Repairing Alesis M1 Active mk2 Monitor Speakers

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Step one - remove the visibly damaged components
Step two - remove all of that crap brown (or black) glue; it dries out, it's corrosive and it turns conductive

R11's the really-blown-up one in the middle of the third photo, right? In that case, there's a good chance the 3842's blown as well.

Do you have a diode-test mode on your multimeter? That would / will come in really handy for double-checking some of the silicon components there.

Step one and two done, see new picture. On the brown spot on the left of the + of C6 a little of the board burned/'eaten away', there's no shortcut and I don't think I have to isolate that extra or do soemthing with that? And yeah, R11 looks blown-up/burned.
My multimter got a diode-test. Which silicon(?) components I have to check? In/out board?
Thanks already for the very quick help!!! I don't have much time for this so won't reactie every time that quick...

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I measured in diode-test both ways;
D6 0,5;>> inside board
D7 0,5; >> in
D8 0,5; 1,2 in 0,7; >> outside
D9 0,16; 0,16 in 0,5;>> out
D14 0,46;>> in
D16 0,46;>> in
D15 0;0 in and out, so have to be replaced, one error more found, thanks :-)

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

The 'hot' 2watt R3+4 should be 47k conform the scheme, if I measure them they are 38k and the colorcode(red,yellow,orange,gold) tell they should be 24k !? I would like to replace them, but for what?
R5 is conform scheme 10 ohm, conform colorcode 5,1 ohm. R18 is conform scheme 100 ohm, conform colorcode 500 ohm. The scheme is officially of the MKII, mine are 620's, allthough the number of the print is the same, maybe the 620 use different values? (Also mine U1 is uc3842 io uc3844, R9 20k io 6k2, Q3 2N5401 io 2N4401). My idea is to exchange them based on the colorcode, is that the right choice?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

The exact values of R3 and R4 aren't critical, as long as they're the same. Although, the lower the value, the hotter they'll get. Those are there only to make sure the voltages across C6 and C7 are divided equally.

R5 isn't critical either, that's there only as a sort of a "fuse". Anything between 5-22ohm should be fine there.

They might've tweaked some of the component values between the Mk2 and the 620, so if the colour codes match with the actual components, stick with those.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Okay, all broken and needed parts are ordered and hopefully I receive everything this week, so I can rebuild this weekend. Thanks for the help so far.

[Shabazxz]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hy. There are 6 months now and they are working fine. With UC3842 it`s ok.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Replaced:
D1-4; R5; D5; U1; R18; D15; C35; R11; Q4(2N5551); R15 (see attached scheme)
But still fuse is burning out directly (without any other visible damage)
What can/have to replace/measure more?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Are the two switching MOSFETs still MOSFETs (and not just pieces of wire, ie. shorted through)?

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I guess that are Q1 and Q2? They look normal. I measured on board the resistance(l=left, m=middle, r=right pin)
Q1: l-m 77kohm, m-r 0 ohm, so broken?
Q2: l-m 660kohm, m-r 660kohm, l-r 10kohm, good?
I measured also my other 620, same problem(fuse), nothing burned or replaced yet
Q1: l-m >>>, m-r 0,6 ohm, so broken?
Q2: l-m 120kohm, m-r 100kohm, l-r 10kohm
In the first speaker Q1+2 are FTP08N50, IPS YHD3, HB001S
In the second FTP08N50, IPS YGH2, HB014S
They all look original and not replaced ever. Isn't that strange?
Should I try to replace only both Q1? And by a FTP08N50 or conform scheme a IRF840?
Thanks again for the help!

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers



See the drawing of the package, and the schematic symbol (G = gate, D = drain, S = source).

Assuming the left / middle / right is viewed from the front, that would mean Q1's shorted drain to source, a pretty normal failure mode. If you look on the schematic, that would mean the rectified mains (about 325v or so) goes through L4, through the transformer primary, through Q1 (which is essentially a piece of wire now), through the source resistor straight to ground. That's gonna blow any fuse.

No, it's not strange - they probably got those FTP08N50's for less money than the IRF840's; the ratings are similar, and neither are anything too special (8A 500v N-channel MOSFETs).

I'd replace both Q1 and Q2, "just in case". Just about any TO220-packaged 8A 500v MOSFET should be fine.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Okay thanks, your assuming is right, so mosfets(IRF840's) ordered in China, so take a while again, but 30x cheaper then buying in my own country(the Netherlands). In China 10 pieces for €1,38 including transport :-)))

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Umm... Yeah, they may be cheap, but you're running the (serious) risk of them being fakes, and then wondering why it keeps blowing fuses again. I'd really steer clear of that, ESPECIALLY in mains-related / mains-connected devices.

http://sound.whsites.net/counterfeit.htm

I wouldn't worry that much if these were, say, output switching devices in a class-D amplifier (which isn't meant for "welding", or continuous full-power use), but i wouldn't wanna take my chances of "shooting myself in the foot" (by creating such headaches, if not even health hazards) just to save 2-3eu.

