EEVblog #1087 - UNBELIEVABLE Alesis Studio Monitor Fault!

An unbelievable fault in Dave's Alesis M1 Active 520USB powered studio monitor speaker.
Can you discover the culprit before Dave does?
This was supposed to be a boring trivial repair of a power LED, but it turned into something much more interesting.

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[Chriva]

I'd say you gave it an upgrade. I'm so damn tired of everything having a blue led

[Wenlocktvdx]

Had this happen with a pocket FM/TV sound radio many years ago. Started with intermittent stereo which eventually failed totally. Turns out the the led was intermittent because it split in half due to the legs being bent when it was installed. This stress the bond wire until it broke. The intermittent stereo had me looking at a faulty chip and first but when I fitted a new led it worked 100%. The led was actually regulating the stereo detection voltage. When the led went open the voltage rose too high and muted the decoder. I actually read of a dead led causing no stereo shortly after that. Think it was in EA mag.

[kmgy]

Clipping will cause the red to blink. So you have your clip function!

[gothroachkinski]

My favorite LED failure was in a piece of machinery from probably 15-20 years ago that I had to deal with recently. Long story short, it has an RGB LED to indicate ready/warning/fault. I am not sure if it was something with the bond wires degrading/shorting or if there were some chemical breakdowns going on, but the LED lit up Yellow, Green, and Yellow. There went a couple of days of working on the entirely wrong part of the machine.

[zaprodk]

Don't cut off hot-snot. Put a drop of isopropyl alcohol on it and it releases like magic!

[flymypg]

Early blue GaN LEDs could have odd behavior, including even color shifts. Just how old is that speaker?

[aabb]

It would be interesting to look through microscope at the crystal of that LED.

[mikesradiorepair]

Actually that's a extremely common fault for LED's. I see indicator LED's going haywire like that all the time. On one second, off the next and then back on. In my experience it's LED's in "cheaper" electronic devices. High end products tend to use better quality LED's.

[MOSkyGuy]

Great video, Dave! I like seeing the step-by-step process as you track this down. Thanks.

[Blacktronics]

>Studio monitor
>Bass boost switch
ok.

[allesklarklaus]

Maybe the red LED leaks in reverse? 1.9V junction voltage for that blue one doesn't seem enough. Or the blue one is leaky in forward bias

[ZomB]

I've seen it a lot with cyan leds like those used in traffic lights. These were damaged by static discharge from not properly handling them. When looking at it under a loupe I see a black fissure-like damage on the die. When I power it it's like a short, but when I increase the current very briefly to 50~100mA I "burn through" the damage and the led lights up again at 10mA but not as bright as a new one. Sometimes varying the current brings back the short circuit or only partially and the led flickers. With the curve tracer I see a curve like resistor in parallel with a diode. A led that doesn't light up at all has a curve so steep that more than 50mA flows before the threshold voltage is reached. When I "burn through" the damage on a led, using the curve tracer, the slope immediately drops and becomes very unstable.

[mattmoreira]

Yeah! I've seen something like that in the past. It's truly hilarious when that happens. Actually, it seems to be a quite common failure mode, if you pass too much current through the poor thing for an extended period.

[KekkopF]

Speach quality is never great when you have a two-driver crossover speaker system. The crossover point typically lies in the middle of the speech band.
Try speech on a high quality 8" broadband driver like the Sica 1100, it blew me away how good it sounded.

[Budmur]

Dave's Law: "Thou shall check voltages.". First corollary: "Thou shall verify current flow."

[Starphot]

Similar problem with white LEDs I used for lighting a portable astronomy dome. PVC pipe and nylon construction dome causes static charges. This was a basic wire-up before adding clamping diodes and capacitors to each LED. These white LEDs started to flicker on and off and turn green in between. The red set were unaffected. No problem since adding the clamping diodes and capacitors since. It is an odd fault with the blue and white LEDs due to the multi-layer construction of the dies.

[sbreheny]

At my company we used to have strange problems with blue LEDs going intermittent or off alltogether. In the end, the actual problem in production seems to have been ESD-induced. However, while we were investigating, we found that a moderate overcurrent through the LED for months would result in just the kind of behavior you show here (flickering which did not seem correlated with any mechanical shock or vibration). If I recall correctly, it was caused by driving maybe 30 or 40mA through a max 20mA-rated LED for about 6 to 10 months which did it.

[ngneer]

He chased a red herring down the rabbit hole. Don't they teach you to not mix metaphors in Australia?

[SidneyCritic]

Maybe the black gunk across the pins of the LED is conductive, and putting most of the power straight to GND.

[zerbt]

I have one led that does something similar after I accidentaly over voltage it