Repairing Alesis M1 Active mk2 Monitor Speakers

[thafx]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

hi all

U1 *powered*
5-7 > 10v with multiple UC3842N & UC8344AN chips
5-8 > less than 1V

*15V is under the official turn-on voltage for the 3842 (which should power up at 16V)*

"are R3 and R4 fine? What about D5 and R5? And Q3 & Q4?"

R3 & R4 replaced with 47k/2w
D5 (420 & 1/OL in reverse),
R5 (11),
Q3 (NPN 570),
Q4 (PNP 520)

^all new components


with C6+C7 [measuring 330v across ] Im now wondering why there is only only 10v after R3,R4,D5,R5, where would the missing 6v come from?

Q1/Q2 tested in & out of circuit (362 on source>drain & 92 after charging gate) no diode drop reading from gate >drain. These are the same readings with new unused chips.

any advice guys?, cheers

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Any ticking sounds coming from the supply board? And does the supply not start regardless of whether the amp board is or isn't connected?

[thafx]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Any ticking sounds coming from the supply board? And does the supply not start regardless of whether the amp board is or isn't connected?

Well Khron, I'm now hearing sounds coming from the speakers as it's finally fixed!

That missing 6v was from a faulty but new C8 (ELNA Silmic II -35w/220uf), it tested normal out of circuit but once replaced with another everything jumped to life.

After years of no results, It's a strange emotional reunion finally listening to the original speakers that inspired my career again so thank you to everybody that gave input, struggled, provided guidance & solutions especially Khron
Thank you! 🙏🏽

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I thought of asking about C8 as well, but...

Spoilers - you do NOT want "audio" caps in switching power supplies. I've yet to see any "audio" electrolytics rated for anything over 85C, and you won't see any low-ESR ones rated for 85C, so... You do the math

Glad to hear it's back to life, though

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What causes the blue flashing light? One of my freshly repaired monitors had a flashing led which went away on itself and is now working again

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Mine still not working. It is driving me nuts.

Replaced almost everything that could be wrong for new components.

I get oscilating 11-16 volts between pin 5-7 of the UC3842.
I get oscilating 0-5 volts between pin 5-8.

I get 171 DC on the primary between pin 5 and ground;
I get no AC voltage between pin 5 and pin 3.

I get no voltage on the secondary at all.

I think my primary is not oscilating but i replaced ALL transistors with new ones.

I've spent way more than i should.

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Mine still not working. It is driving me nuts.

Replaced almost everything that could be wrong for new components.

I get oscilating 11-16 volts between pin 5-7 of the UC3842.
I get oscilating 0-5 volts between pin 5-8.

I get 171 DC on the primary between pin 5 and ground;
I get no AC voltage between pin 5 and pin 3.

I get no voltage on the secondary at all.

I think my primary is not oscilating but i replaced ALL transistors with new ones.

I've spent way more than i should.

I also get no continuity between the inner chassis ground (aluminum heatsink connected to both IRF840) and the actual ground chassis. Shouldn't there be continuity? even if c5 works?

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Usually C8 needs to be replaced when the LED is flashing.

C8 was replaced by HonongX VEHT 105°C 330uF

Flashing came 3 Days afterwards.


I replaced C8 again, but its still flashing and clicking.
Any suggestions?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Replaced C8 with what?

You'll want a known-good, low-ESR capacitor there, from one of the handful of reputable manufacturers out there (Panasonic, Rubycon, Nichicon, Elna, United Chemi-Con), and hopefully from a reputable source as well.

PS: Did they even mis-spell "VENT"?

Usually C8 needs to be replaced when the LED is flashing.

C8 was replaced by HonongX VEHT 105°C 330uF

Flashing came 3 Days afterwards.


I replaced C8 again, but its still flashing and clicking.
Any suggestions?

"I get 171 DC on the primary between pin 5 and ground;"

That's quite normal - you'll note C5 of 1000pF breaking the DC connection between the primary-side "ground" and chassis earth.

Sounds like the 3842 is trying to start, and stumbling. What capacitor do you have in the C8 position (brand, series, value)?

Is the power supply connected to the amp board or not?

Mine still not working. It is driving me nuts.

Replaced almost everything that could be wrong for new components.

I get oscilating 11-16 volts between pin 5-7 of the UC3842.
I get oscilating 0-5 volts between pin 5-8.

I get 171 DC on the primary between pin 5 and ground;
I get no AC voltage between pin 5 and pin 3.

I get no voltage on the secondary at all.

