Repairing Alesis M1 Active mk2 Monitor Speakers

For Friday’s show, we used Alesis monitor speakers that we had in the lab, plus Steve, one of the students, supplied two.

Alesis M1 Active mk2 speakers

During installation, one of the speakers started winking its blue power light and ceased playing sound, and before the show another did as well. Steve found a Studio Central forum post suggesting that the problem was due to a failed electrolytic capacitor that gets baked by a hot resistor right next to it, and a quick peek inside confirmed that it was a likely explanation and fix.

Alesis M1 Active mk2 speaker, circuit boards and interior

After unscrewing, the back panel lifts out and reveals the power supply board mounted vertically on a metal shield, and the crossover/amplifier board mounted flat on the panel.

Alesis M1 Active mk2 speaker power supply board

The naughty capacitor, C8 (actually its replacement after I finished), is in the center red rectangle next to the offending resistor. Another bad electrolytic capacitor whose number I forgot to catch is featured near the top of the board. Both of these tested bad with my Capacitor Wizard in-circuit equivalent series resistance (ESR) tester; all of the other electrolytics on the board tested good.

It was simple work to remove and replace the two capacitors on each board, and it brought both speakers back to life. Thank you, forum posts and Capacitor Wizard!

BTW, are electrolytics supposed to look like this?

Leaky electrolytic capacitor

Two caveats about this repair. First, I should have used 105°C capacitors, but I could only find 85°C caps on short notice, so these will fail quickly and need to be replaced again. At least now it’s known exactly what needs to be done. And second, the forum post suggests moving either the resistor or capacitor to get them further apart, which is a great idea but which I haven’t done yet. I’ve been trying to think up a clever way to stick a little heatsink on a vertically-mounted resistor, which might be a better solution yet.

410 Responses to “Repairing Alesis M1 Active mk2 Monitor Speakers”

  1. prototypicalDave says:

    This is the comment thread that keeps on giving. Just replaced c8 in both my dead monitors and it worked! Thanks everyone for all the information. This is what the ‘net is for.

  2. waran3 says:

    I fixed the power supply.
    https://www.elektroda.pl/rtvforum/viewtopic.php?p=16620577
    There are lots of parts to replace:

  3. Steven Price says:

    I did the one replaced both resistors, and c8, and it come back to life, did the same to the other one, but now it doesnt even flicker. What are the most likely things to be gone, because the time I purchase an ESR meter, I could probably replace the most likely components. Are the 4x Diodes D1-D4 likely to be at fault here?, as Ive tried reading them and they are coming back around 800 mark, but they are in circuit. So Im stuck at the minute what to do next? Maybe the pair of large Caps.

  4. Matthew Stringer says:

    I have a problem with one of these. The C8 cap had died and taken out the oscillator and FET transistor, there was also a bulge on one of the main caps after the bridge. The PSU just pulses about 1Hz, if I hook it up to the rest of the speaker it’s like you’re just switching it on and off continually. Voltages look OK and I’ve not been able to find any duff components, I’ve been using the other speaker to reference it. Can’t see what’s wrong with it, beginning to get frustrated. Any help would be awesome.

  5. Mike O'Connor says:

    So the pair I have have been powered for the last couple of years only being turned off for the week we were away on vacation.

    When I returned and turned them on one let out a “FIIT” and a “POP” with no sound or light. I pulled the plate and had a look at the PS. The caps actually looked good and there was no signs of over heating on any parts. Close inspection of the board and specifically the switching chip showed a small hole in the top. AH HA!

    I look up the chip and see that they bought cheap knock offs of the ST UC3845B. While I am at it I notice that one of the PS input caps is oh so little bulging at the top. So I ordered up rated parts from Digikey as well as the actual ST devices. While I was at it I also bought the cap to replace the 330uF 35V unit as well.

    A note, the parts are not much more for 10 PCS than the required one or two of each so I bought 10 of each part. Total cost for the lot was less than $65.00 USD to my door.

    30 minutes of swapping parts and all together again. Switch it on and all is good!

    Funny thing. Went away for a week and returned. Turned on the speakers again and the other one crapped out!!! Lucky I had the parts on hand. Another hour and I was up an running again.

    Not happy with the fact though that they are so cheap on the PS and the rating is only really 30W of output. This means that the actual sustained output of the speakers is likely to the south of 20W.

    Think the next thing will be to upgrade the PS with a toroidal transformer, bridge rectifier, caps, and regulator board os I can dump the switcher all together. Hell they work and they are expensive. They will more than pay for the parts to upgrade to a good proper supply. I will Ebay them!

  6. moony says:

    good guide. @Matthew I think we’ve got the same issue: while power on I have LED and speaker pulses at about 1Hz. There were two heavily burnt caps: C28 and C29 (the small ones in the corner). I’ve replaced them but still got the 1Hz pulses.
    I’m looking for switching power supply repair info, especially for this “on-off-symptom”.
    I’ll post news here.

  7. Rikk says:

    Hello, I’m a newbie trying to determine if my MK2 problem can be helped by this thread. One of my speakers works just fine at low-ish volume. The blue LED flickers ever so subtly though, and at higher volumes, the woofer will cut out or clip intermittently. Soldering is beyond my skillset, but I know some people, I’d just like to point them in the right direction or at least be able to advise a potential buyer. Any guidance would be greatly appreciated.

  8. David Bullard says:

    Hey! There’s no easy way to tell without giving it try, but the LED flickering is a sign for sure.
    That was one of the symptoms that my speakers developed before the died totally. I also has a very low frequency pulse happening in time with the led, which also pointed to the bad cap. The repair is pretty easy for someone with a mid level of experience and took me about 15 min total. I’m an occasional hobbyist solderer with crappy equipment and it was very easy for me. A pro or mid level amateur should have no trouble. Good luck!

  9. Mike O'Connor says:

    Riik,

    So looking at the crossover/amp board, this is where the LED comes off of, it is across the pair of linear regulators used to supply the crossover section. In this case they are a pair of positive (7815) and negative (7915) 15VDC units. Flickering indicates that one or both of the feeds from the power supply are sagging in output.

    A note, it could be one of the 7×15 regulators but I doubt this as they are pretty reliable when not pushed. These are pretty crappy supplies and are well known for the caps being an issue so there is where I would be looking for starters.

    So I would be willing to bet that the issues are due to bad caps sincethe supplies still seem to kinda work. The parts I would have on hand for the soldering mavens would be at least 4 pcs of 390uF, 200VDC At least 105C or higher rating!!! (C6 and C7) These are the input caps on the line side of the power supply. The other one is close to a pair of power resistors close to the power switch connector. The resistor is R4. You will need a pair of these and the cap is a single 220uF, 35VDC unit. Again at a minimum this should be a 105C rated cap.

    For the inputs (390uF) I bought Rubycon parts and the switching regulator power cap (220uF) I bought Panasonic. These were both from Digikey so the likelihood of them being counterfeit is small. I would not recommend buying electrolytic off of Ebay if it can be helped, especially from China unless you are familiar with the seller which I suspect you would not be.

    Hope this helps.

    Mike

  10. Ignas says:

    Hi, Cleveland king.
    The value of R27 is 0.02 ohm 1Wtt

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