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Old 11-25-2017, 07:10 AM   #261
Khron
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Quote:
Originally Posted by momaka View Post
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.
That's what i had tried to confirm mitigate by having said:

Quote:
Originally Posted by Khron View Post
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.
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Old 11-26-2017, 08:08 AM   #262
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hi guys,

Hope someone can lend me a hand here! My mk2s blew a few months back and I've finally gotten round to opening them up. Bare in mind I'm a complete novice when it comes to electronics.

I can see a trace that's burned off so that's an obvious fix but I'm stumped as to what else may be wrong. I've tested the c8 and c35 caps and they seem fine to me so I started testing other components in the hope that I'd stumble across the fault. This is probably a silly way to go about it but I'm not sure what else to do. Anyway, I took off R3 and R4 to test them and found they measured 38k instead of the 47k that the service manual declares them to be. Excellent! Or so I thought. The colour code (orange, grey orange, gold from what I can see) tells me that it's a 38k resistor.

So then I thought I'd pull out another resistor to see if I could make sense of that measurement. I pulled out R27 and tested that. Service manual says it should be 100ohm but the colour rings (red, black, gold, gold) tell me it's a 2ohm resistor with a tolerance of 5% and it's reading at 2.4ohm which is outside of its tolerance.

I'm so confused. Should I just ignore the service manual altogether? Is R27 bad? Help... please?
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Old 11-26-2017, 12:51 PM   #263
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

1) Photos
2) Tested C8 / C35 how, exactly? And "they seem fine" to you, based on what?
3) What colour code / resistance can you measure on R28? They should be the same (at least in theory).
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Old 11-27-2017, 11:29 AM   #264
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Thank you for replying so quickly Khron, I appreciate it.

1) I've attached some photos, hope it works, not used this forum before. There was a load of gunk there that I cleaned out (didn't realise how much I'd scratched the board until I took the photo... *facepalm*).

2) Sorry, was way too vague there wasn't I. I tested them using an ESR meter.

C8: ESR=0.31ohm, Vloss=2.0% and 342.3uF
C35: ESR=6.8hom, Vloss1.2% and 1081nF

Forgot to say that the C8 cap is 330uF 35v but again the service manual says it should be a 220uF 35v although from what I've read it's ok to have a higher capacitance... ?

3) Colour code on R28 is the same as R27 (red, black, gold, gold) and also measures 2.4ohm

Looks like that whole area around D5 has overheated. C8 has some brown stuff caked on to the bottom as you can see in the photo. D5 was covered with the same or similar brown substance what I thought at the time was burned adhesive but doesn't make sense why D5 would be covered in adhesive in the first place. Measured D5 with a multimeter and got 0.62v so I assume it doesn't need replacing even after being covered with that crap?

Wondering if I should remove the burned adhesive on the board just to the right of R18? I read somewhere that it's conductive once browned but don't know how much truth there is to that. Got some conductive paste to repair the broken trace under R18. Do you think maybe the problem is just that broken trace?

Also, after taking out D5 I noticed that the holes for D5 and R5 are pretty close and looks like they'd join when I replace D5. I didn't notice this before removing D5 so I'm wondering if it's my rubbish soldering skills but then looking at the diagram in the photo it looks like they should connect perhaps? Sorry, so many questions and a complete electronics noob... but I'm enjoying it, so far
Attached Images
File Type: jpg MK1 PCB.jpg (897.0 KB, 17 views)
File Type: jpg MK1 C8.jpg (450.5 KB, 14 views)
File Type: jpg MK1 PCB Reverse.jpg (826.5 KB, 13 views)
File Type: jpg MK1 Diagram.jpg (728.3 KB, 10 views)
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Old 11-28-2017, 05:29 AM   #265
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

As a rule of thumb, if an electrolytic says "VENT" on it, i don't trust it as far as i can throw it. Only (some of) the crap brands have that written on them; the good ones don't - coincidence?

I'd replace that, if only to save myself potential headaches down the road. And i'd recommend you use the method i did - relocate C8 to replace C14 next to the chip (and away from the heatsink), and add C14 back, on the bottom of the circuit board. It's in parallel with C8, so no worries there.

