Repairing Alesis M1 Active mk2 Monitor Speakers

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I think what Micwest meant by his comment above is that he touched his positive multimeter probe to the MOSFET's Gate and the negative probe to Source. When you do this for a N-channel FET out of circuit, this will make the MOSFET turned "ON", so you will get a low resistance / diode reading between Source and Drain. If you de-energize the Gate, you should get open-circuit or a diode reading between Source and Drain, depending on the probe orientation, as you said. So his MOSFET may be okay, if I understood his testing correctly.

That's what i had tried to confirm mitigate by having said:

Touch the gate and source of Q1 together (to discharge the gate) and measure again. If there's any charge left on the gate, that will keep the MOSFET open, and the drain-to-source voltage drop will be lower.

[audioboy]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi guys,

Hope someone can lend me a hand here! My mk2s blew a few months back and I've finally gotten round to opening them up. Bare in mind I'm a complete novice when it comes to electronics.

I can see a trace that's burned off so that's an obvious fix but I'm stumped as to what else may be wrong. I've tested the c8 and c35 caps and they seem fine to me so I started testing other components in the hope that I'd stumble across the fault. This is probably a silly way to go about it but I'm not sure what else to do. Anyway, I took off R3 and R4 to test them and found they measured 38k instead of the 47k that the service manual declares them to be. Excellent! Or so I thought. The colour code (orange, grey orange, gold from what I can see) tells me that it's a 38k resistor.

So then I thought I'd pull out another resistor to see if I could make sense of that measurement. I pulled out R27 and tested that. Service manual says it should be 100ohm but the colour rings (red, black, gold, gold) tell me it's a 2ohm resistor with a tolerance of 5% and it's reading at 2.4ohm which is outside of its tolerance.

I'm so confused. Should I just ignore the service manual altogether? Is R27 bad? Help... please?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

1) Photos
2) Tested C8 / C35 how, exactly? And "they seem fine" to you, based on what?
3) What colour code / resistance can you measure on R28? They should be the same (at least in theory).

[audioboy]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you for replying so quickly Khron, I appreciate it.

1) I've attached some photos, hope it works, not used this forum before. There was a load of gunk there that I cleaned out (didn't realise how much I'd scratched the board until I took the photo... *facepalm*).

2) Sorry, was way too vague there wasn't I. I tested them using an ESR meter.

C8: ESR=0.31ohm, Vloss=2.0% and 342.3uF
C35: ESR=6.8hom, Vloss1.2% and 1081nF

Forgot to say that the C8 cap is 330uF 35v but again the service manual says it should be a 220uF 35v although from what I've read it's ok to have a higher capacitance... ?

3) Colour code on R28 is the same as R27 (red, black, gold, gold) and also measures 2.4ohm

Looks like that whole area around D5 has overheated. C8 has some brown stuff caked on to the bottom as you can see in the photo. D5 was covered with the same or similar brown substance what I thought at the time was burned adhesive but doesn't make sense why D5 would be covered in adhesive in the first place. Measured D5 with a multimeter and got 0.62v so I assume it doesn't need replacing even after being covered with that crap?

Wondering if I should remove the burned adhesive on the board just to the right of R18? I read somewhere that it's conductive once browned but don't know how much truth there is to that. Got some conductive paste to repair the broken trace under R18. Do you think maybe the problem is just that broken trace?

Also, after taking out D5 I noticed that the holes for D5 and R5 are pretty close and looks like they'd join when I replace D5. I didn't notice this before removing D5 so I'm wondering if it's my rubbish soldering skills but then looking at the diagram in the photo it looks like they should connect perhaps? Sorry, so many questions and a complete electronics noob... but I'm enjoying it, so far

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

As a rule of thumb, if an electrolytic says "VENT" on it, i don't trust it as far as i can throw it. Only (some of) the crap brands have that written on them; the good ones don't - coincidence?

I'd replace that, if only to save myself potential headaches down the road. And i'd recommend you use the method i did - relocate C8 to replace C14 next to the chip (and away from the heatsink), and add C14 back, on the bottom of the circuit board. It's in parallel with C8, so no worries there.

If the colour code matches the measured resistance, AND they're both the same, i'd call them good. Highly unlikely they'd both deteriorate to the same exact value, wouldn't you say? And their values aren't exactly critical anyway, they're there more for current-limiting purposes.

Why would D5 be covered in that crap? One reason - zero f**ks given during applying of said goop. But if it measures ok, and the legs aren't too corroded (from the goop), i guess D5 should still be ok.

Yes, remove as much of that adhesive as you can, burnt or not. Yes, it does indeed turn dark, brittle and conductive, as well as corrosive, which is, as i'm sure you can imagine, quite unhealthy for copper traces and component legs.

Conductive paste? I wouldn't, sounds messy, and since that's the drain of one of the MOSFETs, that'll be handling "a bit" of power. Just scrape off the soldermask (green transparent varnish) from the trace, and run a wire (or a resistor leg) where the trace should be / used to be.

