Repairing Alesis M1 Active mk2 Monitor Speakers

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Since I know you are the pro, what Resistor do you recommend from the following? (pics attached)

R15
R24 (Only 1 kind in stock with the parameters offered at DigiKey)

Thank you for ALL your help!!

Skeeter


UPDATE!! I swapped the 22 ohm resistor with the old .22 ohm resistor (which is in terrible shape), and IT WORKS!! Basically, the .22 ohm resistor is barely holding a connection, so I still need to order a new one, and I will still replace the 15K resistor, because it measures only 3K. Wow, did I learn a LOT from you! Thank you for your patience with me.

Thank you! I am rather elated just now

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I have discovered the following resistor issues on the non-working power supply:

R24 is supposed to be 15K ohms, but it only measures 3K in circuit
R15 is supposed to be .22 ohms, but it measures 22 in circuit - this is a resistor that I replaced, and it appears I replaced it incorrectly with a 22 ohm resistor. I have the old one, and it measures .22, but it is in pretty bad shape, including the post is broken off at the body of the resistor, so I don't know if I can get a good reattachment.

So, I plan to order R24 and R15, and see if this fixes the problem.

Skeeter

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

When you say "ground the gate," are you saying connect the negative side of my 12V power source?

Pretty much enough time has passed now that there should not be any charge in your gate left since your last test, so just check it with the ohm meter now like you did before.
But leaving the gate completely open/untouched "Pos probe of the Ohmmeter to Drain, and Neg probe of Ohmmeter to Source"
IF you still get a low ohms reading just discharge it by touching the gate and source together with your finger, but do discharge yourself first, because the gate is very sensitive to static discharges!

With the Component Tester, will I still need to remove one leg of each component to test?

Yes, that is true regardless of what tester is used...

So, I will check each resistor on the 'good' board, and compare it to each resistor on the 'non-functioning' board.
Also, since the color bands on the resistors are getting old and difficult to read, may I use the Parts List from Post #29 as a guide?

Yes you can use the schematic, just keep in mind when you find a difference to try to read the rings too, if you remember post #408 about R3/R4 for example...

Should I resolder the transistors, diode, and MOSFETs before I run through all of the resistors?

No leave them out, the less components are on the board the less false readings will show up...

Thank you again for your considerable help! I wish I had someone in my town who could reliably diagnose and repair this unit. I originally took it to a local electronics repair shop, but after his time and testing, the man said all was functioning propertly. He charged me for his time, but it does not work.

Honestly that is an interesting bit of information, because I tend to agree with him lol

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

This was my precisely my thought yesterday, that everything seems to be working as it is supposed to. So, yesterday, I reassembled everything, and turned on the units to see if the blinking led still persisted. And, yes, still blinking, and no sound from the non-functioning unit. (previously, I had also swapped power supply PCB from the 'good' monitor into the nonfunctioning monitor, and the nonfunctioning monitor indeed did function - so, I am convinced it is an issued located on the power supply board).

When you say "ground the gate," are you saying connect the negative side of my 12V power source?

So, I will check each resistor on the 'good' board, and compare it to each resistor on the 'non-functioning' board.
With the Component Tester, will I still need to remove one leg of each component to test?
Also, since the color bands on the resistors are getting old and difficult to read, may I use the Parts List from Post #29 as a guide?

Should I resolder the transistors, diode, and MOSFETs before I run through all of the resistors?

Thank you again for your considerable help! I wish I had someone in my town who could reliably diagnose and repair this unit. I originally took it to a local electronics repair shop, but after his time and testing, the man said all was functioning propertly. He charged me for his time, but it does not work.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Wow, I must say everything seems to work really well!
Just one thing: with the MOSFETs now out of circuit redoing the test do you have infinite resistance if you ground the gate?

You could do the tedious task of checking every resistor in circuit.
The logic is like this: you should not expect to see a lower resistance reading than stated, because that is not a failure mode of resistors...
So keep testing them and if you see a strange reading you have to take one leg out to test it properly...
If you find nothing wrong:
Maybe a nice "component tester" also known as "transistor tester" would be a good idea to get?
Here is one, I have no affiliation with the seller:
https://www.ebay.com/itm/265808759695

Then you can test all the transistors, MOSFETs etc in a very convenient way...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I did an ohms test UC3844N of the working power supply and the nonworking power supply, using Pin #5 GND for Neg probe, and testing each pin. Below are the results.

