Repairing Alesis M1 Active mk2 Monitor Speakers

[ernie_ohio]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What did you replace it (C8) with? I mean value, brand and series.

Absolute worst case, you could try replacing the UC3844 with a UC3845 - that has a lower turn-on voltage threshold (8.5v vs 16v for the 3844). Otherwise they're functionally identical.

Hi Khron666,

I replaced C8 with a Vishay 220uF 50V High Temp (-55C to +105C) 20% tolerance.
http://www.mouser.com/ProductDetail/...zWi3CsWFFPlKcw

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

"General purpose" - possibly not THE best choice. I would've went with a japanese low-ESR one, at least to be able to rule out as a potential cause of the non-start

I see it spec'ed at 400mA / 120Hz ripple current - the original (as you can see in my post linked below) was spec'ed at 640mA (likely at 100kHz, i don't have the datasheet at hand now).

https://www.badcaps.net/forum/showpo...0&postcount=27

[swarfrat]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

C8 is the bulk filter cap for the UC3844 VCC rail. R3 and R4 are just dividers on the input voltage doubler. I'm really not sure why they chose such a stiff divider just to balance voltage across caps in series on the 120Hz side. I was looking at the schematic to see if I could relocate them, but I'm actually thinking that upping them to 150k or 220k 1/2w, that should cut the combined dissipation down from 1.1W to 1/4w and make C6 less prone to cooking.

I did loose a trace underneath C6 as well, but it just goes to R3 which is parallelling C7.

[budm]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

R4 is not connected in parallel with the cap C7 (R3 is in parallel with C6), R4 is for supplying the start up voltage VCC pin7 of U1, once the power supply starts running the supply voltage will be supplied by T1B winding and rectifier Diode D5, then the current through R4 will be very small.

[swarfrat]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

C7 was a typo, I meant C6 (but can't edit). You're right that R4 isn't paralleling C7, however... once it does startup, It's only 15v or so short of the divider I was describing, and R3/R4 still have about 145v across 47k. I don't see anything about how much current the kick starter needs to operate - just "low". I don't know if upping those resistors to lower heat dissipated right there would prevent it from starting up. If it just has to kick the gate driver over once it shouldn't take much at all. It can't really be required to power up the UC3844 because 15mA doesn't sound like "low" (and the 47k already limits it below that figure.)

[budm]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

There will be about 170VDC on each cap (120 x 1.414, total output in doubling mode = 340VDC) @170 with 47K the power dissipation is about 0.60W. How hot do the resistors get? You can tweak the value of the resistor until it will not start up, then go back down from there.

[swarfrat]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Apparently hot enough to cook the cap.

[budm]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I guess so when the whole assembly is mounted inside a cabinet without any ventilation.

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi Khron666 - can I ask a quick question? I have this problem with my M1's too. A side issue on this is that C33 exploded. It looks like there might even have been flames, going by the soot and the black stringy deposits on the aluminium casing. In the other speaker I see its a small blue cap and the diagram says its 0.1 uF. What's the best cap to replace it with?

A small point. It seems on mine that when R3/4 overheated C8 it also overheated the side of C6. Where the top traces go underneath the edge of the cap casing I think the heat melted them and shorted them causing, amongst other things, the C33 explosion.

Although my one speaker is still working fine, the large C6 and C7 caps are bulging somewhat on both. So I am going to replace all 4 leaving a small gap between them and the pcb. I also want to relocate C8 to either the back of the board (and drill a vent hole in the casing to allow that) or to where C33 is like you did. Any possibility you can post a pic of that?

Well done with your repair by the way. Very creative!

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Just about any other ceramic or film cap should work, of similar value and voltage rating (50-63v).

Have you removed the two main filter caps (C6/7), to check the state of the board / traces right underneith them?

I can't remember if the big caps were bulging on my pair, but i decided to replace them with some Teapo i had on hand, just for peace of mind Although i wouldn't leave any gap between them and the board. Yes, i can see the point of "airflow", but if they wiggle around just on their legs, they might snap the traces connected to them.

Not sure if/when i'll get around to taking any pics though - i'm trying hard to work on my thesis these days, plus work keeps me quite busy. But a friend of mine will buy these off me, as soon as i have the time and energy to reinforce the "shelf" they're sitting on, 'cause the Soundcraft monitors that will replace them are like 5kg heavier EACH

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thanks Khron666. Exactly what I needed to know. I think I can work out what you did from your description regarding the repositioning. Will start stripping components off tomorrow. And order my new parts. Will post more info as I progress. Thanks again!

