The Infamous Alesis M1 Active Mk2 - a hot resistor kills an innocent capacitor

[LEDfoot]

I have a pair of these speakers - Alesis M1 Active Mk2 - and one started malfunctioning with the blue light blinking and a rapid ticking noise emanating through the speaker. I looked this up online and found numerous reports of this happening when a capacitor (C8) - http://www.neufeld.newton.ks.us/electronics/?p=232&cpage=3 , which is adjacent to 2 resistors that get very hot, fails. The official schematic says the two resistors in question are 47kOhm so I purchased the 3W version from Newark to better dissipate the heat and replaced both the old resistors and the blown capacitor. However, after wiring everything up, the speaker didn't work. I reinstalled the old resistors and it did work, which made me wonder if the official values were incorrect. Indeed, they were. The old resistors are both 37.4kOhms to within a few tens of ohms.

I have a few questions:

1. As I understand it, resistors increase in resistance as they age, so the measured values of 37.4kOhms can't be due to aging if the schematic values for these resistors, 47kOhms, is correct. Furthermore, they are very, very close in value to one another (37.41kOhms vs 37.48kOhms - what are the chances they would age nearly identically?), which makes me think they are, in fact, originally 37.4kOhm, 1% resistors - I know the last band, gold, is a 5% tolerance. Why would the schematic say one value and the actual resistors be another? Why such a specific resistor value? Why not 36K or some other much more common resistor? I've tried finding this particular resistor and these are rare as hen's teeth. Orange-Brown-Red-Gold? Orange-Yellow-Red-Gold? Red-brown-red-gold? Thoughts? Original Alesis M1

2. With the new 47kOhm resistors in place for the original fix, rather than not turning on at all, I guess I would have expected the speaker to malfunction in another way like not being as loud. What would be your expectation for what would happen if 2 37.4kOhm resistors were replaced with 2 x 47kOhm?
The schematic, which you will need to make sense of this question can be downloaded here - https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwir0aLq-fn2AhVBLTQIHZo-B1wQFnoECAQQAQ&url=http%3A%2F%2Fforum.vegalab.ru%2Fattachment.php%3Fattachmentid%3D132728%26d%3D1314681208&usg=AOvVaw3XMrdFBxALWknBYiyq_0ej The resistors in question are R3 and R4.

Thanks for your insights!

P.S. I measured the resistance of the original resistors using two different multimeters. I also tried to read the resistor color code but it is faded - what do you think value is?

[james_s]

It's very common for design changes to take place component values to be slightly different than what is shown in the schematic, I wouldn't worry too much about that. A higher wattage resistor will produce exactly the same amount of heat, all else being equal, but if it is physically larger it may run at a lower temperature. If the heat is killing capacitors I would suggest either relocating one or the other slightly, and/or replacing the capacitor with a high temperature long life version.

[LEDfoot]

I'm glad someone who knows what they are doing is having some thoughts similar to my own: I did consider the last-minute design changes and was able to find the schematic that matches the PCB version and date of the one I'm working on. Nonetheless, they don't match one another. :-) As for the higher wattage resistor, I was hoping to spread the same heat over a larger surface and thereby decrease the chances it would harm the nearby components. Finally, I did order high-quality capacitors with the highest heat rating and MTBF hours I could find. It is obviously overkill but hopefully will mean I quit the speakers before they quit me. Thank you very much for taking time to reply. I appreciate your insights!

[james_s]

I think it'll be fine, and I think as long as the new resistors are somewhat close to the measured value of the existing ones that will be fine too. I'm not sure what those resistors are doing, I glanced at the schematic and didn't see them readily, what part of the circuit are they in? Power resistors are usually not precision parts.

[pgo]

Hi,
The resistors look like 39K based on the colours.
The first and third band look the same and based on the schematic value the bands must be orange.
The second band looks like a cooked white.
The measured value is within 5% tolerance of 39K.

The use of higher values is probably causing the low voltage lock-out of U1 to trigger so no supply at all.

Are you sure it is the resistors cooking the capacitor? They seem to be operating well within rating and should dissipate the heat quite well ( about .8 watts each unless I misunderstand the circuit (240*1.4)^2)/(39K+39K). Still they would get hot.  Being well within power spec. I would not expect large values changes with ageing.

I would suspect the heat-sink as much as anything for the heat source. 
Which way is the board orientated - heat rises .

bye

[LEDfoot]

Hello,

Thanks for taking a detailed look at the problem. Based on the schematic, the resistors should be 47kOhms. Am I missing something and they are actually otherwise? Both the schematic and BOM says resistors R3 and R4 are 47kOhms. I can't make sense of the colors on the resistors. I have checked both speakers and the one in question had R3 and R4 measured (out of the circuit) at 37.4kOhms and the other speaker's R3 and R4 measured in the circuit at 35kOhms. I appreciate your interpretation of the colors but am dubious as to whether that is the correct read given that 35kOhms is not within 5% of 39kOhms were that to be the actual value.

