Repairing Alesis M1 Active mk2 Monitor Speakers

[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.

[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

[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

[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

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

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

Did you use a BAV21 or a 4148?

Either should work. That diode shouldn't need to hold up to more than 20V or so, but there's nothing wrong with going with a higher rating there.

The BAV21 is rated for 250mA while the 1N4148 is only rated for 100-150mA or so. Allowing for some headroom is of course not a bad thing; you're free to use a 1A diode if you wish, as long as you make sure it's a fast-recovery (or perhaps Schottky) type.

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I used a BAV21 from my second board that is working.

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I got it working.

Swapped D5 from the working board and it powered on

Thanks a lot!

Did you use a BAV21 or a 4148?

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Any suggestions? Anything I could measure?

I got it working.

Swapped D5 from the working board and it powered on

Thanks a lot!

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

R3 and R4 are showing 6kOhm. The value rises slowly to 30kOhm and above when measuring.
R5 5.6 Ohm

--> i get the same readings on the working board


Q3 & Q4 look good, compared with the working board i get the same readings between the pins.

D5 gives same reading on both boards

Any suggestions? Anything I could measure?

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Although it checked ok on the component measurer, I replaced c8 (previously 330uF 35v) with a new one similar with a 330uF, 50v.

i just noticed that i might have fried my 3842 because it is measuring -169v on pin 7 now and -162 on pin 2.

no tick sound.
it is connected to the amp board yes.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Is your C8 ok? 15V is under the official turn-on voltage for the 3842 (which should power up at 16V), but if it slowly rises to that and the chip takes a gulp of current and empties C8, then it would look like the voltage is just ramping up and down.

Can you hear any periodic "tick" sound (like every second or so)? And is the output connected to the amp board?

[Camarada78]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hello,

I've read the entire thread and found lots of common with mine, did lots of the suggestions and changes and still i got no power.

I'm getting the 340ish volts on the primary, i'm getting the 11-15v on the IC3842, i changed fets, oscialating transistors (4401 and 4403), changed suspicious diodes and so on.

But i get no voltage on the secondary
Secondary diodes and caps are okay, tested outside the board.

I'm not getting any voltage of the opto, 0.2v to be precise, the same voltage i'm getting on the secondary.

Is it possible that the T1 is burned? even though i checked the resistance and they seemed fine (and reconized as inductors on a component tester).

I think the fets are not oscilating for some reason.

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

R3 and R4 are showing 6kOhm. The value rises slowly to 30kOhm and above when measuring.
R5 5.6 Ohm

--> i get the same readings on the working board


Q3 & Q4 look good, compared with the working board i get the same readings between the pins.

D5 gives same reading on both boards

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

With 3.3V instead of 16V or so, no, i quite doubt Q6 or Q7 are to blame.

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

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

5-7 3.3v
5-8 0v

Im not an expert but from my guess Q6 and Q7 are next candidates for replacement?

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes, with the circuit powered. With no power, how can you measure the chip's supply voltage, and internal reference voltage?

I thought maybe its possible to power the board with a lower voltage for safety reasons.
Give me a second, i will measure it

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Yes, with the circuit powered. With no power, how can you measure the chip's supply voltage, and internal reference voltage?

[filth]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What voltages do you get between pins 5 and 7 of the UC3844? And between pins 5 and 8?

With 220v connected and switched on? Can i remove the UC3844 and measure the voltages between the pins then?