I have most of the parts required for the replacement of the power supplies with an external 300W linear upon the next failure. Just waiting on the connectors and the wire for the umbilical cords. A single 300W toroid with separate dual 15V and 28V windings is right on the money after full wave rectification. Gonna due dual mono after the transformer and give each pair their own DC rectifiers and cap banks. I will put woofer and tweeter cap banks in each speaker in place of the power supply boards to feed the amp board. Kinda done with these total crap switchers after having to repair them for second time.

So an update on the speakers. They have been working for the last 5 years only being shut off for one week a year over the last 5 years. The other day I turned them off only to have them not come back on when I turned them on a few days later.

When I originally effected the repairs I did replace the cheap caps with much better units that were higher spec and better temp ratings. I replaced the resistor that was running hot and extended it further off board and away from the caps. I also had to replace the switching regulator at the time and used the real device rather than the cheap no name crossover chip that was in them originally. I seldom drive the speakers hard so heat should not have been an issue.

I think I am going to be replacing the troublesome, and WAY UNDERSIZED switchers in the box with a 300W torroidal and a pair of linear regulators for each of the amp pairs. Feed to be brought in via the EIC connector port that I will replace with a blank and Anderson power pole connectors sealed up with silicon. I will use the power switch to simply control the mute circuit. the supply will be replaced with a board with large filter caps fed from the external supply. That should remove the vast majority of heat and provide far more headroom too. I may fix the supplies and simply sell them but what a pain in the ass.

https://maker.pro/forums/threads/the-alesis-m1-active-mk2-problem-capacitors-dying-from-resistors.296898/post-1843403

At minimum, if the PSU is repaired by replacing C8, it should also be re-housed in a ventilated/heat-sinking enclosure and mounted on the exterior of the rear of the speaker. It’s very easy to run the cable between the PSU and the amplifier/crossover board through the hole for the kettle lead, and the PSU board’s mounting plate is held to the speaker’s rear panel with bolts, so that whole assembly can be removed into whatever new housing you choose. No excessive modification of the speaker body is necessary, just ensure the kettle lead port and power switch port are covered with something rigid and sealed to prevent pressure leaks.

Info for anyone looking to repair them by fully replacing the PSUs, or by turning the speakers into passives:

Alesis M1 Active MK2

Woofer: 75w, 8 ohm

Tweeter: 25w, 4 ohm

Crossover: 2kHz

“Specifically in the case of the M1 Active Mk2, the wide, mid-frequency
polar response of the tweeter can be utilized down to a low 2000 Hz
thereby mating more cohesively with the 6.5″ non-woven carbon fiber
(NWCF) woofer. This low crossover point is only possible through the use
of a steep, electronic fourth-order (24 dB per octave rolloff) high pass filter.”

Tweeter PSU output: 18V

Woofer PSU output: 36V

Tweeter amplifier: LM2876

Datasheet: https://www.ti.com/lit/ds/symlink/lm2876.pdf

Woofer amplifier: LM3886

Datasheet: https://www.ti.com/lit/ds/symlink/lm3886.pdf

PSU cable output to crossover/amp board:

-V_LO (green)
+V_LO (red)
AMP_GND (white)
SIG_GND (blue)
-V_HI (green)
+V_HI (black)

Just wanted to say thanks, I followed your advice and successfully made them passive speakers today. Very happy and cost probably just a bit more than one of the power units. Also think they sound so good. Probably better than the mk3 I got to replace these.

My left Alesis M1 Active mk2 speaker no longer powers on. The right one is fine.

Is anyone here interested in purchasing either or both?

Thanks!

-Susan