Repairing Alesis M1 Active mk2 Monitor Speakers

[Skeeter]

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.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Wow, I must say everything seems to work really well!
Just one thing: with the MOSFETs now out of circuit redoing the test do you have infinite resistance if you ground the gate?

You could do the tedious task of checking every resistor in circuit.
The logic is like this: you should not expect to see a lower resistance reading than stated, because that is not a failure mode of resistors...
So keep testing them and if you see a strange reading you have to take one leg out to test it properly...
If you find nothing wrong:
Maybe a nice "component tester" also known as "transistor tester" would be a good idea to get?
Here is one, I have no affiliation with the seller:
https://www.ebay.com/itm/265808759695

Then you can test all the transistors, MOSFETs etc in a very convenient way...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

This was my precisely my thought yesterday, that everything seems to be working as it is supposed to. So, yesterday, I reassembled everything, and turned on the units to see if the blinking led still persisted. And, yes, still blinking, and no sound from the non-functioning unit. (previously, I had also swapped power supply PCB from the 'good' monitor into the nonfunctioning monitor, and the nonfunctioning monitor indeed did function - so, I am convinced it is an issued located on the power supply board).

When you say "ground the gate," are you saying connect the negative side of my 12V power source?

So, I will check each resistor on the 'good' board, and compare it to each resistor on the 'non-functioning' board.
With the Component Tester, will I still need to remove one leg of each component to test?
Also, since the color bands on the resistors are getting old and difficult to read, may I use the Parts List from Post #29 as a guide?

Should I resolder the transistors, diode, and MOSFETs before I run through all of the resistors?

Thank you again for your considerable help! I wish I had someone in my town who could reliably diagnose and repair this unit. I originally took it to a local electronics repair shop, but after his time and testing, the man said all was functioning propertly. He charged me for his time, but it does not work.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

When you say "ground the gate," are you saying connect the negative side of my 12V power source?

Pretty much enough time has passed now that there should not be any charge in your gate left since your last test, so just check it with the ohm meter now like you did before.
But leaving the gate completely open/untouched "Pos probe of the Ohmmeter to Drain, and Neg probe of Ohmmeter to Source"
IF you still get a low ohms reading just discharge it by touching the gate and source together with your finger, but do discharge yourself first, because the gate is very sensitive to static discharges!

With the Component Tester, will I still need to remove one leg of each component to test?

Yes, that is true regardless of what tester is used...

So, I will check each resistor on the 'good' board, and compare it to each resistor on the 'non-functioning' board.
Also, since the color bands on the resistors are getting old and difficult to read, may I use the Parts List from Post #29 as a guide?

Yes you can use the schematic, just keep in mind when you find a difference to try to read the rings too, if you remember post #408 about R3/R4 for example...

Should I resolder the transistors, diode, and MOSFETs before I run through all of the resistors?

No leave them out, the less components are on the board the less false readings will show up...

Thank you again for your considerable help! I wish I had someone in my town who could reliably diagnose and repair this unit. I originally took it to a local electronics repair shop, but after his time and testing, the man said all was functioning propertly. He charged me for his time, but it does not work.

Honestly that is an interesting bit of information, because I tend to agree with him lol

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I have discovered the following resistor issues on the non-working power supply:

R24 is supposed to be 15K ohms, but it only measures 3K in circuit
R15 is supposed to be .22 ohms, but it measures 22 in circuit - this is a resistor that I replaced, and it appears I replaced it incorrectly with a 22 ohm resistor. I have the old one, and it measures .22, but it is in pretty bad shape, including the post is broken off at the body of the resistor, so I don't know if I can get a good reattachment.

So, I plan to order R24 and R15, and see if this fixes the problem.

Skeeter

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Since I know you are the pro, what Resistor do you recommend from the following? (pics attached)

R15
R24 (Only 1 kind in stock with the parameters offered at DigiKey)

Thank you for ALL your help!!

Skeeter


UPDATE!! I swapped the 22 ohm resistor with the old .22 ohm resistor (which is in terrible shape), and IT WORKS!! Basically, the .22 ohm resistor is barely holding a connection, so I still need to order a new one, and I will still replace the 15K resistor, because it measures only 3K. Wow, did I learn a LOT from you! Thank you for your patience with me.