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, here's an update. I have the offending components stripped off the board. I have also done some cleaning around the impact zone. I discovered that it may not in fact have been the big cap that shorted the traces. Right next to the large cap are two points where the top trace and the bottom trace are connected through the board, I guess just by solder. It seems that this might be the original source of the problem. Whatever, where these points are the heat has burnt a hole through and charred the surround so its like a biscuit. Some of the trace has also been burnt away. The same is true where C33 exploded.

I was wondering whether I should actually make a complete new pcb seeing as I have the trace diagrams, and transfer all the components onto that. But then I thought of Khron666's beautiful hacker repair and decided I'll simply wire past the area of devastation, like they did with my bypass surgery.

Made a silly mistake ordering the big Nichicon 390uF caps. RS has two very similar on their website. One is dia 22 height 30, the other is dia 30 and height 22. The images are the same for both, and I ordered the "fat" one rather than the "tall" one by mistake. Was going to do this over the weekend but now I need to wait till Monday for the replacements to come. They are taking the others back.

Beware of that one. Apart from that RSOnline are excellent - huge selection and very quick delivery (if you are in the UK).

If anyone wants to see pics of this exercise for reference let me know and I will post them. I am taking pics all along the way.

So let me get on with it...

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Actually, (fortunately) i'm not the one with the charred PCB But yeah, as long as the distances between the high-voltage traces are maintained, wire links should work just fine. Perhaps a dab of hot glue to secure them in place, when all the rework is done?

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

My humblest apologies Khron666! Reading back I discover my inefficient memory conflated two sets of information. It was in fact the brilliant LuigiDJ who set the bar so high for creative repair of these speakers. I guess it is actually possible to wire the whole setup with breadboard (which would probably not fit into the speaker case so would have to be external) but LuigiDJ's demonstration of human ingenuity gets my prize for now.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

No worries If i were in the same situation, i likely would've chosen the same route

And while yes, it would be possible to breadboard the whole thing, i'd really advise against it. It would take forever to ensure the proper clearance / creepage distances between traces / pads, in the primary, high-voltage sections

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Just noticed something interesting about this board. If any of you has yours open or has the trace diagrams of it, take a look at C34's legs (its next to the big transformer in the middle). On the underside of the board the trace joins the two legs together which would render the cap entirely pointless. Its the same on both boards (L and R), as one might expect. Anyone else's like this?

Or am I going mad?

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Lol. Also, on the top of the board it doesn't connect to anything. On the underside there is only one trace connected to it. Its a cul de sac. Is there some mystery of electronics that needs explaining to me that I am missing here?

Btw the trace diagrams are the same so I guess they are all like this. Another design flaw?

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

"Electrostatic shield" it says on the circuit diagram and it is indeed drawn as just a loop off the ground with the cap in it, nothing else. Perhaps one of you brilliant electronics people might tell me what it does?

[Ozyris]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well I have now run out of ideas. I have replaced the obviously faulty components except for the two large caps which remain to be soldered. Before I put them in I thought I would test the good board and the "damaged" board against each other. They both now have exactly the same components in them as I am replacing the big caps on each.

I have been testing resistance between various points to try and see if I have got rid of the short that caused the big bang. The main thing I find is that at odd points across the "bad" board I am finding no resistance at all whereas on the "good" board I am finding 40 - 50 ohms between the same pairs of points, or 100% resistance. I am not finding any open circuits on the "bad" board where there is not a matching condition on the "good" board.

Without stripping everything off the "bad" board and testing all the components, and testing the board itself for shorts between the layers (where its electrically burnt), I can't think of how to take this further. Other than to make a new pcb and rebuild the thing from ground zero.

As a replacement board ready to plug in would cost around £70 plus postage I don't think that would be cost or time effective. Might be fun and educational.

Thoughts anyone?

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

"Electrostatic shield" it says on the circuit diagram and it is indeed drawn as just a loop off the ground with the cap in it, nothing else. Perhaps one of you brilliant electronics people might tell me what it does?

That cap indeed does look very pointless to me as well. Maybe the designers were reading some black magic theory books? I really don't know. All I know is, current always picks the shortest path to ground. So with the cap's legs shorted like that, I don't think any function will be affected if you remove that cap. But I am not a "certified" electronics person, so take that as it is .

I can't think of how to take this further.

Put the new caps in and use the series light bulb trick, then power it up and see what happens. The series bulb trick should prevent the thing from setting off a nuclear explosion should bad components still remain somewhere. You'll probably need a 100W light bulb. But if you start with 40W, it won't hurt anything either (the speaker PSU might just not power on, that's all). Also, the light bulb needs to be incandescent. Can't use CCFL or LED (plus, good luck finding one at that wattage).