If the colour code matches the measured resistance, AND they're both the same, i'd call them good. Highly unlikely they'd both deteriorate to the same exact value, wouldn't you say? And their values aren't exactly critical anyway, they're there more for current-limiting purposes.

Why would D5 be covered in that crap? One reason - zero f**ks given during applying of said goop. But if it measures ok, and the legs aren't too corroded (from the goop), i guess D5 should still be ok.

Yes, remove as much of that adhesive as you can, burnt or not. Yes, it does indeed turn dark, brittle and conductive, as well as corrosive, which is, as i'm sure you can imagine, quite unhealthy for copper traces and component legs.

Conductive paste? I wouldn't, sounds messy, and since that's the drain of one of the MOSFETs, that'll be handling "a bit" of power. Just scrape off the soldermask (green transparent varnish) from the trace, and run a wire (or a resistor leg) where the trace should be / used to be.

Well, what good would "one leg in the air" for either R5 or D5 do, hmm? No physical connection = no electrical connection (since this isn't RF black-magic-voodoo stuff), soooo...

PS: Investing a couple quid in some decent solder-wick (aka. desoldering braid) is priceless - works great for cleaning holes like those.
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Old 11-30-2017, 10:43 AM   #266
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Take note that the Driver IC can be powered from a 15-19V bench supply. Current limit will be a good idea. This is just to troubleshoot and see that the IC oscillates.
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Old 11-30-2017, 08:32 PM   #267
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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Originally Posted by Khron View Post
If the colour code matches the measured resistance, AND they're both the same, i'd call them good. Highly unlikely they'd both deteriorate to the same exact value, wouldn't you say?
Also, some multimeters have considerable offset at low resistance measurements. It's not uncommon to see up to 0.2-0.5 Ohms on a decent meter and up to 3 Ohms on a very cheap meter. So always keep that in mind (best way to check is to touch the multimeter leads together and see what resistance it shows on its lowest scale - that's usually your offset for low resistances below 10-30 Ohms.)

Quote:
Originally Posted by Khron View Post
Yes, remove as much of that adhesive as you can, burnt or not. Yes, it does indeed turn dark, brittle and conductive, as well as corrosive, which is, as i'm sure you can imagine, quite unhealthy for copper traces and component legs.
I concur.

That tan glue is absolutely horrible. While it doesn't usually go conductive enough to pass considerable amounts of current, it can pass enough current to turn on a transistor or pull a signal low or high - and that alone can destroy your power supply.

And like Khron said, it corrodes traces, so that's another good reason to remove it.

Quote:
Originally Posted by Khron View Post
PS: Investing a couple quid in some decent solder-wick (aka. desoldering braid) is priceless - works great for cleaning holes like those.
And a stainless steel sewing needle. You may find one or the other easier. Me, I am definitely better with needle (no I don't do drugs, lol) and can't use wick for almost anything except BGA rework. So you may want to try both methods and see which one works easiest for you.
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Old 12-08-2017, 10:17 AM   #268
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Hey, so I've been looking at one of these for a couple weeks now trying to get it working. When I got it from a friend the fuse had blown, RT1 and R15 had exploded.
I swapped out C6, C7 and C8 as they had domed. It seems to be the newer revision as C8 is on the other side of the heatsink to the service manual I found online.
I swapped RT1 and R15, measured Q1 and found it was shorted. Swapped Q1 and Q2 but as I removed Q1 the trace lifted with it (goes from Q1 drain to Q2 source through a via on the board). Ran a wire where the trace used to be to reconnect that.
Next I checked the diodes on the bridge rectifier, measured continuity both ways on them and some erratic readings so swapped the lot of them.

So maybe in haste, thinking that I'd found the main problem (Q1 shorting seemed to be the cause) I swapped the fuse and tried to power it up. Fuse blew again straight away. Checking Q1 again it is fine, reading about 490 between drain and source, same as Q2. Diodes all seem fine as well.