Thank you! I am rather elated just now

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

R15 is supposed to be .22 ohms, but it measures 22 in circuit - this is a resistor that I replaced, and it appears I replaced it incorrectly with a 22 ohm resistor. I have the old one, and it measures .22, but it is in pretty bad shape, including the post is broken off at the body of the resistor, so I don't know if I can get a good reattachment.

Famous words:

P.S: Did any of the resistors you replaced measure bad, or why did you replace them?

In the voice of Homer Simpson: Doh!

R24 is supposed to be 15K ohms, but it only measures 3K in circuit

Do you mean it measures correctly in the other unit?
I would expect that Q7, RT2, R17 and C35 skews the results.
You can try to reverse the probes, Q5 might have been forward biased when you read 3K...
If you get perfect readings on this resistor on the other unit both ways around maybe C35 has failed open in it...

what Resistor do you recommend from the following? (pics attached)

I recommend a metal film replacement for R15: RR01JR22TB

For R24 you can always go with better ratings, here is a 1% one: RNF18FTD15K0
But you can just lift a leg on the current one and measure it, likely it is ok as I explained above.

I would also order some braid and a flux pen to wet it with, then clean up some of the solder, especially R27 seen here for example.

UPDATE!! I swapped the 22 ohm resistor with the old .22 ohm resistor (which is in terrible shape), and IT WORKS!! Basically, the .22 ohm resistor is barely holding a connection, so I still need to order a new one

Nice! But just a word of caution: if R15 where to loose connection the MOSFET's could explode, so do not run the unit more than necessary until you have replaced that resistor!

What about caps for the other unit? I would recommend to replace C8 & C35 in it too!

Finally your unit unusually has Rifa capacitors as the Y2 safety rated capacitors.
I don't like these because they usually fail shorted, this is not a safety hazard when they are in live > neutral connections.
But C37, C36, C3, C4 and especially C5 are connected to ground, if they would fail shorted (like they usually do) it would be considered a safety hazard.
If you see all other units posted about in this looong thread they are all safety rated blue disc ceramic capacitors, not paper capacitors.
So if you want you could replace those with something like this: CD45-E2GA102M-VKA (just check the lead spacing I choose is correct: 10mm and that they would fit in general.
Also after 20 years probably the X2 capacitors (C1 & C2) are bad too, if you wish you can replace them with these: F862BP224K310ZV054 (again double check lead spacing is 15mm).

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Wow! You continue to be a tremendous help to me! Thank you.

R24 is supposed to be 15K ohms, but it only measures 3K in circuit

Do you mean it measures correctly in the other unit?
I would expect that Q7, RT2, R17 and C35 skews the results.
You can try to reverse the probes, Q5 might have been forward biased when you read 3K...
If you get perfect readings on this resistor on the other unit both ways around maybe C35 has failed open in it...

R24 does measure 15K ohms out of circuit.
If C35 has failed, wouldn't that keep the unit from functioning, just like when C8 failed?

I recommend a metal film replacement for R15: RR01JR22TB

Will do!

I would also order some braid and a flux pen to wet it with, then clean up some of the solder, especially R27 seen here for example.

I have the braid, but will get the flux, too, to help.

Nice! But just a word of caution: if R15 where to loose connection the MOSFET's could explode, so do not run the unit more than necessary until you have replaced that resistor!

Good advice! I will wait to install the new parts before using the studio monitors.

What about caps for the other unit? I would recommend to replace C8 & C35 in it too!

I replaced C8 & C35 (not C38) on the non-functioning board. I purchased a set for the functioning board, too, based on the recommendations from others in the forum. So, I will do this when I receive the parts order that includes the .22 ohms resistor. (by the way, the R15 you recommend is physically smaller - will this still be okay?)

Finally your unit unusually has Rifa capacitors as the Y2 safety rated capacitors.
I don't like these because they usually fail shorted, this is not a safety hazard when they are in live > neutral connections.
But C37, C36, C3, C4 and especially C5 are connected to ground, if they would fail shorted (like they usually do) it would be considered a safety hazard.
You can watch this video to get an idea about their failure:
https://www.youtube.com/watch?v=XAbrU17hLTM

Are you recommending that I replace C37, C36, C3, C4 and C5 too?