I think my primary is not oscilating but i replaced ALL transistors with new ones.

I've spent way more than i should.

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Oh ok. I used this one:

https://www.ebay.de/itm/5x-Elko-330u...53.m2749.l2649

This one should be good?

https://www.ebay.de/itm/2-Elko-Panas...0AAOSwv0tVZz08

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

As i mentioned, you don't want some random general-purpose capacitor in that position. Not all 105C rated capacitors are low-ESR, BUT all low-ESR capacitors are rated for (at least) 105C.

Those Panasonic FR's should be fine (assuming they're genuine).

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Replaced C8 with what?

You'll want a known-good, low-ESR capacitor there, from one of the handful of reputable manufacturers out there (Panasonic, Rubycon, Nichicon, Elna, United Chemi-Con), and hopefully from a reputable source as well.

PS: Did they even mis-spell "VENT"?



"I get 171 DC on the primary between pin 5 and ground;"

That's quite normal - you'll note C5 of 1000pF breaking the DC connection between the primary-side "ground" and chassis earth.

Sounds like the 3842 is trying to start, and stumbling. What capacitor do you have in the C8 position (brand, series, value)?

Is the power supply connected to the amp board or not?

Yes, PS is connected to the amp board.

The C8 i put is a Epcos/TDK, B41821 85º, 220uF/63v

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

1) Try it with the amp board disconnected.

2) See my previous post.

As i mentioned, you don't want some random general-purpose capacitor in that position. Not all 105C rated capacitors are low-ESR, BUT all low-ESR capacitors are rated for (at least) 105C.

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

1) Try it with the amp board disconnected.

Same thing. voltages as mentioned. +170 and -170 where it should.

2) See my previous post.

That was the only one i had already. The other i have is a chinese Topmay-40+105º 330uF 50v. These topmay i have showed 0.00ohms on the ESR (component tester).

For better ones i have to order online. No store nearby has them.

Thanks for your patience khron

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Oh ok. I used this one:

https://www.ebay.de/itm/5x-Elko-330u...53.m2749.l2649

This one should be good?

https://www.ebay.de/itm/2-Elko-Panas...0AAOSwv0tVZz08

I replaced C8 with the cap from the auction above. The LED is still flashing
What could I do / check next?

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

1) Try it with the amp board disconnected.

2) See my previous post.

I fixed mine, Khron.

Thanks a lot for the help.

In the end the problem (besides all the stuff that burned and was replaced) it was a short on C35 due to overheating and the holes being too close to each other). tha prevented the oscilation.

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ive given up wasting more time on this crap and im selling mine. One is working, one with flashing led.

Bought a pair of Adam Audios instead.

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I fixed mine, Khron.

Thanks a lot for the help.

In the end the problem (besides all the stuff that burned and was replaced) it was a short on C35 due to overheating and the holes being too close to each other). tha prevented the oscilation.

After 4 weeks working, it stopped again.

Huge bulge in one of the Elna main caps (390uF). replaced it, nothing.
Everything seems normal, took out of the board and measured

I'm getting normal -170/+170

i´m getting no DC oscilating between pin 7 and pin 5. In fact i get a 0.2V constant difference between them. I suspect that c8 is bad again.

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

After 4 weeks working, it stopped again.

Huge bulge in one of the Elna main caps (390uF). replaced it, nothing.
Everything seems normal, took out of the board and measured

I'm getting normal -170/+170

i´m getting no DC oscilating between pin 7 and pin 5. In fact i get a 0.2V constant difference between them. I suspect that c8 is bad again.

I'm getting -178/+178 volts that should be normal.

I replaced the caps with another c8 105º low ESR one and still no oscilating voltage between pin 5 and 7. I get constant 0.153 volts.

Could this be a faulty q1, q3 or q4? In cirtuit they dont appear faulty (no shorts whatsoever).

jesus i should've bought the main PS board since the begining. this circuit was blackened by the burnt cap.

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Could this be a faulty q1, q3 or q4? In cirtuit they dont appear faulty (no shorts whatsoever).

In-circuit measurements are unreliable, especially when just relying on finding shorted silicon parts. BJTs, in particular, can often fail open-circuit or high-impedance.

On that note, I nearly pulled out my hair recently while troubleshooting an ATX PSU. The culprit was a partially-shorted diode that showed lower voltage drop than a normal diode, but still high enough to make me think it was OK when in reality it wasn't. I did all kinds of unnecessary tests when all I should have done is pulled all silicon parts that could have gotten damaged and measure them out of circuit. So when it doubt, start pulling parts out.