#68 Alesis M1 Active Mk2 rehabilitation, part one

Over on the Badcaps.net forums, a thread was started all the way back in 2010, about power supply issues in these arguably "classic" low-cost active studio monitors. It even ended up being stickied just this winter, so it's been quite active and useful. It served as motivation for myself, to take up hunting for faulty ones - not sure if i had gone through three pairs, but at least two pairs i revived and sold off to a couple friends, some 7-8 years ago, and to this day i've yet to hear back about any further issues, so i take that as a good sign. In the mean time, i still contributed with thoughts and advice there, which helped lead to several successes.

Recently, a fellow forum member there got fed up with his failed attempts to revive his pair of monitors, and got himself a pair of new ones. In that context, we struck a deal for the Alesis'es, and a few weeks later, here they are.

The service manual (including schematics as well as board layouts) can be found right over here (at least for now).

"New Tide" brand combo-XLR input connectors. Never heard of before, but i guess it's a thing? On the other hand, the two "bumpers" out on the sides, to protect the volume pot in case of the speaker falling on its back or something, is a nice touch.

Given the not-exactly-stellar track-record of these, even before i first powered up the things, i chose to disconnect the power supply from the audio board, as well as give the power supply a going-over first. Speaking of which...

Now, one must keep in mind that both these had gone through at least one round of repair attempts, so some of the things that may look odd or out-of-place are not "factory". As, for example, the sizeable Panasonic FR electrolytic "floating" on its long leads, well above the board. Or the lone diode that had been (re)soldered onto the bottom of the board. Also, the four 1N4007 diodes rectifying the mains had been replaced, but the soldering could've been done a smidge better.

As i had done in the first(?) pair of these that i had revived, many years ago, i chose to relocate a few of the capacitors, in the name of long-term reliability. Namely C14, moved to the bottom of the board, soldered straight to pins 5 and 7 of the UC3844 (filtering its Vcc supply line; the stock one didn't survive the removal so was replaced with a 100nF film cap); C11, also moved to the bottom (filtering the UC3844's 5V voltage reference); and C8, the bulk filter / smoothing capacitor, moved into the footprint of C14, further away from the MOSFET heatsink, and closer to the "open" side of the board, in a far less crowded area.

Just to be on the safe side, i decided to do the first power-up through my isolation transformer / series-tungsten-lightbulb combo, to prevent any disasters. The results were... interesting, to say the least. The supply to the UC3844 looked to be a solid 17V, its reference voltage 5V, so all well on that side. The output though, slightly different story. The "+V_HI" measured in at 59V, while the "-V_HI" showed -56V. Juuuuust a touch on the "toasty" side, especially since the LM3886 feeding off those rails, which drives the woofer, is rated for an absolute maximum of +/-47V with no input signal. Not to mention that the bigger filter capacitors on those rails are rated at 50V. Surprisingly enough, they hadn't / haven't yet domed up or shown any visible signs of damage. At least temporarily, they will get replaced with some 63V caps (if i have any in my parts bin), to eliminate shoddy feedback to the power supply controller as a cause.

Further investigation showed that the above-mentioned LM3886 was actually shorted through (V+ to V-). Whether that was the cause of the power supply overvoltage, or just a consequence, i can't yet know, but nor is it terribly relevant at this point. On the bright side, i still have some "brand new" (ie. unused) samples i got from National Semiconductor maybe half my life ago, for some projects that never came to be, so that side of things is sorted. Fortunately the LM2876 driving the tweeter seems to have survived unscathed (the lower-voltage supply rails, even overvolted as they are, are still just within the +/-36V absolute maximum ratings).

Starting with the simplest thing first though, replacing the first line of smoothing capacitors on the secondary. Stuck in a pair of Panasonic FR 560uF / 50V for the "hi" rails, and a pair of FR 270uF / 35V for the "lo" rails. Quick power-up test revealed... no change. So lousy capacitors were not to blame (this time around). 

After consulting with a good friend, i then swapped out the TL431, since all the feedback circuitry virtually revolves around it, but that did not help things either. Which then led me to start measuring the resistors around it. The first couple turned out good, but when i got to R30... My bench multimeter started from 2Kohm or so, and went up at a moderate pace. All the way past 100K, and i lost my patience when it passed 200K. Removed it from the board, and whaddya know - wiiiiiide open. I think we have the suspect in custody!

Swapped in a known-good 100K resistor, flicked on the power and... The "lo" rails are at a much more nicely-behaved +/-18.5V, while the "hi" rails are at a civilised +/-37V. All because of one crappy little 1/8W resistor, no less. Now, to swap out the shorted-through LM3886...

Hooray for "hoarding" free samples from 15-20 years ago. Since one of the power amp chips needs replacing, that's all the more excuse to clean off the heatsink and both mica isolating pads, and slap on some fresh Arctic Cooling MX-4. With that done, as one would hope (if not even expect), the thing works again!

Now then, time to turn our attention to its pair. As was the case of the first one, several screws on the back are missing, but that's of no consequence. But, umm... wow... This one looks like it's gonna be a weeeeee little bit more involved, to say the very least.

[To be continued...]