Well, what good would "one leg in the air" for either R5 or D5 do, hmm? No physical connection = no electrical connection (since this isn't RF black-magic-voodoo stuff), soooo...

PS: Investing a couple quid in some decent solder-wick (aka. desoldering braid) is priceless - works great for cleaning holes like those.

[diydidi]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Take note that the Driver IC can be powered from a 15-19V bench supply. Current limit will be a good idea. This is just to troubleshoot and see that the IC oscillates.

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

If the colour code matches the measured resistance, AND they're both the same, i'd call them good. Highly unlikely they'd both deteriorate to the same exact value, wouldn't you say?

Also, some multimeters have considerable offset at low resistance measurements. It's not uncommon to see up to 0.2-0.5 Ohms on a decent meter and up to 3 Ohms on a very cheap meter. So always keep that in mind (best way to check is to touch the multimeter leads together and see what resistance it shows on its lowest scale - that's usually your offset for low resistances below 10-30 Ohms.)

Yes, remove as much of that adhesive as you can, burnt or not. Yes, it does indeed turn dark, brittle and conductive, as well as corrosive, which is, as i'm sure you can imagine, quite unhealthy for copper traces and component legs.

I concur.

That tan glue is absolutely horrible. While it doesn't usually go conductive enough to pass considerable amounts of current, it can pass enough current to turn on a transistor or pull a signal low or high - and that alone can destroy your power supply.

And like Khron said, it corrodes traces, so that's another good reason to remove it.

PS: Investing a couple quid in some decent solder-wick (aka. desoldering braid) is priceless - works great for cleaning holes like those.

And a stainless steel sewing needle. You may find one or the other easier. Me, I am definitely better with needle (no I don't do drugs, lol) and can't use wick for almost anything except BGA rework. So you may want to try both methods and see which one works easiest for you.

[ultramag]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hey, so I've been looking at one of these for a couple weeks now trying to get it working. When I got it from a friend the fuse had blown, RT1 and R15 had exploded.
I swapped out C6, C7 and C8 as they had domed. It seems to be the newer revision as C8 is on the other side of the heatsink to the service manual I found online.
I swapped RT1 and R15, measured Q1 and found it was shorted. Swapped Q1 and Q2 but as I removed Q1 the trace lifted with it (goes from Q1 drain to Q2 source through a via on the board). Ran a wire where the trace used to be to reconnect that.
Next I checked the diodes on the bridge rectifier, measured continuity both ways on them and some erratic readings so swapped the lot of them.

So maybe in haste, thinking that I'd found the main problem (Q1 shorting seemed to be the cause) I swapped the fuse and tried to power it up. Fuse blew again straight away. Checking Q1 again it is fine, reading about 490 between drain and source, same as Q2. Diodes all seem fine as well.

I have little to no experience with power supplies but it would be nice to get this working again. Does anyone have any ideas on where to look next? I've clean as much of the glue/fixative from the board as possible (or where appropriate).

My friend noted that before the speaker finally blew, it would cut out a the light would flicker if you turned the volume up past 50% on the back.

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

https://oscarliang.com/ctt/uploads/2013/10/mosfet.jpg

http://www.savel.org/stuff/20050525g.gif

(Just to avoid confusions between the meanings of "left", "right" etc)

In the diode-check mode, it does matter which probe goes where.

"No reading" would mean something like "OL" or "1 " (depending on how your meter indicates "out of range"); if it shows all zeroes, or "0.00x", then that's a wire / short-circuited - slight difference there, i'm sure you'll agree.

thanks for the clarifying images, Q2 and Q1 give ma about a diode drop when measuring source to drain. Q1 does the same for gate to drain. Q2 gives no reading on gate to drain (reads 1).

Is the gate to drain supposed to give a diode drop? Or is Q1 defect (i did replace this one so sounds weird).

EDIT -- i read on and noticed you mentioning

On a good MOSFET, you'd only expect around a diode-drop with the red probe on the source, and the black probe on the drain. So if you're getting 0.57v both ways between the drain and source, that's definitely gone bye-bye

This leads me to thinking Q1 got faulty again, but now to found out what caused this. Any suggestions on that?

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Is the gate to drain supposed to give a diode drop?

Nope! - Not outside a circuit, anyways. In circuit, there may be a protection diode connected to the Gate, in which case that's what you might be reading across.

But in a MOSFET, the Gate is electrically isolated from both Source and Drain (there is a small capacitance between Gate and Source, but that's it). So outside of a circuit, there shouldn't be any resistance or diode drops at all from the Gate to Drain/Source.

[superrushfan]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

great discussion. does anyone have a schematic for alesis M1 #9-40-1386-a , i think it is the first version of this marked 10/06/98 94vo type 1. its a little bit different than the schematic on page 2. my symptoms are smoke coming out of the tweeter but resistance check shows 3.2 ohms .

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Amp chip's blown? Sounds like either oscillation, or a considerable DC offset's going on...

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Nope! - Not outside a circuit, anyways. In circuit, there may be a protection diode connected to the Gate, in which case that's what you might be reading across.