But that's just me, i'm weird like that...

PS: Legit ones aren't THAT expensive anyway.
Here are results from Farnell for 8-15A 5-600V MOSFETs in TO220 (normal and isolated) packages, in ascending order of price:
http://nl.farnell.com/w/c/semiconduc...w&sort=P_PRICE

PS2: Found the Dutch Mouser too:
http://nl.mouser.com/Semiconductors/...d&Ns=Pricing|0

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Oh, okay. I thought 99% of all this stuff is coming from China, also the ones I buy here in local shops? Aren't they the same?
And i thought these mosfets are 'just' for the powersupply unit, so won't impact the quality of the speakers. But if they are doing a hardjob in this part, I will reconsider to buy some in NL...

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Most electronics are indeed at least assembled in China, but not necessarily the components. All i'm saying is, silicon on eBay straight from China is... questionable / risky And for just a couple bucks, when it's stuff you plug into the wall, i'm not sure it's worth the risk, in my humble opinion.

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

But still fuse is burning out directly (without any other visible damage)

Use incandescent light bulb in series with the mains to your speakers. Like this:
https://www.badcaps.net/forum/showpo...4&postcount=70
That should limit the current to prevent your fuses and other parts from blowing up again, should there still be something else wrong in the PSU.

but not necessarily the components.

Well, actually most components are made in China or Taiwan too. But there is a difference between "cheaply made in China" and "made in China for legit company X". And then there is also the whole thing with counterfeit and rejected stock making its way on the market. For example, when a legit manufacturer makes a bunch of parts, but determines that some of them don't pass quality control, they typically discard them for recycling. In China, however, many times those recycled parts will find their way back on the market again, and sometimes even sold as legit parts, despite being recycled/reject parts. With that said, you could get lucky that perhaps the legit manufacturer only found a few parts that didn't pass QC and labeled the whole batch as "faulty/reject" - in which case, you could end up with a good reject part that performs just the same as the normal non-reject part. Or you may not be so lucky and the part could blow at half-capacity.

So that's the thing about cheap parts from China/Taiwan: you never know what you are going to get in terms of quality. With low voltage and low current parts, of course, the danger is much smaller. For example, I wouldn't worry about getting a cheap LM358 dual op-amp on eBay.

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hey guys,

found this topic after one of my speakers blew out my fuses and decide to have a go at fixing it.

I've replaced the diodes (d1-4) the big black caps and C8, c35 and the fuse after finding a youtube repair video. This wasn't enough as it then burnt up r15 and rt1 mosfet exploded :/

I've replaced these, the diodes again, and Q1 as well after using a multimeter to reference some parts with the still working speakerboard.

Now nothing happens, power on doesn't blow any fuses, but also no blue led and no sound.

How would i go about next steps? I'm no engineer but i'm thinking using a multimeter again just don't know where and how. Are there any suspectable parts i should look into?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, that's one reason to not just replace the fuse, before determining the cause of it blowing. Also, the "shotgun" approach to replacing parts willy-nilly is... inefficient, to say the least.

The least things to be done would've been to measure the suspicious components before replacing them, for example. I mean, what's the use of replacing parts that are, in fact, fine?

What voltage (if any) is present between pins 7 and 5 of U1?

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes you're correct in that this approach isn't the most efficient. I just thought i'd give it a shot since it seems a common problem with these speakers. On my 2nd replacement round I did actually measure components and determined they were broken
I will measure the voltage soon, and see if I can notice anything.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Replaced:
D1-4; R5; D5; U1; R18; D15; C35; R11; Q4(2N5551); R15 (see attached scheme)
But still fuse is burning out directly (without any other visible damage)
What can/have to replace/measure more?

Okay, mosfet replaced (Q1 and Q2, chinees versus, sorry), again D1-4, C1+2(I don't know why). First issue solved: fuse won't blow out anymore, but no audio, no powerled. Between below D3/4 and above D1/2 I measure 318V=. U1, between 5 and 7 1,1v. On connector J5(not attached during measuring) I don't measure any voltage between pin1 and others....
so any suggestion what to measure/replace more?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Supply voltage for the PWM chip is (obviously) way too low.

Are R3 and R4 ok?

And C8, R5, D5?

If all those check out, i might start suspecting the PWM chip...