Repairing Alesis M1 Active mk2 Monitor Speakers

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok did some measuring, counted the pins counterclockwise starting at the 'half moon' on the chip.

5 to 7 shows me 350-400 and 5 to 8 shows 650-700. I know a diode should give a drop of about 600-700, so does this means the chip is in bad shape?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

http://www.ti.com/ods/images/SLUS223...pp_slus223.gif

5 to 7 does indeed read a little bit low, but definitely not what i'd call "toast".

Are the two MOSFETs (Q1/2) ok? Viewed from the front, diode test with the red probe on the left-most leg and the black probe on the middle pin (or the metal tab) should give about a diode drop, if all is well.

Do R3 and R4 still measure at least close to their designed values? What about R5 and D5?

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

thanks for the reply, sorry for my slow replies but I'm doing this in the little spare time I have (as do most of us probably ).

I measured Q1 (was replaced by me before already), it's fine. Q2 on the other hand does not give any reading, so it looks like it's broken.

R5 and D5 are what they should be. Funny thing with R3 and 4 is that the reading seems legit, but the resistance keeps climbing if i measure for a longer amount of time. Not sure if this is my inconsistent measurement or normal behaviour for these resistors.

So replacing Q2 with a working unit seems the way to go?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

So measuring the source-to-drain voltage drop on Q2 gives a (near) zero reading, or..?

R3/4 make sense, since they're across the big main capacitors. When they're empty, the voltage your meter uses to measure resistances, charges those capacitors, which look like low resistances from the meter's point of view.

[diydidi]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I repaired two of these units today. Wasn't too difficult.

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

So measuring the source-to-drain voltage drop on Q2 gives a (near) zero reading, or..?

Q1 gave about a diode drop measuring left leg, same goes for right leg. Q2 gives no reading with the left leg (as in, no drop), right leg does give about a diode drop.

Seems like Q2 is partially defect then right?

[Khron]

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.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

If you want small quantities of good quality caps, try this seller - i've ordered a few times, after seeing him(?) recommended here, and shipping took under 2 weeks.

To be on the safe side, yes, i'd replace those two transistors; maybe even Q5 as well, at least to eliminate some variables.

I replaced Q3,Q4,Q5 and C8 by the NCC KY serie.
But still not powering on.
Voltage on U1 pin 5-7 swings between 10-15 volt...
I'm lost again, some tips to measure now?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What about between pins 8 and 5?

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What about between pins 8 and 5?

Swinging between 0 and 1,9 volt...

No sorry, wrong side of R9.
Hard to see, between pin 5-8. My autorange meter is not quick enough, so swinging from 0 to OL, max seen 42volt, swing freq 1-1/2Hz? or this is limit of my meter?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

That should be 5V, when the chip is on and working, so.....

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

That should be 5V, when the chip is on and working, so.....

So... measured some things...
Q7 measure shorten. Replaced and working!!!
My first 620 repaired! Thanks very much for the help!!!

Up to the second one. That one blown fuses also, but no visible damage. I've learned a lot, so will first start by myself and measure some things and will come back with my conclusion :-)

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Oh, great then, congrats on that one!

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Okay, repairing M1 620 number2. Issue blowing fuse, no visual damage, first time parts will be replaced.
I measured most transistors and diodes and found the following components brocken:
D1-D4: shorten
Q1 shorten
D16 shorten
D6 and/or Q5(between b-e) shorten (will measure again as soon as I dismounted them, measurements still on board)

I would like to replace these parts and preventive C8 and R3&R4 (higher value and will mount them on backside of print for heating issue.

Do I forget something important?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Wow, that's quite a lot of stuff blown on that one...

It's gonna be interesting to see if all those still measure bad, out of circuit.

If Q1 is blown (which is likely), that wouldn't necessarily mean that D16 is shorted as well. If you follow the schematic, D16 and Q1 are part of a "loop" that includes half the primary of the transformer, so when you measured D16, the much lower voltage drop of the Q1-turned-into-a-wire would show.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

D1-D4 also shorten outside the circuit, D16 shorten outside.
Q5 and D6 looks good outside
Even Q1 doesn't looks that bad as inside. Diodetest D-S give 0,2v and 0,575v the other way(which is propably not good but not total shorten). All other way of diode measuring are OL on Q1
D16 is a superfast MUR160 600v 1A. I don't have that way spare, wil a FR107 do?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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.

Yes, that FR107 should be fine.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

After touching gate and source both ways on Q1 are 0,57v so still good?
But I replaced already all parts and placed a FR107 on D16.
And speaker working again!!! That was a quick one!
And probably replaced too much, I guess only D1-4 and D16 where the problem...

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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

Glad to hear the other one's back to life too

[momaka]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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

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.