Repairing Alesis M1 Active mk2 Monitor Speakers

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Got a multimeter (and the knowledge / skill to use it)?

Dear Guys,

my mk2's went wrong too.. i read this entire topic, but have you got information about, where can i order a completely new power supply?

AL9-79-B601

All the shops, where i find says out of stock

yes, i tried to repair, first changed the condensators, but i'm not a good engineer

thanks!
Szabolcs

[kuf]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Got a multimeter (and the knowledge / skill to use it)?

off course, i've, but what happens, if other components are wrong? i read in other forums,that maybe ic or diodes are faulty too.

So ok, i've a try for myself.. tomorrow i change the r4 resistor.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, that's the point - finding the problem and taking care of it.

All that buying a whole new power supply (if you could even do that) would solve, is postponing the problem.

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Morning all. Especially Khron…

I too have acquired a pair of these speakers, one as faulty.
A little bit of background: I am a keen hobbyist, have been repairing- and recapping HiFi for years, am good at soldering, have a good Fluke meter with capacitance- and diode tests and know how to use it.
So far on the broken speaker I have found the following:
The obligatory C8 is….actually ok (!!!)
U1 s/c
Q1, Q2 s/c
R11 is toast, o/c
R15 o/c
Q3,4,7 s/c
D14, s/c

Now I have ordered the parts and as I am planning on doing the C8 to C14 and moving C14 under the board mod, I have both power supplies next to each other on the bench.

Now as we don't want any accidents later as a matter of course I measure every component on the board.
The one thing I struggle with, is actually understanding circuits, so I wanted to run this one by the hive mind:
Bearing in mind that the healthy board is complete and the bad one has a lot of things missing, including the above named parts and all electrolytic, and a lot of the non-electrolytics near R10.
On the bad board I read just under 1k for R10 in circuit, it should be 15k and reads 15k out of circuit. R10 also reads 1k against earth (HS1)?
On the bad board I read 15k across R17 in circuit (should be 15k), on the good one I read 3.2k in circuit? Out of circuit they both read fine (15k)
It looks to me like R17 and R24 form a voltage divider, with the thermistor RT2 in line with R17. I have had RT2 out and it reads 59 ohms, should be 100ohms, but I believe at 120C? On the healthy board it reads around 70 ohms, so not that much different.
I have tested just about every resistor, diode, capacitor and transistor, some out of circuit, and can't find any more faulty components.
Is the difference due to the missing components (ie U1?).
Or do I need to keep looking?
I don't want to solder the bad board back together only to find out that there is still something going on. Could any of the inductors be gone?
Many thanks

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Re: R10 - are those measurements taken before or after removing the toasted U1?

R17 and R24 actually form, at best, a current divider, and even that's only temporarily (until C35 charges up). Same question as above, though.

R17 and the thermistor do indeed form a voltage divider.

The difference in measurements may well have to do with the U1 being in circuit, on the good board.

Quite unlikely for any of the wiring to be blown - silicon parts are usually way more sensitive and prone to failure than copper. That's why the running joke in this field is that semiconductors usually sacrifice themselves in order to save the fuses...

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok that great to know. I mean the voltage divider/current divider knowledge.

Measurements are with U1 removed from the circuit. So I'll just wait until the bits arrive from Farnell and see what it all reads after refilling the board with fresh bits.

Thank you muchly, Khron, and if you have the time and patience maybe you could try and have a guess why those bits mentioned went kaput?

It would help me understand circuits better (which is the thing I struggle with).

Many Thanks

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

And I kind of answered my own question re the inductors myself, but it is always good to get some moral support...

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi, one last question as otherwise we are good to go, Khron.

I seem to have read a post by you somewehere here not t use wirewound resitors for R15, due to induction?

Induction remains a dark art to me, can you confirm and/or explain why it is nescessary to use a non-wirewound resistor in that place?

Many thanks

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Errm, maybe not the last question. So R10 reads just under 1k in circuit with R15 in place (on the bad board).

Once I take R15 out, everything reads fine. I have tried a couple of different resistors in that location, all causing the same result???

Logic tells me it must be R15 but somehow it must be something else in that part of the circuit?
Unless it is because all the resistors I tried are wirewound. That's what I ordered from Farnell (and I had lying around) and until I get something else I won't be able to find out.

Please help if you can.

Many thanks.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

It's perfectly normal. If you look carefully, R10 is in parallel with R11+R15 (which, incidentally, add up to virtually 1k).

https://www.digikey.com/en/resources...eries-resistor

https://en.wikipedia.org/wiki/Curren...hunt_resistors
"The parasitic inductance present in the shunt affects high precision current measurement. "

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, I have 2 boards, side by side.

The other, working one read 15k across R10.

Does that meant there is something wrong in that circuit?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

The 0.22 ohm current-sense resistor may well be open-circuit, especially if, say, the bottom MOSFET is blown as well.

[mercedesjenz1]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thanking you again, sir.

It was actually much easier than that (I knew that this was a good, working board). I had actually unsoldered one side of R15 and forgot!

I will have a look at those links you posted, I knew about Ohm's law but had clearly not used my brain, so will read up on that again.

Many thsnks, sir!

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Good luck with the repairs, then.

PS: No need to "sir" me - the Queen hasn't knighted me, i'm not even one of her subjects, and i'd like to think i'm not quite THAT old..... yet

[gpj1136]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I purchased the revised boards because I did not have time to deal with the problem last time. The original boards blew within a few hours of each other. Strange after years of use.

