Tuesday, November 11, 2014


One of my mission companions would say that a lot. He threatened (jokingly) to fix people. He was Tongan and barely spoke English or Filipino but we had fun and got some good work done.

Regardless of that, my Presonus Firebox had been making a strange whining sound for a while. I started a session a few weeks ago to finally begin a new song but (as usual) didn't get very far before fatherly duty called and the session sat for at least 2 weeks or so. This happens as regularly as I record which is roughly every few weeks. I eventually passed through the studio to grab something out of our food storage in the next room and realized that the firebox was completely dark. "Strange," I thought so I went and tried to restart JACK. The firebox wheezed and whined and eventually the blue LED slowly lit up. "Not good," I thought (I usually think very tersely), but I had to get that food upstairs and get dinner on for the kids.

I didn't read too much into it at first since I was pretty busy. But when I came back to it after a few days, I tried it again with the wall power. It powered up more quickly but still not anything like the instant blue light it used to bring up and it was still very audibly whining during operation. I tried plugging the firewire cable into the other ports on the computer and the interface. I tried changing some jack settings and eventually came to the discovery that it was making this audible noise even when nothing was plugged into it except AC power, not even attached to the computer. It sounded like some communication noise I've heard before, like a uart running at 9600 baud or something, but I reaffirmed that it wasn't a good thing. I had noticed it making this noise quietly for the last few months but now it was very noticeable. Playing with my condenser mic a little showed that whatever had changed raised my noise floor significantly.

The firebox works just fine in Linux using the FFADO drivers. It's not feature complete since I can't do the hardware monitoring that I'm pretty sure the firebox is capable of, but it's plenty good for me and my one man band recording methods. It has clean preamps (relative to the internal sound card of my laptop) and is useable at whatever rates I need (usually just 24bit 44.1khz though). And finding a new audio interface that works on linux is no small task. It was especially painful to think of needing to replace it because I'm finally about to have enough money in the budget to buy my first studio monitors, and even just meeting another guy in some middle school parking lot to buy a replacement interface for $60 again would threaten putting off the monitors for some time more.

So with heavy heart I pulled it apart this morning. Actually I had already fixed a broken chair I'd been meaning to get to and a tape measure that I dropped off the roof while re-shingling a few months ago, so I was on a roll. But I pulled it apart without too much trouble and tried to see what was going on. Luckily the damage was fairly noticeable. The insulation on the wire that connects the headphone jack to the upper PCB had melted to the cap on the lower board.

Most of the info I could find about problems with the firebox had a cap completely blow with some charring etc. This seemed much less dramatic and I was concerned that this one slightly marred cap wasn't fully the problem. But it was the best I had to go off of.
The damaged capacitor, Notice the melted plastic and the top seems slightly bubbled

I had a Nicholsen PW(M) cap that was 470uF but it was 25V instead of the 10V that the Chang cap I was replacing. The PW series aren't audio grade but I think the Chang wasn't either since this was near the power section anyway. I was glad to be upgrading the rated voltage of the cap, but this meant the new one was much larger. I had to get creative with the placement to keep it out of the way of the headphone jack and not touch the chassis or other components.

The new capacitor in place. You can also see the slightly melted insulation on the middle white headphone jack wire.

 The soldering was fairly trivial. Wick the old solder, pull the part, solder in the new one. I put some electrical tape on the headphone jack wires to help prevent them getting further compromised. I got lucky because I had left one of the leads just a bit too long and the first time I plugged it in they were shorting to the chassis, so when I plugged it in I got no power light at all. In retrospect I was lucky it didn't blow up something, but it was very disappointing. I took it all apart trying to figure out what had gone wrong and ohmed out all the transistors I could find to see if any had shorted.

When that proved fruitless I plugged it in without the chassis and it worked! I then just added one screw at a time and tested if it still turned on. Next screw. Test. Etc. When I got to the front part of the chassis attached, thats when I realized I didn't have good clearance on that lead of the new cap. I trimmed it and put the front on. Test. SUCCESS! I continued doing this through the rest of the assembly to be safe but in the end I had it running perfectly silently, fully assembled, there on the workbench.

I think this is the first time I've had a complex electronic item, took it apart, and was able to fix it without a schematic. It felt so awesome! Fixing the house, the chair, my remote control helicopter motor... mechanical issues are easy to diagnose and fix. Electronics though take either detailed documentation and knowledge, or a little luck. So when it works, fix is very sweet.

I took it home and just as a test hooked up my condenser, cranked the gain and added a simple amplifier plugin for more gain. Silence. I am back to my original noise floor! From now on I think I'll shut down my computer between sessions and use the AC power instead of just bus power.

Now I just need to actually make a recording again.

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