Super Freak, Super Freak

So we now turn our attention back to the Arturia MicroFreak that I had picked up on a whim. After a bit more investigation I was able to determine a few things.

First off, thanks to the thermal camera I picked up, I was able to narrow down the failure of the 6v +/- supply to the regulator itself, and not any of the chips it powers.

Secondly, I was able to figure out that it's actually just a 6v supply, not a 6-7v supply. I misinterpreted a page on the datasheet regarding how the positive and negative voltages are calculated across the sense resistor bridge.

Thirdly, I discovered that the supply actually only powers three chips in the VCO and VCF circuits, not the audio output. The latter is actually driven by a 1.8v linear regulator, which is apparently sufficient to drive low-impedance headphones. Go figure.

Anyway, the full repair is documented here:

And I'm glad to report that the fix worked great. The synth has actually been performing solidly for a few weeks now and shows no signs of failing again, though who knows how long that'll last. Hopefully forever.

Also the rose bush is in full bloom

Which is nice.

Getting Freaky

So I was flipping through the seedier parts of eBay the other day, and happened upon a listing for a "broken" Arturia Microfreak synthesizer. I tossed a lowball bid on a lark and, wouldn't you know it, scored it for less than $100 including shipping. It usually sells new for $350, for comparison.

The stated problem? It was boot-looping and refused to flash. So I power it on and wouldn't you know it, it's stuck on this screen.

So that's a good sign. I've been getting pretty bored of buying "broken" audio equipment only to find that it was actually working perfectly.

Anyway, I figured maybe I could get a quick "if your microfreak is boot looping, just do this, dummies" YouTube video out of using one of the magic-key-combo rescue procedures to reflash the device, but just as a quick test I plugged it into my MacBook Air to verify that it would, indeed, not flash using the normal procedure.

Oh.

Yup. Flashed just fine, no special konami-code needed.

But the story doesn't end here. There's no onboard speaker on this model, so I plugged it into my mixer to give it a test run and... nothing, no sound. Well, not exactly nothing, more like "horrible ground loop buzzing". So I swapped out the included power supply for a USB cable (it can run off of either) plugged into the USB outlet on my power strip, and got less ground loop buzzing this time, but still no usable audio. So for try number three I grabbed a USB wall wart and plugged it into that one and finally cleared up the ground loop buzzing, but still got no love in the form of producing actual audio out.

Ok, well, that's not great. Now I'm starting to suspect that something really is wrong with this device.

So I pop it open and take a poke around. The main 3.3v power supply is doing just fine, as evidenced by the display and lights and so on all responding normally, but it's nice to confirm that directly.

Using only 3.3v to drive the audio output wouldn't result in much volume though, so poking around a bit further lead me to find a split-supply boost converter in the middle of the board. Checking the sense resistor bridge, it seemed to be configured to deliver around 6 to 7 volts plus and minus (I didn't check which was which, but I'm assuming they configured it for +7 and -6 so that it would be centered on +0.5v so that there wouldn't be any DC offset for a 0-1v audio signal).

On the backside of the board, these three test points are connected to the positive, ground and negative supply rails generated by the boost converter.

So let's check one of them.

Ok, that's definitely not 6-7 volts. How's the other one look?

Ah, right. Yeah, that's not at all the voltage it should be.

I did a bit more probing and the regulator is oscillating at around 440kHz, which seemed like a reasonable value, so the regulator chip itself likely isn't to blame. However, when I powered down the unit and measured the resistance across the test points, one of them read a reasonably high value, but the other, the one showing only 1/4 of a volt, measured 11ohms. That is suspiciously low.

So likely what happened is that the output amplifier got a few too many dancing pixies rammed back up its arsehole and lost all its magic smoke. Reading up online, this is apparently quite a common failure with this synth when people accidentally forget to turn off the 48v phantom power on their mixer and plug this in, and I guess Arturia didn't think to include any protection circuitry. (And no, I did not leave the 48v phantom power on MY mixer turned on)

Anyway, I didn't feel like chasing this down any further right then, so I sent an email off to Arturia tech support to inquire about obtaining a replacement board, and we'll see where that goes.

To be continued...