Of the revised boards one just blew with about 1/20th the use. This time I will replace the caps on the good one before it blows, and repair the three bad boards.

I just want to thank you guy's for this excellent thread (repair manual really)
Especially Krohn, and mamoka. You have gone to great lengths to help people with these monitors. If I find anything that hasn't been covered already I will add what I can.

[gpj1136]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

After taking the Monitor apart I notice that when the speaker is in an upright position. The caps are above the R3 and R4 resistors. It seems having them on their sides might not only be better for sound. Might be a bit of a stretch , but heat does rise, so in an upright position the caps might get hotter.

[james2k2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi All,

I've actually followed the advice on this forum to repair one of my units previously (replaced C8 with a low ESR Panasonic and the resistor nearby due to charring). I also stood off the cap to move it away from the heat a little.

The speaker in question is a spare 3rd one I have and I went to use it today and found it not to work properly anymore. The LED lights, tweeter works fine, but no woofer.

I have done lots of voltage testing and everything is in spec. The PSU is outputting a very clean +/- 37V on the HI and +/- 18.1V on the LO.

I've also tested the mute output to ensure Q3 on the amp board is OK and yep, seems fine. I've checked the output of the two +/- 15V regulators and all OK too.

Any ideas? Just a fried Audio IC - Seems unlikely IMO if it previously worked?

Cheers,
James

[AnjinMeili]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

First, let me thank everyone for putting this wealth of information together, related to this old monitors. Keeping things running is an awesome motivation for tearing things apart

I have a pair of Alesis M1 Mk2's here that a friend of mine is very attached to. They start to heatup and the performance is no longer what it was. Lots of crazy noises, hums, pops, hiss, or just stop. I immediately thought it would just be a cap issue, before I started digging into it, because it is one of the things that do fail in older amps and power supplies.

As I tear this pair down, they both suffer from overheating in the load resistor R37 that appears part of the RC snubber circuit bridging the secondary outputs of the flyback transformer. This half of the flyback feeds the +V-Hi that powers the woofer amplifier section.

While I notice in the thread above, damage to this area is common, I am not seeing the other related issues in this pair. I have a guess that when used at low to reasonable volumes, this circuit dumps far more energy into the RC snubber than the designers accounted for. But I am only making a SWAG at it from my rather rusty understanding of analog circuits

I would think simply using a higher wattage load would do. No need to change the resistance value, but moving to a more substantial dump would most likely do the trick. But before I just jump in and start changing things, I thought I might ask opinions. My limited, and as noted, quite rusty understanding, is that the combination of R37+C38 form an RC snubber that keeps things grounded so to speak, a little bungie cord that keeps things calm, at the price of lots of heat load. Is this evaluation correct?

Thus, simply increasing the wattage, while also sinking the load resistor to the metal chassis, should stop this from simply repeating?

Thanks in advance for any debate and discussion. I appreciate the wealth of information already laid out. And worry not, I will be replacing the caps anyhow, since I already got them torn apart

Cheers
Anjin

[DarthBoz12]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hello,

I'm new here and found the board through searching for this very issue. I have been trying to get my M1 Active Mk2 working again. I tried the C8 cap replacement, but it didn't fix the problem. The blue light pulses and the woofer pops at a regular beat when turned on, but that's all I get.

The caps I used were these:
https://www.digikey.ca/en/products/d...1MH20D/4843724

But after reading this thread, it seems they may not be up to the task, so I have ordered some low-ESR caps to try:

https://www.digikey.ca/en/products/d...5AG4AA/2712539

So, while I wait for those to be delivered, I have another (self caused) issue that I was hoping someone may be able to help me with. While trying to clean off some of the dark, hardened goop that was put all over the components, I managed to break off the leads of T2. The schematic lists this as only "Transformer 1:1 Gate Drive". Being somewhat of an electronics novice (I've had a few courses and understand the basics, but that's about it), I am unable to find a suitable replacement. I would appreciate it if anyone could point me in the right direction, or even where to get components and a guide to rewrap it, if that is possible.

Thanks!

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

So, while I wait for those to be delivered, I have another (self caused) issue that I was hoping someone may be able to help me with. While trying to clean off some of the dark, hardened goop that was put all over the components, I managed to break off the leads of T2. The schematic lists this as only "Transformer 1:1 Gate Drive". Being somewhat of an electronics novice (I've had a few courses and understand the basics, but that's about it), I am unable to find a suitable replacement. I would appreciate it if anyone could point me in the right direction, or even where to get components and a guide to rewrap it, if that is possible.

I can't see too many details on the picture you uploaded how, where, and which the wires/windings are broken... but if it's just a few turns, you can use magnet wire to connect to the broken winding(s) and extend/re-wrap however many turns there missing/broken off. Just make sure the two separate windings are well electrically isolated from each other as they were originally. Or if you have enough magnet wire of the same (or thicker) gauge/diameter, then fully re-wrap the inductor with the same number of turns as before. Whichever the case, just mind the direction of the windings while re-wrapping, as that, and the number of turns, is the only thing that matters. As long as the core is not cracked, you don't need to replace that transformer.

And if you are in a real pinch and don't have magnet wire, then good regular wire (with PVC insulation) could work too.

As for the rest of the main issue with the speakers... I haven't posted that much in it and don't remember the details too well. But try reading through the entire thread first (I know, it's 16 pages!) just to make sure you didn't miss anything.