Working Power Supply
Pin 5 to 7 = 28 Ohms
Pin 5 to 1 = 37 Ohms
Pin 5 to 2 = 46 Ohms
Pin 5 to 3 = 47.5 Ohms
Pin 5 to 4 = 35 Ohms
Pin 5 to 6 = 31 Ohms
Pin 5 to 8 = 35 Ohms

Non-Working Power Supply
Pin 5 to 7 = 28 Ohms
Pin 5 to 1 = 37 Ohms
Pin 5 to 2 = 46 Ohms
Pin 5 to 3 = 47.5 Ohms
Pin 5 to 4 = 35 Ohms
Pin 5 to 6 = 31 Ohms
Pin 5 to 8 = 35 Ohms

The measurements are remarkably consistent!



I next tested the MOSFETs on the PCB, with Gate + Drain touched by the Pos probe of the Ohmmeter, and the Source touched by the Neg probe. Both Q1 & Q2 still measured 18.5 ohms.

Finally, I removed both MOSFETs from the PCB, connecting 12V to the Gate, Pos probe of the Ohmmeter to Drain, and Neg probe of Ohmmeter to Source. Both MOSFETs measured between .7 and .8 Ohms.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

UC3844N - is there a test to determine if it is bad?

Not really, you could take some measurements, for example ohm out VCC pin 7 to GND pin 5 and then compare to your working unit.
In fact you could ohm out all pins in relation to ground and compare with the other unit.

Q1 & Q2. I have a question...if I only connect a 12V positive lead to the gate, how is any current transmitted?

It does not need to source any current, just "present" a voltage.
P.S: To make it clear in your "Test B" the positive probe should be touching both the Drain & Gate, and negative on Source.
If that didn't work: Sorry I never owned a analog meter and was making some assumptions about how big a voltage it sources in ohms mode.
Please watch this video: Electronic Basics #23: Transistor (MOSFET) as a Switch


I'll refer to what is shown at the 2:35 time mark: no path to ground is necessary, only the transfer of some capacitive energy.
I tried with a large MOSFET I have here, I can get it down to 0.55ohm by just touching the gate with my bare hand, but of course it is not in a circuit where other components drain it away.
I'm a bit afraid to tell you to connect the negative of your external power supply, that is because current could go to paths it should not and destroy things...
So it would be much better to remove the MOSFETs from the circuit and then test them with your 12v power supply and a light bulb like we did before, just referring to the video also so you see what you need to change...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

UC3844N - is there a test to determine if it is bad?

Q5: yes, PNP (I think I mistyped the kind), and forward = open; reverse = 30 ohms.

Q1 & Q2. I have a question...if I only connect a 12V positive lead to the gate, how is any current transmitted? I am calling the 12V application "Test A" below, and the non- 12V application "Test B." Nonetheless, I found the following:

Q1: Test A (with +12V to gate). Without the 12V = 18 ohms; With 12V = 18 ohms - no change (but, don't I need a ground for 12V to apply?)
Q2: Test A (with +12V to gate). Without the 12V = 18 ohms; With 12V = 18 ohms - no change

Q1: Test B (Pos lead to Gate, Neg lead to Source) = 875 ohms
Q2: Test B (Pos lead to Gate, Neg lead to Source) = 390 ohms

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I tested the 'good' power supply, and found that each of the transistor measurements were exactly the same as the problem power supply, with the exception of Q7. On the 'good' power supply, Q7 measured open circuit in the forward test for PNP, and 30 ohms on the reverse (on the problem power supply, Q7 was 300 ohms forward, and open reversed).

This could be pointing in the direction that the UC3844N is what is bad, I'm really not sure though...

Q5 Out of circuit NPN: Forward = open; Reverse = 30 ohms

Q5 is not NPN but a PNP transistor, can you please just double-check this, as with the info you provided it would be bad.

D6 Out of curcuit: Pos to Neg = 30 ohms; Neg to Pos = open circuit. Forward polarity = lamp lights; Reverse polarity = lamp did not light (note: I only removed the pos lead from the PCB for this test)

Good!

