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The Alesis M1 Active Mk2 Problem - capacitors dying from resistors

tampere

Apr 5, 2022
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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), 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 Resistors.jpg

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. 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.

Labeled M1 Board.jpg
 

Audioguru

Sep 24, 2016
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The schematic "Cannot be downloaded securely".
My pc does not know what kind of file type it has. Maybe it should be a PDF.
 

tampere

Apr 5, 2022
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Here's a pdf of the service manual / schematic.
 

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  • ALESIS M1 ACTIVE MKII.pdf
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kellys_eye

Jun 25, 2010
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Change C8 - it's DUD!

The resistors are probably ok and don't need changing but if you want to.....meh. They're only there to 'balance' the voltage across the two main smoothing capacitors (C6/C7).

Measure the power supply output and see if it is within spec. I reckon replacing C8 will fix whatever issues you're having.
 

Audioguru

Sep 24, 2016
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The schematic of the amplifier show that R3 and R4 feed the "mute" pins of the amplifier ICs with a low current.
 

kellys_eye

Jun 25, 2010
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The old resistors are both 37.4kOhms to within a few tens of ohms.

The schematic has them at 47k. The 37.4K you measured was possibly as a result of them originally being fitted as 36k devices (one of the E24 range) and this fits with the discolouring if the middle band was originally blue. The original resistors are probably still ok despite their discolouring.

Although the value found is different from the schematic value it may be as a result of the controller IC1 minimum current requirement.
 
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