3 and Q4 might be OK too. However, Q1 may or may not be good. One way to know is to measure the resistance of R15 (0.22 Ohms). You should get a very low resistance on your multimeter - same as if you were to short the probes together (i.e. lowest resistance your multimeter can measure.) If R15 is good, then Q1 is probably OK too and there is something else preventing the PSU from starting up. In that case, check diode D5 and resistor R5 (10 Ohm). These supply the UC3844 with power when the PSU is in steady-state operational mode.

If D5 and R5 are OK, check the output rectifiers on the secondary side, D10 through D13, along with the filter electrolytic capacitors, C17 through C24. If these are good, see if the PSU runs when disconnected from the amplifier board (measure voltage output across C18, C20, C22 and C24.) For t his test, it might be a good idea to connect a 100-300 Watt halogen bulb in series with the Live (240V AC in your case) line on the input. This is to prevent damage to the PSU, should some component fail suddenly.

If no voltage on the output at C18, C20, C22, and C24, time to check more components on the primary side. Start with Q3, Q4, and Q5. But before that, make sure the primary caps, C6 and C7 are discharged. BE CAREFUL around these - a faulty primary could have these still charged close to 340V total, which can definitely hurt you if you touch the wrong parts or damage your multimeter. You can discharge these safely with a low-power device, such as a hot glue gun, soldering iron, or portable lamp. Just connect the plug from one of these across each cap, and that should discharge the caps pretty gracefully. Once C6 and C7 are discharged, then check Q3, Q4, and Q5.
Q4 is a PNP transistor. With black MM probe on its Base terminal, you should get around 600 mV reading on your MM when you touch either the Emitter or Collector terminals with your red probe. For Q3, you should get the same readings, but just repeat the test with the red and black probes reversed compared to Q4 (since Q3 is an NPN transistor.)
Q5 may be a bit tricky. Due to transformer T2A, you'll probably see a short-circuit / very low resistance / 0 mV reading between Base and Emitter terminals. But for Base and Collector terminals, measure it like Q4 and you should get around 600 mV again, if I'm not mistaken.

[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.

[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

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

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

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

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

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

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

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

[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!

[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

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).

[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!

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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