Hi I Thought we'd take a look at my Alesis M1 Active 520 USB Studio Monitor speaker These are the speakers that I use for all of my video I didn't have done for many years now and I like it not only because they as a small and compact the 520 means it's a five inch main driver here but I like it for the fact that I it's got the volume and power switch on the front and I'm always using this and I don't have to dick around with like the software or an external box or anything like that. It's got the headphones on the front which I use for podcasting and it's USB As the name says, it's actually a USB interface as well which is handy. It just means that there's you know, one less box on my bench I don't need an external DAC or to use the crappy sound card inside the computer. so I rather like these up you, there's not too many like a USB monitor speakers on the market and they don't make this one anymore.

They do make the 320 USB which is a three inch driver version but anyway and I do like the fact that it uses a nice beefy XLR connectors for the to go to the other speaker. all of the driver is inside this one, the other one is just a speaker. it's got bass boost and a rear port on the thing and also totally I Really like this because I do video editing where like speech is everything you know like I don't have any music I don't do any music editing or anything like that. And apparently the crossover in these has been designed to avoid the mid-range of the voice.

so you know they're potentially a bit better than some others on the market in terms of our voice reproduction and stuff like that. Anyway, I think they're a cool little Monda speaker, but there's a problem with this one and that when I turn it on, it's supposed to have a funky blue light across here. There's obviously a LED backlight. This is just like a light pipe and it's supposed to turn red when it clips.

but I I don't think I've ever seen that cuz I've never over driven these things. but yeah, supposed to have a funky blue light so doesn't anymore. but it still sounds fine. but I thought I'd just crack it open because I haven't opened these before so we'll do a teardown and have a look.

What's wrong with that LED light shall we? now? what I'm interested in is that little board tucked up under there that has the power switch and the connection to the lead and the lead pipe. So I will remove the knob there and we can get down into the nut down in there. Oh, I need a socket driver for that yeah pair of needle nose pliers? She'll be right. Actually, as it turns out, I'm not the least bit interested in that it doesn't have the lead.

There's actually another lead here actually going down into the same hole down in there, which goes to the lid. So that's the one I'm interested in, don't It's a jewel color. the only two wires going in, but there's both red and blue. Nothing there.

but if we put it backwards and there you go, it lights up red. but blue is the problem. Hmm. can we get that off? All right? Oh yeah, whoa.

we're in like Flynn All right, the lids have to be tucked up and hots knotted all within there, which is a bummer. and they just go into the light pipe. But yeah, I assume it's just a jewel back to back. they back to back red and blue lead.

Okay, so what I've done is just put the power supply into constant current. Our mode 5 volts our compliance or output voltage that's just the technical term and 10 milliamps. So because the 121 multimeter is only capable of a couple of milliamps due to the 2.2 ha, there we go: Blue, Tada. No problems whatsoever.

if I change the polarity red so there you go, what current will it operate down to? and if I set it to 1 milliamp, which is the absolute minimum resolution of the power supply. Obviously having the I think it's about 2.2 K or something in series with the inside, the multimeter wasn't enough to light that up. That's a rather weird one, so that's okay unless isn't like an intermittent contact. I'm not seen.

Our problem seems to be down there. There's our connector for the driving that lead, so there's a little Teo 9220 next to that is that is that driving that thing and our switching? the polarity? Hmm, that could be the culprit. Hey I just decided to plug it back in. Sweet.

Look, look there we go. It's come good. Is it just a Dickey connection? but it went off on its own. Yeah, it's no.

no, it's fading is it? Yeah, there's something something not good there. I swear it was brighter before it's my imagination. Something weirds going on. So I kids I Don't think it's a maquette.

that's not a connection issue. Let me give it a bit of a give it a bit. a bit of a wiggle. Yeah, no, nothing to do with the connections.

So it's got to be electrical. It suckers. electrical. Yeah, it's definitely switching off and on.

It was off a second ago, trust me and it just flickered back on and there is some like some sort of flickering in there as well. So I don't know I depends on how they're driving this thing. we might have to reverse engineer the circuit, but we've got some blue. they're just switched off.

What I'm doing now is actually I'm overloading the thing to the hilt I'm feeding in a one Kilohertz sine wave an overload, another. don't even get the red clip in anymore. but I'm not driving the speaker here. but I was actually driving the speaker before at full volume and I didn't get the red clip.

So I think that part of it is buggered too. But yeah, just switched off and on zippity-doo-dah This thing like changes all the time. That's the problem with like faults like this. If you can't consistently reproduce it, then it can often be hard to track them down.