Q1 (as NPN): Forward = 18 ohms; Reverse = open circuit
Q2 (as NPN): Forward = 18 ohms; Reverse = open circuit

I didn't ask you to make these tests, because these are N-Channel MOSFET's, not transistors.
They are voltage controlled devices, they need a positive voltage at the gate and then they will turn on pretty much like a light switch.
So you can put your meter in ohms, positive lead on Drain and negative on Source.
Then apply +12v to the gate, you do not need to hook up the negative to anything.
If the MOSFET works the meter will read a very low resistance when you apply 12v and it will be completely open before that.
Actually instead of the power supply you can try to just put a wire from the positive lead of your meter to the gate, that is probably enough to turn it on because Vgs(th) voltage is only 3v

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I tested the 'good' power supply, and found that each of the transistor measurements were exactly the same as the problem power supply, with the exception of Q7. On the 'good' power supply, Q7 measured open circuit in the forward test for PNP, and 30 ohms on the reverse (on the problem power supply, Q7 was 300 ohms forward, and open reversed).

I will get to work on the other recommendations soon.

Thank you again.



About 30 minutes later, and I have the rest of the results:

Q1 (as NPN): Forward = 18 ohms; Reverse = open circuit
Q2 (as NPN): Forward = 18 ohms; Reverse = open circuit
Q5 Out of circuit NPN: Forward = open; Reverse = 30 ohms
D6 Out of curcuit: Pos to Neg = 30 ohms; Neg to Pos = open circuit. Forward polarity = lamp lights; Reverse polarity = lamp did not light (note: I only removed the pos lead from the PCB for this test)

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Q3/Q4 results: Their bases are connected together, so equal results are expected.
Q6 results: This transistor has a 100ohm resistor (RT2) between base and emitter so this result could be normal.
Q5: This transistor does seem to be shorted because there are two diodes (D6 & D7) between base and emitter, they should not be forward biased though, however the transformer winding T2A also in circuit does not care and will conduct so it makes sense... It needs to be tested out of circuit. Since it has long leads I would recommend to just cut them close to the board, that will make it easier to remove the remaining legs with the soldering iron + tweezers.
Q7: This is the only reading that looks normal

What I would do is remake these tests on your other unit, it will give you a good reference to compare against.
Also we have not tested the Q1 and Q2 yet and we should, but first I want you to show what power supply you have available for testing, the one you used previously (its ratings).
As for doing voltage tests: a multimeter in ohms mode does a voltage test: it outputs a specific voltage and the meter has an internal resistor.
Via Ohm's law it calculates the voltage drop and shows you a reading...

Finally a note about D6: you said you tested all diodes and they had correct behavior, well it should not be possible with D6 due to transformer winding T2A the test lamp should always have been on, that diode needs to be tested out of circuit just like Q5...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Wow! I am learning a lot from you! I followed your instructions exactly, and here are the results:

1. Q3 NPN diagram. (base with Pos lead and Emitter with Neg lead). Measurement = 30 ohms. Reversed (base with Neg lead and Emitter with Pos lead) = 30 ohms again. I did this and each one below in the same order as you described.

2. Q6 NPN diagram. Forward = 150 ohms. Reversed = 30 ohms.

3. Q4 PNP diagram. (base with Neg lead and Emitter with Pos lead). Measurement = 30 ohms. Reversed (base with Neg lead and Emitter with Pos lead) = 30 ohms again. This is exactly the same result as Q3 (of course it was NPN).

4. Q5 PNP diagram. Forward = closed circuit. Reverse = closed circuit.

5. Q7 PNP diagram. Forward = 300 ohms. Reverse = open circuit.


Then, I verified by doing the measurements again, and got the same results with the same order of testing each transistor.


Do I need to test with voltage tests? I did a bit more reading from a couple of websites so I might understand transistor testing a little more, and these sites had a variety of tests, not all which I am prepared to undertake!

So, with the results from 1-5 above, are my transistors all bad?

Skeeter

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you again.
I inspected the board carefully. The darkened areas are still the same as has always been, and I checked all tracers for good connections again.
Q1 Q2, Q3, Q4, and Q5 all appear excellent - nothing to suggest a problem.

Ok, lets try to test Q3, Q4 & Q5 in circuit starting with Q3 an NPN transistor:
With everything off and allowed time to discharge connect your meter in ohms mode, positive lead to the base of the transistor and negative to its emitter.
You should expect a voltage drop of one diode here, or around 0.7v that we previously tested on your analog meter and you saw around 24 to 28ohms.
Reversing the leads it should show as open.
You can also test Q6 while we are at it, it is the only other NPN transistor on the board.
Testing of Q4, Q5 (and Q7 for good measure) is the same, except you have to reverse your test leads since it is a PNP transistor, report what you find, as tests in-circuit can be tricky and also give false results...

I kept the board disconnected from the amp board, and left it running for more than an hour. I could detect a slight warmth from the board, but nothing hot. It could have been the C6 and C7 just slightly warm to the touch, but certainly not even close to being considered hot.