Thanks for your comment on the possible effect of a higher value (though matching the schematic as I read it) resistor. That jibes with what happened when I tried to power the circuit.

The problem of those resistors and the adjacent capacitor is well-described so much so that replacement of C8 is the default action when a person experiences the flashing blue light and ticking noise with this model speaker. While the cap is beneath the resistors in question, it is within millimeters, so while the convective heat rises, the radiant heat still warms this cap. Of course, Alesis used the cheapest components in this speaker so even an acceptable heating given good components would take its toll on these cheap versions.

I don't doubt that the resistors are in spec with regard to heat dissipation, though the darkening at the base of both of these resistors on the PCB suggests significant heating is occurring. However, given how many corners were cut on cost, I think there may be added value (while I'm messing around with the board any way) to use a resistor with a higher wattage rating simply to dissipate heat over a larger area and, at the same time, to relocate the resistors away from their fickle neighbor.

Thank you again for your sage insights!

[james_s]

If in doubt, go with the measured value. If the circuit works with resistors that are about 37k then go with something around that, 39k is a good guess based on the colors. I missed that it was not working, try reinstalling the original resistors and see what it does. As I mentioned, it's quite common to find incorrect values on a schematic.

[Haenk]

There really is no reason to change any values (as it did work before, why change them).
But, as you know, the cap is utter junk, find a good branded one (Nichicon e.g.) with long life rating and 105oC.
That should be good enough.

(The cap did not blow up due to the resistors, but due to being junk ever since the factory put it in...)

[Audiorepair]

Hello,

I don't doubt that the resistors are in spec with regard to heat dissipation, though the darkening at the base of both of these resistors on the PCB suggests significant heating is occurring. However, given how many corners were cut on cost, I think there may be added value (while I'm messing around with the board any way) to use a resistor with a higher wattage rating simply to dissipate heat over a larger area and, at the same time, to relocate the resistors away from their fickle neighbor.

I have always laid the new cap down on the PCB between R18 and the big cap.
Bend the legs neatly at right angles, sleeve them, then silicone the cap body to the PCB.

It's a bit of a bodge, but then so is doing anything with the existing resistors.

I don't think the resistors heating up themselves and the PCB is actually much to worry about, both can take a bit of discolouration.

[LEDfoot]

That's a great suggestion. What is the silicone for? Vibration? I noticed a number of the capacitors and inductors are both soldered and adhered to the PCB and, often, to each other.

[LEDfoot]

I believe it is in the power section and, according to the BOM, are only 5% tolerance.

[LEDfoot]

As I get in deeper and deeper in this project, I'm reminded of the axiom, "The enemy of good is great." Now I can't get the originally malfunctioning speaker to work even with the original resistors. I knew I could potentially brick one or both but wanted the DIY experience.

[LEDfoot]

When I initially opened up the malfunctioning speaker, knowing that this model was highly-rated at the time I bought it, I thought I would find top-tier components but when I started reading the names on the passives, I didn't recognize any of them. Then I consulted this article - https://www.tomshardware.com/reviews/power-supplies-101,4193-5.html - on the various levels of component quality and nearly all of the ones used in this speaker are at the bottom of the list. When I purchased replacement parts for the capacitor in question as well as others that looked like they were starting to bulge or leak, I bought from the top of the list AND found ones that were both rated for high temps (105C to 125C) and had longer MTBF ratings. In the end, there's no question this is overkill but I'm using this as a learning experience and the difference in cost between crap and quality is so minimal, it didn't make sense to save a few dollars. Thanks for your insights!

[Audiorepair]

That's a great suggestion. What is the silicone for? Vibration? I noticed a number of the capacitors and inductors are both soldered and adhered to the PCB and, often, to each other.

Silicone is a really useful glue.  It stands high temperature, sticks to everything, and is flexible so a great shock absorber.
That cap will never break off the PCB.  Ever.

Unlike hot melt glue, which only sticks to a few things, and mostly not that well.

[LEDfoot]

Silicone is a really useful glue.  It stands high temperature, sticks to everything, and is flexible so a great shock absorber.
That cap will never break off the PCB.  Ever.

Unlike hot melt glue, which only sticks to a few things, and mostly not that well.

That's awesome! Do you have a brand or type you like for this purpose?

[Audiorepair]

Silicone is a really useful glue.  It stands high temperature, sticks to everything, and is flexible so a great shock absorber.
That cap will never break off the PCB.  Ever.

Unlike hot melt glue, which only sticks to a few things, and mostly not that well.

That's awesome! Do you have a brand or type you like for this purpose?

Even the cheapest is more than adequate for electronic gluing purposes.

