Repairing Alesis M1 Active mk2 Monitor Speakers

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you. I am not extensively experienced, as I can see are you. However, I will do my best to determine these voltages. (BTW, I assume the voltage between C7 and C8 is to be 35V. Correct?)

In addition, I measured resistors that I expected may have issues, based on heat darkening areas on the PCB, and replaced those which did not measure according to the parts list by Khron on Post #29. Was it okay for me to measure these while still on the PCB (but, of course, not connected to power)?

Original Resistor measurements on PCB:
R15 was 0 ohms
R3 was 40K ohms and was next to C8, the problem capacitor
R4 was 45K ohms and was next to C8, the problem capacitor
R27 was 100 ohms but showed heat darkening on the PCB

Thank you. To be continued...

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you. I am not extensively experienced, as I can see are you. However, I will do my best to determine these voltages. (BTW, I assume the voltage between C7 and C8 is to be 35V. Correct?)

No, don't mix up C6/C7 with C8.
When I asked you to measure across C6 & C7 this is because this is the mains input, your mains voltage is 115VAC between live and neutral.
Note how there is a jumper (J4) that also connects this voltage directly between capacitors C6 & C7: this is called a voltage doubler circuit.
Normally your rectified AC voltage would be: 115VAC x √2 = 162VDC
However since you have the doubler circuit it is actually 115VAC x √2 = 162VDC x 2 = 325VDC which is the voltage I asked you to measure: this is to make sure this circuit actually works.
P.S: There is also a jumper called J3 behind it, I assume there is some cable there otherwise the circuit is not complete and will not work.

In addition, I measured resistors that I expected may have issues, based on heat darkening areas on the PCB, and replaced those which did not measure according to the parts list by Khron on Post #29. Was it okay for me to measure these while still on the PCB (but, of course, not connected to power)?

Original Resistor measurements on PCB:
R15 was 0 ohms
R3 was 40K ohms and was next to C8, the problem capacitor
R4 was 45K ohms and was next to C8, the problem capacitor
R27 was 100 ohms but showed heat darkening on the PCB

Measuring resistors on the PCB is fine but it must be assumed when a reading is incorrect that the most likely reason is because it is in circuit:
So it needs to be desoldered and measured out of circuit to know if it is ok or not.
In addition the schematic posted is for a specific revision of the PCB, and components are often changed from revision to revision.
So the correct way to check a resistor is to read its color bands and make sure it is within their tolerance rating:
https://www.digikey.com/en/resources...tor-color-code

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.
Now if this where to not take place I guess the voltage here could get dangerously high, for example if the UC3844 does not work:
Theoretically C8 could charge up to 325VDC in that case and it would cause it to bulge and fail but I diverge...
Anyway the actual rating for R3 & R4 given in the schematic (47k) might be different in your unit.
You should stay with whatever original ratings your resistors had. (According to their color code).
That said it tells allot if the resistors where damaged, showing another value:
For example if R15 that you mentioned you replaced would measure open it means that Q1 and or Q2 turned on for too long, either because they have failed, or Q3/Q4 or Q5 have failed or the UC3844 controller...

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you for this extensive info. Since our previous correspondence of posts #400/401, I have measured the following:

1. Negative of C7 and Positive of C8, the voltage pulses about one time per second between 12V and 14V.

2. For UC3844N pin GND and pin 7, the voltage is 2V.

3. The connector from PWRAMP/XOVER PCB board:
Wire colors in order:
Black (I used this as Ground)
Green = 22V
Purple = 12V
White = 12V
Red = Reverse polarity, pulsing 5V to 6V
Green = 35V

Am I to understand since Neg C7 and Pos C8 voltage issue, that this points to a particular problem?

Thank you again!

[Skeeter]

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.

[Skeeter]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

C6 & C7 Findings:
I have an analog multimeter (I know...so old school, but I have had it for many years, and electronics for me is limited!), so my measurements are approximate.
C6 measures approximately 160V across Neg to Pos
C7 measures approximately 160V across Neg to Pos
Across C6 and C7, it appears to measure approximately 320V.

So, posts #403, #404, #405 are my findings since your most recent message to me.

I am interested in figuring this out, and your knowledgeable help is greatly appreciated! Thank you!

[Per Hansson]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

start after the bridge rectifier, for example across negative of c7 and positive of c8, you should have around 325vdc.

c6 & c7 findings:
I have an analog multimeter (i know...so old school, but i have had it for many years, and electronics for me is limited!), so my measurements are approximate.
C6 measures approximately 160v across neg to pos
c7 measures approximately 160v across neg to pos
across c6 and c7, it appears to measure approximately 320v.

Perfect!

If you do then check across UC3844N pin 5 GND and pin 7 VCC, you should have around 15VDC, you can also measure this across C8 instead if it is more convenient.

2. For UC3844N pin GND and pin 7, the voltage is 2V.

This is very bad, but it does not correlate with what you wrote right above that point, please re-check the voltage and only measure it across C8 for safety.
(UC3844N pin 5 GND and pin 7 VCC is the same thing as across capacitor C8)

If you do then measure all voltages on the connector that goes over to the PWRAMP/XOVER PCB board.

3. The connector from PWRAMP/XOVER PCB board:
Wire colors in order:
Black (I used this as Ground)
Green = 22V
Purple = 12V
White = 12V
Red = Reverse polarity, pulsing 5V to 6V
Green = 35V

Am I to understand since Neg C7 and Pos C8 voltage issue, that this points to a particular problem?

Thank you again!

I didn't ask you to measure across C7 & C8 so you can ignore that reading, it is invalid.

I tried to make sense of the outputs from the schematic but it is hard because there are no voltages listed nor colors of the cables...
So I made this list, please check it and correct any mistakes I made.
On the power supply side pins 3 & 4 are connected together but on the amplifier board they are not according to the schematic, please check this.
In any case these wires (3 & 4) are where you should base your measurements from, i.e. where you should put the multimeter negative probe.
Please check the voltages as it is right now, then disconnect the amplifier board and check it again, if the voltages are different post both.

J5:	J7:
1 Black	1: -V_LO (minimum -18v for LM7915 linear regulator) LM2876T power op-amp for Tweeter
2 Green	2: +V_LO (minimum +18v for LM7815 linear regulator) LM2876T power op-amp for Tweeter, Also connected directly to blue LED
3 Purpl	3: AMP_GND
4 White	4: SIG_GND
5 Red	5: -V_HI LM3886T power op-amp for Woofer, also connected to mute circuit 
6 Green	6: +V_HI LM3886T power op-amp for Woofer

[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)

[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!

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

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

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

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

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

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

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

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

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

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

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

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...