L3 color is only the manufacture colors. I checked to see if the windings had good continuity from post to post, and all is good. I checked all of the inductors, and all windings are good. However, I was just making sure of continuity only.

Ok, it does not sound like anything out of the ordinary.

Now, one issue that I think I need to mention, but did not appear to affect this process:
Prior to any of this work on the power supply board, I found that the same speaker/monitor damaged my computer audio output. Upon investigation, I found that a post on the amp board was grounding to the back plate of the monitor, and sending 35V through the speaker input to the computer's audio output. So, I isolated the post with a couple of layers of electrical tape and sheathed the screw that mounts it to the back plate. That solved this issue. Do you think it may have caused a problem on the power supply board? The attached photo shows the problem post, now fixed.

Possibly this could have damaged the op-amp U1-B since it is powered from +/- 15v but your input signal exceeded that.
However there is also a 10k resistor protecting the input so it might be fine.
Besides the issue we are troubleshooting exists without this board connected so no need to worry about it now...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you again.
I inspected the board carefully. The darkened areas are still the same as has always been, and I checked all tracers for good connections again.
Q1 Q2, Q3, Q4, and Q5 all appear excellent - nothing to suggest a problem.

I kept the board disconnected from the amp board, and left it running for more than an hour. I could detect a slight warmth from the board, but nothing hot. It could have been the C6 and C7 just slightly warm to the touch, but certainly not even close to being considered hot.

L3 color is only the manufacture colors. I checked to see if the windings had good continuity from post to post, and all is good. I checked all of the inductors, and all windings are good. However, I was just making sure of continuity only.

The areas of the board that are darkened remain darkened from the start of this process. They are even darker from my de-soldering of the replaced components. The tracers that were disrupted from de-soldering, and where I had to add just a bit more solder to bridge the connection all checked out again.

Now, one issue that I think I need to mention, but did not appear to affect this process:
Prior to any of this work on the power supply board, I found that the same speaker/monitor damaged my computer audio output. Upon investigation, I found that a post on the amp board was grounding to the back plate of the monitor, and sending 35V through the speaker input to the computer's audio output. So, I isolated the post with a couple of layers of electrical tape and sheathed the screw that mounts it to the back plate. That solved this issue. Do you think it may have caused a problem on the power supply board? The attached photo shows the problem post, now fixed.

Skeeter

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ok, it sounds like your analog meter sources enough voltage to actually turn on the diode then from those readings, so it can be assumed to be correct.
Good that you tested the diodes with the light bulb to confirm, I'm kind of surprised because that then confirms D5 is okay, it would make the most sense that it was faulty when nothing else is blown...

Have you inspected the board carefully to see if anything looks burnt or cracked?
In particular these components: Q3, Q4, Q1 and then Q5 and Q2.

If you leave the board running with the amplifier board disconnected does anything get hot on it?
P.S: Be very careful checking this, as we are working with high voltages here!
Please read this thread first for safety reasons because your primary side heatsink does have high voltage on it with respect to earth ground: https://www.badcaps.net/forum/showthread.php?t=96508

I also want you to inspect inductor L3, it looks very strange from your previous pictures here, with one winding being yellow and the other black:

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I measured the resistance of D5 in both directions. Neg to Pos is open, while Pos to Neg shows 28 ohms resistance. I compared this to the "Good" power supply D5 on PCB, and the pattern is exactly the same.

Also, just to see the result, I measured the resistance of D1, D2, D3, and D4. Each one is open Neg to Pos, while each one shows 24 ohms Pos to Neg.

I also ran the light bulb test you recommended. I have a small 12V power supply, and used a 12V bulb with test leads and my probes to reach the diode. D5 did light the bulb in forward direction, but did not light the bulb in reverse direction. I used this test for Diodes #1-16 on the PCB, and all functioned just as you described.

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you again!

R5 measures approximately 12 ohms. I see on the parts list from Post #29 that it is supposed to be 10 ohms.

Sounds fine!

I am unable to measure the diodes, because my ancient analogue multimeter does not have a diode measurement option. So, is it possible to continue to figure this out without the diode measurement? I read another forum post a week or so ago, and the writer replaced a set of four diodes...something about following a Graetz Bridge or part of a Graetz Bridge. Should I plan to replace these, including D5?

Graetz Bridge is just a really bad Google translation from Polish, it should say "diode bridge"
It is the four diodes D1, D2, D3, D4 that form the diode bridge on the incoming but those are fine, they produce the 325VDC if you recall...

