Tag Archives: compressor

The DBX 900 Series

Way back in the stone age, we used some equipment in the studio that was really neat stuff… very high-quality circuitry, mostly discrete, handbuilt equipment that I used to record some pretty good records, even though you’ve never heard them. One of my favorite pieces of outboard gear that we had in our rack was a DBX 900 rack. This was a 19″ wide power supply with spaces for nine 5 1/4″ x 1 1/2″ modules. Two other companies made similar systems- Valley Audio (we had a Valley rack as well, and we liked it better than the DBX) and API. Of the three, only the API Lunchbox has survived, and now there are hundreds of modules manufactured that can fit API racks, and Radial Engineering has just released their own version of a rack cage and power supply for 500 modules. Very nice, but very pricey.

My DBX900 rack, almost identical to the one we had at my studio 30 years ago. It's taken several years to put this together.

In contrast, a DBX 900 rack can be found used as low as $150 on eBay (though I’ve seen some crazy prices lately) and individual modules go for around $100-200 each. One other company besides DBX made modules for the 900 series, and that’s Aphex. These folks make some GREAT compressors, but as far as I know, only the  9721 Dominator and 9651 Expressor is available for the 900 rack, though there may have been others. Because of size constraints, the Dominator doesn’t have all the controls of the 19″ rackmount version, but it’s a very sweet compressor.

The DBX 904 noise gate module for the 900 series rack

Don’t overlook modules that need repairs, either. I was lucky enough to find some 900 units on eBay that were being sold as non-working, parts-or-repair-only. I took a gamble and got these for about $30 each. One was a 905 parametric EQ. This one turned out to be an easy fix… a disc capacitor had blown itself in two.  Thanks to DBX’s unselfish policy of publishing their schematics, I was able to figure out the value and replace it, and it works! Sometimes a blown part is a symptom of other problems, but so far, nothing else has smoked.

Next up was a DBX 903 compressor limiter. This one exhibited some general wierdness in terms of the signal, and the LED meter on the front was out. The 900 series all use the same meter driver board, which helps. I traced the signal at the meter board’s input, and confirmed that it was getting a signal, so I pulled the board and swapped it with another from a working unit… problem fixed. Now we know the problem is somewhere on the meter board. Examining with a microscope showed a resistor with a tiny burn mark around the middle. Replaced the resistor, and presto! The meter board works.

The business end of the 904. The board on top is the meter driver.

The last problem piece is a 904 noise gate, the donor for the working meter board used to correct the compressor. This one was a bit tougher to fix. Reinstalling the now-repaired meter board shows that the unit is detecting properly and gating a signal according to the settings, but there’s no output.  I poked around with a meter, and got about a .4v signal at the input. I started working my way back from the output, and found a non-polarized electrolytic capacitor that wasn’t passing any AC. So to test my theory, I removed C3– a 4.7uF non-polarized electrolytic cap– and replaced it with a pair of 10uF capacitors wired back-to-back, i.e., negative tied together, positive side out… and it works, passing a clean, gated signal!

After a proper 4.7uF NPO cap arrived from Digi-Key, it was soldered in and tested… that fixed it. While I was at it, I ordered enough Nichicon capacitors to re-cap the entire board… fortunately, DBX designed their circuit boards to accommodate either radial or axial-leaded caps, so either type will fit.

The problem cap. Replacing this fixed the unit. Fortunately, the problem wasn't under the big square metal case... that's a DBX discrete voltage-controlled amp (VCA), which is mostly transistors packed very tightly onto another circuit board. Repairing one of these would be really difficult, and replacement would be impossible.

That’s the other big advantage of this old stuff… it’s possible to fix it when it breaks. When parts fail on new gear… likely built overseas, by robots, using surface-mounted components… you’re pretty much done and replacing the entire board is the only option. Sometimes sourcing parts for vintage gear is difficult or sometimes impossible… germanium transistors and diodes come to mind. But vintage gear can almost always be rebuilt, and if the quality is there to begin with, it’s worth the effort.

The 904 fully recapped. The job wasn't that difficult or expensive, but you do need to be careful and take your time.