But we know there's something wrong with that lead driver thing. both. It looks like the both the power and the clip aspect to it. So it's time to measuring stuff.

So what we're gonna do is take a look around this Op-amp down here. Drive Transistor The three pins. There's our connector that goes off to the lead. You can see that the collector and the emitter directly goes across the lead.

so it actually yeah, shorts that out. but it basically goes across a diode here into at pin seven of this Op amp here. So the Op amps power from 30 volts there. The interesting thing about this is that there's it's actually switched off at the moment for switching on.

There we go I'm It's exactly the same. So that Op Amp is always are powered up because this is not a mains power switch is basically a soft power switches directly across the lead. and that's when it's So this is when it's off, its minus 1.5 and then it's minus 2.9 basically when I switch that power on. So in theory that should be enough.

but I've got the lead disconnected at the moment and I've got the lead connected. Switch it on. There you go. Two volts.

It's obviously not enough to drive that blue LED that there's something in the circuitry that's a starving the supply the current supply on this thing I think and that's why we can't see it. Just measure a diode up here. 0.7 No worries, that goes through to the base of the driver down there and that's not a problem because it's all in circuit. but we're basically getting that 0.7 Yes.

I Know the battery indicators on here I know what I'm doing I'm a professional. welcome to Dave CAD Reverse Engineering. Please excuse the crew D If the model didn't have tried to build a scale or to paint it, here's our two: LEDs there are back to back like this: a node cathode I've done them with the correct polarity blues, opposite polarity. Like that one side of them is going down to ground that transistor.

There actually are shorts them out. We'll see why in a second and it goes through a lead dropper resistor here to an Op-amp now. I've actually measured this and when the power is both off and on, we've got negative fifteen volt basically saturate point seven. It's saturated at the negative rail so that is the correct polarity if this is not.

- 15 volts the lead dropper. that's the correct polarity for the blue LED relative to ground here. because this is ground is actually a higher potential than - 15 volts. The blue LED is going to light up simple and likewise if are clipping happens I haven't like reverse-engineered all this side.

This is obviously coming from the clipping circuit and as soon as it clips and wants to turn on the lid, it then drives this high like this and that just drives the red LED like that down to ground. So plus 15 volts boom and the blue LED is back to front. but it's not presented with the high voltage because it's going to be limited by the 1.8 volts on the lead, the drop on the red LED here and vice versa so they don't damage each other so that's no problem at all. So what's this trainee doing here? Well I haven't done seen where this goes off to but somewhere on the power board.

so obviously when you are switch the pair off this actually turns on the transistor and then shorts it out. Oops sorry I forgot to draw in that's actually a Zener like that. otherwise the polarity would be backwards on this. This has to have breakdown in order to do that.

Anyway, when you switch the thing off, it just turns on this transistor via this pull up here and then just switches both LEDs off. It doesn't matter what the Op Amp can be doing, it can be flapping around in the breeze over here, oscillating to buggery. and it's not going to matter because this transistor is going to permanently short out both of those LEDs So when you turn it off, the LEDs go at both. Let's go off.

So this is a pretty basic stuff I Mean we could actually remove that transistor and it should work because it's basically going and negative plus/minus are 15 volts there. If we actually suck that out, then it like it must work. So I've actually measured the output voltage here. It is 14 point 7 volts but that's without the lid plug.

So let's plug the LED in. And if we still get fourteen point seven volts here - 14 point 7 volts that blue LED must come on. If it doesn't then there's obviously something wrong with this transistor here which is maybe partially on and then causing that blue LED to go off. And that kind of makes sense because we're getting seen it going or higgledy-piggledy so you know it's something.

something in here I Suspect it's not the Op-amp Let's measure that. There you go - 13 volts and the blue LED I can assure you is not on. So therefore that transistor must be conducting and causing that blue LED to switch off. We should be able to see that here if we actually probe the base voltage - 1.3 huh? That's interesting.

What's going on I'm gonna suck out that Trenee. There we go. Got it out. Let's test it.

Base Emitter There we go. Point Seven, Six. And the good thing about the fifteen volt? Our diode range here, so we could swap that around and actually test the reverse breakdown: The emitter base rate down voltage datasheet says six, but it's a highly variable number. There you go.

Seven Point Seven. So it breaks down so it's okay. Okay, this is getting ridiculous I Suck that transistor out and the output of the Op-amp is minus Thirteen Point Two volts and I've got the lead connected And there's the voltage across the lead. There, it is there.