The main suspect at this stage is D5, because nothing else seems blown up!
(Usually when this power board fails it explodes and takes out allot of components, as is evident looking back in this thread and that Polish one you found...)
Try to just measure resistance across it, it should read open in both directions.
You could also make a super simple circuit to test it:
For example a battery of any voltage you wish, and either a regular incandescent lamp suitable for that voltage, or a LED but then with a suitable dropper resistor in series.
Then just try hooking the diode "D5" into the circuit: in the forward direction nothing should change, and in the reverse direction the light should stay off...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you again!

R5 measures approximately 12 ohms. I see on the parts list from Post #29 that it is supposed to be 10 ohms.

I am unable to measure the diodes, because my ancient analogue multimeter does not have a diode measurement option. So, is it possible to continue to figure this out without the diode measurement? I read another forum post a week or so ago, and the writer replaced a set of four diodes...something about following a Graetz Bridge or part of a Graetz Bridge. Should I plan to replace these, including D5?

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I have attached a photo of the parts I replaced, with measurement descriptions, now measured while now removed from the PCB. Possibly all the resistors were okay? The new parts ordered and soldered on the PCB were in accordance with Post #29, because I found it difficult to identify a few of the color rings on the resistors.

Yes, most likely your R3 & R4 resistors where rated at 38K when new (orange, grey, orange, gold)
looking back through the thread I found many others report the same value.
I'll refer back to what I wrote previously in this quote:

An example here is resistors R3 and R4: These resistors charge capacitor C8 (from the 325VDC measured previously) up to around 16VDC.
At that point the UC3844 controller kicks in and starts regulating the voltage through transformer winding T1B via resistor R5 and diode D5 to 15VDC

From your voltage measurements quoted below it sounds like the UC3844 controller does get charged up by these resistors to the "kickstart" voltage where it is suppoosed to start working (oscillating).
However when it tries to hand over control via transformer winding T1B via resistor R5 and diode D5 it fails and the voltage sags: the process then repeats.
So please measure the resistance of that resistor, and do a forward voltage check of the diode with a multimeter that has a diode check function.
P.S: Since your new resistors have a higher resistance they let through a lower kickstart voltage, so it's actually harder for the unit to start.
But I don't think that is the issue, as you mentioned changing those resistors had no effect, still it's worth to keep in mind...

Thank you again.
Voltage across C8 is pulsating about one time per second between 12V to 14.5V.
I also verified that UC3844N (U1) pin 5 and pin 7 are the same - pulsating between 12V to 14.5V. The same measurement occurred with and without amplifier PCB attached.

Below are the connector measurements:

A. With amplifier board connected to power supply board:

1 Black1: Pulsating about one time per second between 3V to 5V, Reversed polarity.
2 Green2: Pulsating about one time per second between 0V to 1.5V.
3 Purple3: GND
4 White4: GND
5 Red5: Pulsating about one time per second between 3V to 5V, Reversed polarity.
6 Green6: Pulsating about one time per second between 5V to 7V.


B. Power supply only (disconnected from amplifier board):

1 Black1: 12V - Reversed polarity
2 Green2: 11V
3 Purple3: GND
4 White4: GND
5 Red5: 21V - Reversed polarity
6 Green6: 21V (normal polarity)

This sounds good, the voltages are lower than expected with the amp board connected because of the problem I explained above.
I also found the expected voltages in a post by Khron now: the V_LO is supposed to be +/- 18v like I wrote.
And then V_HI is supposed to be +/- 36v

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you again.
Voltage across C8 is pulsating about one time per second between 12V to 14.5V.
I also verified that UC3844N (U1) pin 5 and pin 7 are the same - pulsating between 12V to 14.5V. The same measurement occurred with and without amplifier PCB attached.


Below are the connector measurements:

A. With amplifier board connected to power supply board:

1 Black1: Pulsating about one time per second between 3V to 5V, Reversed polarity.
2 Green2: Pulsating about one time per second between 0V to 1.5V.
3 Purple3: GND
4 White4: GND
5 Red5: Pulsating about one time per second between 3V to 5V, Reversed polarity.
6 Green6: Pulsating about one time per second between 5V to 7V.


B. Power supply only (disconnected from amplifier board):

1 Black1: 12V - Reversed polarity
2 Green2: 11V
3 Purple3: GND
4 White4: GND
5 Red5: 21V - Reversed polarity
6 Green6: 21V (normal polarity)