If you see all other units posted about in this looong thread they are all safety rated blue disc ceramic capacitors, not paper capacitors.
So if you want you could replace those with something like this: CD45-E2GA102M-VKA (just check the lead spacing I choose is correct: 10mm and that they would fit in general.
Also after 20 years probably the X2 capacitors (C1 & C2) are bad too, if you wish you can replace them with these: F862BP224K310ZV054 (again double check lead spacing is 15mm).

Thank you. I will work on these, too!


ALSO:
Since heat seams to be a problem in the Alesis M1 Active MI, I am considering a small fan for each unit, such as a 2" or 55mm ultra quiet and slow RPM electronics fan for the back of the speaker enclosure, or a surface mount fan for the main backing plate. What do you think? I could mount on the backing plate and drill a series of small holes, or mount on the lower back of the speaker enclosure with a whole which corresponds to the size of the fan. I expect the power source to be a separate lead and plug into wall power. It may also be a thermostatically controlled fan. I just want to avoid sound issues created by the fan. What are your thoughts?

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Attached is my parts list, so far, not including any cooling additions.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

R24 does measure 15K ohms out of circuit.

Great!

If C35 has failed, wouldn't that keep the unit from functioning, just like when C8 failed?

No this capacitor is not as important, based on its location in the schematic it is there to provide an initial change in the base voltage to transistor Q7, changing the feedback behavior of the UC3844N.

I replaced C8 & C35 (not C38) on the non-functioning board. I purchased a set for the functioning board, too, based on the recommendations from others in the forum. So, I will do this when I receive the parts order that includes the .22 ohms resistor. (by the way, the R15 you recommend is physically smaller - will this still be okay?)

Yes it is fine, I just picked this one based on availability and price, you are free to select any of the others, here is the parametric search I used.

Are you recommending that I replace C37, C36, C3, C4 and C5 too?

It is not a necessary thing, but if they fail shorted and you don't have a ground available any exposed metal of the unit would be a real shock hazard.
If the unit is grounded well they will just fail violently and then go open circuit like in the video linked from my previous reply.

ALSO:
Since heat seams to be a problem in the Alesis M1 Active MI, I am considering a small fan for each unit, such as a 2" or 55mm ultra quiet and slow RPM electronics fan for the back of the speaker enclosure, or a surface mount fan for the main backing plate. What do you think? I could mount on the backing plate and drill a series of small holes, or mount on the lower back of the speaker enclosure with a whole which corresponds to the size of the fan. I expect the power source to be a separate lead and plug into wall power. It may also be a thermostatically controlled fan. I just want to avoid sound issues created by the fan. What are your thoughts?

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

Attached is my parts list, so far, not including any cooling additions.

Part list looks good!
P.S: I would not change R15 on your other still working unit, only the capacitors.
The reason it fails is solely due to one of the MOSFET's failing, so as long as they are fine no reason to change it... (But still good to have an extra on hand).

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Great!

No this capacitor is not as important, based on its location in the schematic it is there to provide an initial change in the base voltage to transistor Q7, changing the feedback behavior of the UC3844N.

Understood.

Yes it is fine, I just picked this one based on availability and price, you are free to select any of the others, here is the parametric search I used.

Very good. Thank you.

It is not a necessary thing, but if they fail shorted and you don't have a ground available any exposed metal of the unit would be a real shock hazard.
If the unit is grounded well they will just fail violently and then go open circuit like in the video linked from my previous reply.

I will order them and install. Thx

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

One of Khron's photos showed the C8 with long leads, which is exactly as I have done. So, I will start by just bending away from R4, just the same, and skip the cooling fan idea.

Part list looks good!
P.S: I would not change R15 on your other still working unit, only the capacitors.
The reason it fails is solely due to one of the MOSFET's failing, so as long as they are fine no reason to change it... (But still good to have an extra on hand).

Will do!

Thank you very much! This has been quite the adventure for me!

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

After doing all the repairs, I have a new problem:

I have successfully connected my computer to both of my Alesis M1 Active Mk1 monitors via a bluetooth adapter with RCA outputs. I run the RCA cable to the speakers, and have high quality RCA to unbalanced 1/4" plug adapters. Everything works great! However, when I connect exactly the same to my computer with a Behringer UCA222 USB audio interface, the blue led light on the front of the monitors blinks, the same way they did before I replaced C8. When I revert back to the bluetooth adapter, everything works fine. However, the bluetooth adapter is an older SBC codec, the Logitech Audio Adapter, and sound quality is not so good.

