Repairing Alesis M1 Active mk2 Monitor Speakers

[Valet2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Here's my problem.

https://youtu.be/ooU_z3K5wHU

I've had the same problem with other monitors, and it's not a potentiometer. The issue remains even when I unplug the input and deattach the power cord (the monitor keeps working for a second using capacitors).

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Cold solder joints? Crapped-out electrolytics on the audio board?

Does it keep doing that while playing audio, as well?

[Valet2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes, and this low-freq noise interfere with the audio, making it much quiter and with much less lows.

I've looked inside - all caps look fine.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Just because they "look fine", doesn't mean they are fine.

That being said, even though it's less likely, silicon parts can occasionally fail, but here we're looking for something that's common for both the woofer AND the tweeter circuit. The tweeter's making those crackling sounds, right?

If that's the case, it's gonna be something between the input connector and the inputs to the filters. On page 10 of the service manual, if it's not the potentiometer, it could be the connectors between the two boards (J1/J2), or U1A/B and the passives around them.

That is, assuming the power supply isn't the actual culprit...

[Valet2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Nope, tweeter is silent - woofer is sick.

Will check the power supply then. I'm glad I have 3 monitors btw.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Oh, right - the video was a little bit confusing then.

Then you'll want to focus on the woofer side of the crossover (U4A-D and associated components), as well as the woofer amp (U5).

If the power supply was at fault, you might expect the tweeter to show some symptoms as well. But on the other hand, it has all the high-passing in the active filter.

If you put your finger on the dome, i assume / hope you can't feel the same low-frequency stuff the woofer's putting out, right?

[Valet2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yeah, it moves up to a centimetre back and forward at a random rate.

Maybe it affects the hi-freq section as well, but since the tweeter is unable to do a low-freq (plus that crossover) I can't tell for sure. Need an oscilloscope.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I just realized something - since the noise is low-frequency, there's a (greater) chance that a multimeter could pick it up on the AC range.

That way, you could check a ballpark-value for the ripple on the power amp supply rails, for example. Or on the input of the power amp. Or the supply rails for the opamps, and so on.

[VikHU]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi all, just popping in to stay thanks to everyone who has contributed to the information in this thread (especially Khron ).

I was able to resurrect my M1 620 which has popped its fuse. It took two tries before I came across it and I'm still not 100% sure but I think replacing U1 did the trick. R3 and R4 had burn marks so this time I've also replaced them as well as cleaning the burned up glue that partially covered them. Fortunately I didn't lose any traces. Before replacing U1 and the two resistors, I've replaced the fuse, RT1 and the four rectifier diodes (D1-D4) but they blew again (3 out of 4 anyway), with RT1 exploding.

This time I also rigged up an extension lead with a lightbulb and nervously plugged it in. The bulb came on for a second then went out and the speaker worked perfectly

I've now replaced C8 and C35 in both speakers. They were both out of spec, especially C8. Over 10 years of constant heating by the resistors certainly didn't help. But I think the real reason for the failure was the glue residue which shorted high voltage onto pin 7 of U1. Would it be a fair assumption?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

That's quite likely, i guess.

Depending on what revision power supply your pair has, you may or may not want to relocate one of those two capacitors like i had to (for future-proofing's sake).

[VikHU]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

That's quite likely, i guess.

Depending on what revision power supply your pair has, you may or may not want to relocate one of those two capacitors like i had to (for future-proofing's sake).

I opted for leaving the legs of C8 as long as I could (the full length of the shorter leg or about 2cm), and bent it well away from R3 and R4, almost into the gap between the heatsink and L2. I've also replaced the 37k resistors with 47k ones on the board I've repaired, which should make them dissipate about 20% less power. Even next to the heat source the original C8 cap lasted for over ten years, I can live with that

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, in high-frequency situations (which switchmode power supplies are), you usually want to minimize the caps' ESL (equivalent series inductance), which means leads as short as possible.

You might have a bit of digging to do in this thread, but i think i've detailed the little "swapsies" i did in the (first?) ones i fixed, back in the day.

[VikHU]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

By the way... having spent a fair bit of time studying the schematic for this power supply, I still don't understand some parts.

One of these is how Q2 gets turned on. Is it T2 that drives it? Or is it a pin 3 of T1? And what does Q5 do?

The other grey area is the circuitry below U1 on the schematic. I'm guessing that Q6 is opened by RT2 if Q2 overheats, which would disable the PWM output by forcing pin 1 of U1 to ground. What do D15, Q7 and C35 do? Do they implement some kind of slow startup by charging C35 via R24 which would make Q7 "switch off" and stop pulling pin 1 of U1 low via D15, enablng the PWM to start?

[VikHU]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, in high-frequency situations (which switchmode power supplies are), you usually want to minimize the caps' ESL (equivalent series inductance), which means leads as short as possible.

You might have a bit of digging to do in this thread, but i think i've detailed the little "swapsies" i did in the (first?) ones i fixed, back in the day.

I agree that moving it closer would be better, but I'm not convinced that the inductance of the component legs would make that much difference. The UC3844 datasheet says this about pin 7: "A bypass capacitor, typically 0.1 μF, connected directly to GROUND with minimal trace length, is required on this pin. An additional bypass capacitor at least 10 times greater than the gate capacitance of the main switching FET used in the design is also required on
VCC."

The cap with minimal trace length is C14, there is no such requirement mentioned about the big cap, which is C8. And isn't the sizing requirement of the big cap only relevant if the chip directly turning on a FET, where you need to overcome gate capacitance quickly to go from fully closed to fully open, to minimise the amount of power dissipated in the interim stage? But in this design, the FET is driven by a pair of BJTs, which are presumably easier to turn on, by virtue of being current and not voltage controlled?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

My reasoning was sort of "for both purposes" - both minimizing lead length AND getting that cap out of that crowded hot-spot. Two birds with one stone, and all that

Even though the datasheet doesn't specifically mention such details, it belongs to the "best practices" category (regarding board layout).

That current still needs to flow into and out of the gate capacitance of the low-side FET, even if the UC3844 isn't the (direct) driving element.

[Valet2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Okay, I've replaced ALL capacitors. But the problem remained.

The noise remains even after I unplug the power cord, until the input caps are discharged.

https://youtu.be/slcRNiSDMn0

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well then, the problem's not with the capacitors. Resistors are somewhat unlikely to cause that sort of behaviour, so i'd say it's some silicon that's likely the culprit.

I don't suppose you have access to an oscilloscope, do you?

[Valet2]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well then, the problem's not with the capacitors. Resistors are somewhat unlikely to cause that sort of behaviour, so i'd say it's some silicon that's likely the culprit.

I don't suppose you have access to an oscilloscope, do you?

I have some oldschool one

Will measure voltage on regulators. The problem is when I come home I feel so tired. Will bring the board to my work then...

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Oldschool is plenty - since the problem is pretty low-frequency, just about any scope will do.

My thought was to try to work back from the woofer output back through the power amp input and then the crossover, and see where the "bad signal" starts.

[kuf]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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