I have little to no experience with power supplies but it would be nice to get this working again. Does anyone have any ideas on where to look next? I've clean as much of the glue/fixative from the board as possible (or where appropriate).

My friend noted that before the speaker finally blew, it would cut out a the light would flicker if you turned the volume up past 50% on the back.
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Old 12-10-2017, 05:40 AM   #269
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Quote:
Originally Posted by Khron View Post
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.
thanks for the clarifying images, Q2 and Q1 give ma about a diode drop when measuring source to drain. Q1 does the same for gate to drain. Q2 gives no reading on gate to drain (reads 1).

Is the gate to drain supposed to give a diode drop? Or is Q1 defect (i did replace this one so sounds weird).

EDIT -- i read on and noticed you mentioning
Quote:
Originally Posted by Khron View Post
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
This leads me to thinking Q1 got faulty again, but now to found out what caused this. Any suggestions on that?

Last edited by markovitch; 12-10-2017 at 05:45 AM.. Reason: found more info
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Old 12-14-2017, 02:53 PM   #270
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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Is the gate to drain supposed to give a diode drop?
Nope! - Not outside a circuit, anyways. In circuit, there may be a protection diode connected to the Gate, in which case that's what you might be reading across.

But in a MOSFET, the Gate is electrically isolated from both Source and Drain (there is a small capacitance between Gate and Source, but that's it). So outside of a circuit, there shouldn't be any resistance or diode drops at all from the Gate to Drain/Source.
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Old 12-31-2017, 10:16 PM   #271
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

great discussion. does anyone have a schematic for alesis M1 #9-40-1386-a , i think it is the first version of this marked 10/06/98 94vo type 1. its a little bit different than the schematic on page 2. my symptoms are smoke coming out of the tweeter but resistance check shows 3.2 ohms .
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Old 01-01-2018, 10:06 AM   #272
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Amp chip's blown? Sounds like either oscillation, or a considerable DC offset's going on...
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Old 01-05-2018, 08:58 AM   #273
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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Originally Posted by momaka View Post
Nope! - Not outside a circuit, anyways. In circuit, there may be a protection diode connected to the Gate, in which case that's what you might be reading across.

But in a MOSFET, the Gate is electrically isolated from both Source and Drain (there is a small capacitance between Gate and Source, but that's it). So outside of a circuit, there shouldn't be any resistance or diode drops at all from the Gate to Drain/Source.
ok, so this means my replaced Q1 is faulty. What would be the best way to continue? I could replace Q1 again, but my gut tells me it will break again since there is another component faulty which I still need to identify. I have a working speaker that i can cross measure stuff, but I'm just not sure where to begin
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Old 01-05-2018, 10:03 AM   #274
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Default Re: Repairing Alesis M1 Active mk2 Monitor Speakers

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ok, so this means my replaced Q1 is faulty. What would be the best way to continue? I could replace Q1 again, but my gut tells me it will break again since there is another component faulty which I still need to identify. I have a working speaker that i can cross measure stuff, but I'm just not sure where to begin
Get two more replacement MOSFETs for Q1 first.

Next, check transistors Q3 and Q4 - these are the drivers for Q1. If one of them is bad, they can cause Q1 to get stuck ON and burn out.

Once Q3 and Q4 are verified to be okay, install the first replacement MOSFET for Q1 and use the incandescent series bulb "trick" in place of the fuse to prevent the power supply from blowing up Q1 and any other components again.

Incandescent bulb trick:
https://www.badcaps.net/forum/showpo...4&postcount=70

If the bulb stays fully lit, that means Q1 is stuck ON and may need to replace UC3844 IC. On a normally working SMPS, the incandescent bulb will usually glow briefly for a moment or two and then go OFF or very dim. Of course, that will also depend on your bulb's power rating. I recommend at least 60 Watts. Around 100 Watts is what I use for testing larger SMPSes like the ones for PC and audio gear.

While testing Q3 and Q4, also check D14, Q2, D6, D7, and R18. If these are bad, that could make excessive current flow through Q1 as well.

Last edited by momaka; 01-05-2018 at 10:07 AM..
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