Don't use one of those pointy nozzles, the silicone will harden inside it in one day rendering it useless next time.
Instead, place a piece of plastic bag over the tube opening, excluding all air, and then screw the pointy nozzle on to seal it.

Next time you come to use it, you just remove the nozzle and the bit of plastic bag, and hopefully remove a small plug of hardened silicone from the top of the tube and away you go again.


Otherwise one tube of silicone will not last more than one tiny job.

[amyk]

This is a SMPS designed for 240V with a voltage doubler at its input when configured for 120V operation.

The two big resistors are used to supply power to the UC3844 to start the circuit running, after which T1B should continue to power it through R5 and D5; I would also check those two components to see if they're within value, and also relocate C8 away from the bootstrap resistors.

2x37k is definitely much lower than what I've seen in other designs using UC384x and related family - the bootstrap resistors are usually >100k total. Here's some examples:

https://danyk.cz/iz350_en.html

https://www.exxoshost.co.uk/atari/last/psu/PSM_5341_MegaSTE-TT030_PSU.pdf

[LEDfoot]

This is a SMPS designed for 240V with a voltage doubler at its input when configured for 120V operation.

The two big resistors are used to supply power to the UC3844 to start the circuit running, after which T1B should continue to power it through R5 and D5; I would also check those two components to see if they're within value, and also relocate C8 away from the bootstrap resistors.

2x37k is definitely much lower than what I've seen in other designs using UC384x and related family - the bootstrap resistors are usually >100k total. Here's some examples:

https://danyk.cz/iz350_en.html

https://www.exxoshost.co.uk/atari/last/psu/PSM_5341_MegaSTE-TT030_PSU.pdf

Thank you so much - this is the type of information I'm looking for to help me better understand not just the failure but the circuit itself. To your point about being 120/240, a fairly bulky jumper is in place to default the circuit to 120, which I unsoldered and replaced with a piece of soldered wire so I could use the recovered space to give breathing room to the resistors.

What is T1B? I will definitely check these components - I assume that can be done while in situ in the circuit, right?

Thanks for illustrating your response with examples. I checked out the datasheet for the UC3844 and TI indicates it operates at 500kHz?! That seems fast for the very limited understanding I have.

[amyk]

Check the resistance of R5, and that D5 still behaves like a diode.

T1B is a winding from the transformer.

The UC3844 can operate up to 500kHz but based on the values in the schematic, I think the designers were going for 140kHz. Here's a detailed description of designing using it: https://www.ti.com/lit/gpn/uc3844

[LEDfoot]

Ok. Thank you. I will check those values. Earlier tonight, thinking I had reassembled everything correctly, including returning the old resistors to their original place (just to check functionality) I plugged it in and nothing happened. I looked at the solder joints and compared the working circuit with the non-working one. I noticed the 250V 5A fuse had blown, so I replaced it with the only fuse in that ballpark that I had - 125V, 6A - and it blew when I turned it on a second time. Any thoughts on what else might have failed leading to the fuse blowing? BTW, feel free to tap-out if your kind involvement has taken you beyond what you're interested in offering. :-)

[BradC]

Silicone is a really useful glue.  It stands high temperature, sticks to everything, and is flexible so a great shock absorber.
That cap will never break off the PCB.  Ever.

Unlike hot melt glue, which only sticks to a few things, and mostly not that well.

That's awesome! Do you have a brand or type you like for this purpose?

Make sure it's neutral cure. Most common Silicone is acetic acid cure and that'll do "bad stuff"(tm) to the speakers.

[amyk]

Check for shorts. Due to the high voltages involved, one failure can easily take out other parts in a SMPS. You should use a "dim bulb" tester at least, or ideally a variac, to make this less likely to happen.

If you know what the output voltages and currents are, it might be easier to fit an entire replacement PSU...

[james_s]

Fuses don't just blow, something is clearly wrong. I'd start by checking the chopper transistor for shorts.

[cdev]

I am guessing you already saw this..

https://www.badcaps.net/forum/showthread.php?t=10910

Its strange that its causing a power supply failure and thats whats causing the problems.

 I'm curious about why they are so picky. If I owned them I would put them on a variable supply and tweak the voltage and current a bit to see what happens.

You may just need to replace the two power supplies and a cheap supply may be adequate. I dont know.

[amyk]

It seems the PSU was badly designed from the beginning; I'm not even sure what topology this --- it looks like a mix of half-bridge and regular flyback, especially with that odd upper switching transistor. Posts in that thread mention units working fine with 2x47k bootstrap resistors, so I suspect the OPs unit may have a bad D5/R5 and be barely on the edge of working. There's also discussion about using a UC3845 instead of a '3844. Main switching FETs and their driving transistors appear to be common failures too. I'm no SMPS expert, but it's a weird design and yet doesn't seem to be cost-optimised.


The voltages are apparently +/-25V and +/-35V, which might be hard to find in an off-the-shelf unit.