But in a MOSFET, the Gate is electrically isolated from both Source and Drain (there is a small capacitance between Gate and Source, but that's it). So outside of a circuit, there shouldn't be any resistance or diode drops at all from the Gate to Drain/Source.

ok, so this means my replaced Q1 is faulty. What would be the best way to continue? I could replace Q1 again, but my gut tells me it will break again since there is another component faulty which I still need to identify. I have a working speaker that i can cross measure stuff, but I'm just not sure where to begin

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

ok, so this means my replaced Q1 is faulty. What would be the best way to continue? I could replace Q1 again, but my gut tells me it will break again since there is another component faulty which I still need to identify. I have a working speaker that i can cross measure stuff, but I'm just not sure where to begin

Get two more replacement MOSFETs for Q1 first.

Next, check transistors Q3 and Q4 - these are the drivers for Q1. If one of them is bad, they can cause Q1 to get stuck ON and burn out.

Once Q3 and Q4 are verified to be okay, install the first replacement MOSFET for Q1 and use the incandescent series bulb "trick" in place of the fuse to prevent the power supply from blowing up Q1 and any other components again.

Incandescent bulb trick:
https://www.badcaps.net/forum/showpo...4&postcount=70

If the bulb stays fully lit, that means Q1 is stuck ON and may need to replace UC3844 IC. On a normally working SMPS, the incandescent bulb will usually glow briefly for a moment or two and then go OFF or very dim. Of course, that will also depend on your bulb's power rating. I recommend at least 60 Watts. Around 100 Watts is what I use for testing larger SMPSes like the ones for PC and audio gear.

While testing Q3 and Q4, also check D14, Q2, D6, D7, and R18. If these are bad, that could make excessive current flow through Q1 as well.

[remondo]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

While testing Q3 and Q4, also check D14, Q2, D6, D7, and R18

Hi, I have the same issues as everyone else here and I'm in the process of tracking down the issue(s).

Do I need to remove these components from the board to test them?

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

^ If it's not an issue for you with desoldering those components, then yes, remove and check them anyways. Better to check more parts and find nothing is wrong than to skip a few, then replace the bad one(s) and find out the PSU has the same issues again due to one of the skipped parts also being bad.

[ancient geek]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Anyone got any advice on the correct value of L4 of the PSU for this unit. Mine was all burnt, taking out a bunch of other components. The schematic says 25uH, and this is what I have installed, but it gets very hot (>100C) and has fewer turns than the original.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Not all inductors are created equal.

I don't suppose you can tell anymore what colour the core might've been, can you? Yellow on the outside, inside and one flat side, and white on the other flat side, perhaps?

[niripas]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi guys. I was lurking around for year here. I love the forum and I finally had some spare time to check what's wrong with my Alesis M1 Mk2. In the past there was no time to fix them and they were usually blowing up just after warranty and it was actually cheaper (and faster) to buy new ones than to send them for repair. So I ended with 6 of them Reading this thread I spent a few hours yesterday just to see what's wrong with them and most of that time - cleaning them. And I cannot emphasize enough of one of the advices in this thread:
REMOVE ALL THE GUNK YOU CAN. In my case 3 out of 4 speakers had good C8, but the gunk was dry and conductive. It measures between 80k and 200k Ohm when dry. Out of those 3 - one is the newer version with C8 moved away from the hot spot, but all of them developed fault in the very same place. I don't know If I will have time to diagnose and repair those and basically toroidal transformer + two bridges and some caps looks like a much faster solution. I'll remove the rest of the gunk (which is pain in the rear) and then start to read this thread again to look for pointers
If anyone is not yet convinced why you should remove the gunk - I am attaching a few pictures what happens under the gunk if you let it dry.

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi niripas, welcome to the forums!

REMOVE ALL THE GUNK YOU CAN. In my case 3 out of 4 speakers had good C8, but the gunk was dry and conductive. It measures between 80k and 200k Ohm when dry.

Yes, that's very typical of this conductive glue. Although the resistance is not enough to cause major current flow to melt traces, it is more than enough to bias transistors and thus wreck the circuit. It is also corrosive and will eat traces away over time.

So I concur with what you said - always remove that glue from electronics, regardless if it has gone conductive or not.

basically toroidal transformer + two bridges and some caps looks like a much faster solution.

That's actually not a bad idea. I was thinking of suggesting the same to someone else with a different speaker setup in another thread. Sometimes these PSUs may not be worth the trouble. However, this applies only if sourcing the transformers you need (with the right voltage) is possible.

If you do go this route, please post about your mod here, as I'm sure it would be beneficial to others who might want to try it. In all honesty, I'm starting to believe that an SMPS in a speaker enclosure is just a bad idea... as is placing the amplifier inside it - at least from reliability standpoint.

I'll remove the rest of the gunk (which is pain in the rear) and then start to read this thread again to look for pointers

If you try to repair the original power supply... or rather once you've made an attempt, use a series incandescent light bulb (60-100W should work) with the AC line to limit the current, should something try to go bad. In a case like that, the light bulb will become fully lit and stay that way. A bulb that is off or just slightly starting to glow means at least there are no short circuits.