Minus 1.9 and the blue LEDs not on it. What gives? There's no other path there. That's ridiculous. Well, you can see itself here.

There's the ground, there's then which the land. Here's the other side of the ladder. It goes over to this trace. Here, it's no transistor in there.

Remember, this is a single sided board. goes into a dropper resistor and through to pin seven of the Op-amp in assembly. Op Eva's - Thirteen Point Two. Let you turn on it doesn't Well, I think I'm done.

There's nothing wrong with that circuit at all. The only conclusion I can come to is that there's something wrong with the LED in there. Wow Wow Yes, I've measured the resistor. It's 1.5 K 13 volts.

Do the math. There's the Mongrel. it's embedded inside the plastic like this, so some sort of. it's almost like a Oh No is that is that epoxy in there or something? Anyway, I had a little hot snot around here I've cut all that off.

Yeah, it's really stuck in there. It's integrated in the whole thing. Obviously the rest of it's just a light pipe. So I there's one thing I can try rather than just barge in another.

Let what I can do is just snip the leads here, actually reverse them and then I'll have a red power LED all the time and a blue one. Otherwise I'm verbal an on blue one because there's something wrong with the blue. I mean it. It could be something.

you know. It's something weird going on with the dye I don't know. a little micro attachment, a little bond wire issue or something like that? No idea. Anyway, this is fascinating.

Watch this. I'm gonna put another 1k resistor in parallel with the 1k on there so powers turned on. you notice that leads off like nothing I physically do to it can make it. Come on, but watch this.

I'm gonna put 1k across there and it's on right and I disconnect it and it's a it stayed on for a bit and I actually got it before I got it - like latch on. It's something weird that's happened with that lid. Oh and it stays on. It stays on.

It flickers, it flickers. right. and I've monitoring the voltage and the voltage is the same look. flickers on and off.

I'm not touching anything. It's not. Well, no, it's not physical. look.

it's electrical. Wow The last thing I would have expected inside. okay, if it's fair enough, but some sort of electrical problem that we can solve by basically putting more current through it like that. So to fix this I could just that technically change the value of that resistor and everything's hunky-dory but like it's just gonna get worse.

Like it could. Certainly, it was almost guaranteed. Look at that. like what? Unbelievable fault.

Seriously, how lucky are we to find a fault like that? That is just that's insanely rare. So after all that, there really wasn't a problem. I Don't think there's a problem with the circuit at all. And of course you know you go down the rabbit hole because like, we were able to light the LED up and you know you just assume.

Okay, it's connected. It's not a physical, you know, jiggled the wires and stuff. It was fine and dandy. So you know the lid works.

So it must be a problem with the LED driver. So you go down that rabbit hole chasing a bloody red herring and then you come back up the rabbit hole. I Need to find that damn blizzard fault. Unbelievable.

This thing is hilarious. It's just there. You go back to front and got a nice red power LED I Like it up here. the blue clip LED Um, who cares? I don't use the clip functionality anyway.

Oh yeah, I could have like wide in a proper blue LED but then physical around all that sort of stuff. It was easier just to reverse the damn thing. and yes, I know that the blue LED in Reverse is actually protecting the reverse voltage of the red. LED So that may be a problem in the future, but now I'll deal with that.

Maybe I could add like another LED inside in parallel on the wires or or something like that. I mean this is a red, It differentiates. it is one that's been hacked and I so did that poor innocent back in and sorry I suspected you and there you go. I've got a nice blue red pail lid I like it fixed, we know in a chicken dinner.

so there you go I Hope you like that. While we were very lucky to find out, like a rather obscure problem like that, something wrong inside the lid. If you've seen this, had any good ideas like I've seen him fail inside. maybe like an intermittent bond wire contact, but it seemed to be electrical.

Maybe cast pass current through what it's making it like. you know, like a little diode connections. So like a little point contact diode connection or or something's really. you know there's a something physically wrong inside that thing that manifests itself in terms of current.

You put more current through it, it's fine and sort of stays Eric almost like latches kind of thing for a bit and then switches back on. So that was absolutely fascinating. And if you remember back in the video where we were passing current through that thing lighting it up, that kind of gave us an indication it didn't work down at those lower currency. probably should have like you know, like sort of smelled a rat back then, but like hey, the lead came on.

Let's say you know and you know I real. It's for the circuit. you know, boom that problems checked. So it's real obvious in hindsight, isn't it? I'm But that's where it leads you.

And yes, ironically, I'm gonna go edit this video right now using this puppy. So if you like that and found it interesting, please give it a big thumbs up. And as always discussed down below, catch you next time.