So, is the Behringer UCA222 just not compatible with the Alesis M1 Active Mk1? What should I do? What recommendations do you have?

Thank you.
Skeeter

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

However, when I connect exactly the same to my computer with a Behringer UCA222 USB audio interface, the blue led light on the front of the monitors blinks, the same way they did before I replaced C8. When I revert back to the bluetooth adapter, everything works fine.

Does the Behringer work with any other speakers?
And does the same thing happen with both speakers or only one of them?
If only one of them, is it the same speaker as you mentioned in this quote:

Now, one issue that I think I need to mention, but did not appear to affect this process:
Prior to any of this work on the power supply board, I found that the same speaker/monitor damaged my computer audio output. Upon investigation, I found that a post on the amp board was grounding to the back plate of the monitor, and sending 35V through the speaker input to the computer's audio output. So, I isolated the post with a couple of layers of electrical tape and sheathed the screw that mounts it to the back plate. That solved this issue. Do you think it may have caused a problem on the power supply board? The attached photo shows the problem post, now fixed.

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Does the Behringer work with any other speakers?
And does the same thing happen with both speakers or only one of them?
If only one of them, is it the same speaker as you mentioned in this quote:

Yes, the Behringer USB sound card works with other speakers.
Yes, the same thing happens to both Alesis monitors. I have not allowed the connection to be on for more than 5 seconds, because I do not want to damage the Alesis M1 monitors. The blue LED light blinks, so I expect it is a warning.

Thank you.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, no I have no idea why that might be, it does seem very strange!
If you want to trouble shoot it further you could measure the voltage across C8 when it happens.
Or perhaps better: the output voltages to the amplifier board, specifically green to purple wire.
If they really do get shutdown it is super strange, it would point to that rail being shorted.
As I wrote in post #406 the blue LED is connected directly to +V_LO line (before the voltage regulators).

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes, I am disappointed that I am having this issue! If I need to replace the Behringer unit for another, I can understand, since I did not spend too much $ on it. However, I just don't want to guess and buy another sound card only to find it will not work, either!

I will measure both of these. Can you please tell me the voltage that should occur across C8 when operating properly (and/or improperly)?
Just to clarify: you want me to measure Green (neg) to Purple (pos)?

While connected to the Behringer USB sound card:
The Green to Purple pulses approximately 3 to 4.5 volts.
C8 pulses 13 to 15 volts.

I can hear a clicking sound simultaneous with the pulses of voltage and the blue LED light.
Also, the Behringer USB sound card USB indicator light pulses at the same rate.
All of these are on the unit with the new parts.

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I have also found the following:
The Behringer line out measures 24K ohms. (my Alesis monitors blue light blinks and sound does not work with this unit, even though it works with other speakers)
The Logitech bluetooth device measures 15K ohms. (my Alesis monitors work with this unit)
My laptop headphones out measures 7K ohms. (my Alesis monitors work with this unit)

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

The voltages you should have on C8 and purple to green you can measure when the Behringer is not connected...
But to get a pretty good idea you can look at the picture in this sticker
https://www.badcaps.net/forum/attach...6&d=1651073329

As for C8 this is the VCC voltage for the controller, "you should have around 15VDC" as I wrote in post #400 but it is mostly an educated guess, so do check it without the Behringer...

All in all with the results you have posted I would say the +V_LO line is being overloaded by the Behringer, it is really hard to say why that is.

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

The voltages you should have on C8 and purple to green you can measure when the Behringer is not connected...
But to get a pretty good idea you can look at the picture in this sticker
https://www.badcaps.net/forum/attach...6&d=1651073329

As for C8 this is the VCC voltage for the controller, "you should have around 15VDC" as I wrote in post #400 but it is mostly an educated guess, so do check it without the Behringer...

All in all with the results you have posted I would say the +V_LO line is being overloaded by the Behringer, it is really hard to say why that is.

Without the Behringer:
C8 voltage measures 17.5VDC.
Purple to Green = 19VDC

With the Logitech bluetooth adapter:
C8 voltage measures 17.5VDC
Purple to Green = 19VDC

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yea there must be something odd with the Behringer to cause that.
I guess you could try e-mailing or calling them to see if they have any ideas.