ICEinLAVA's Domain

Of all the skills a live sound operator uses, system troubleshooting is the one skill that needs to be the sharpest. It is a touch ironic that no sound person ever wants to apply these skills, but when the need arises, the pressure fix the problem in time requires the utmost system knowledge and plenty of hands-on experience.

Whether it is a circuit board or a complete sound system, the principles of technical troubleshooting apply. The first thing to have a grasp on is the documentation of the system circuit layout. In Printed Circuit Boards (PCBs), a circuit schematic diagram or “photofact” sheet provides the ins and outs, the intersection points (nodes) and expected signal types and amplitudes at key nodes. While most experienced sound system technicians do not keep paper copy system schematics, most do commit to their brains a mental picture of the sound system and the flows of signal cabling.

Divide and Conquer

Divide and conquer is not a battle strategy for the Caesars of the Roman Empire, but a basic mental tool for quickly getting to the root of the problem. Given a chain of signal processing circuits with no clue as to which element corrupted the signal (no signal, noisy signal, distorted signal), the divide and conquer strategy says to start at the midpoint of the chain and check for proper signal conditions. If the signal is still corrupted, then take the first half of the chain and probe its new mid-point for signal goodness. If the midpoint signal was good, go to the new midpoint of the latter half of the chain and check for signal goodness. This process repeats until the cable or signal-processing block (mixer, processor, amplifier, etc.) is pinpointed. Figure 1 shows a generic sound system block diagram showing typical signal chains.

In sound systems, not everything has a single start and end point. Because mains and monitor speakers tend to split early, creating two chains, troubles that affect both systems quickly narrow the problem down to the sources (mics, DIs, etc.), cabling, and source phantom power supplies. By considering the main/monitor console(s) as the convenient “midpoint” for divide and conquer, getting good signal to the console and having the means to monitor it via metering and headphones is a crucial first step. Besides our human senses and headphones, good troubleshooting tools for your briefcase should be your dynamic talkback mic and some kind of audio signal generating device. For me, I keep a Neutrik Mini-rator handy, but there are other smaller, cheaper alternatives. Later in this missive, I will note some other handy items for troubleshooting and field repair.

Always a Plan “B”

As you zero in on identifying the suspect cable or piece of gear that is malfunctioning, you should have a “Plan B” in case the item cannot be put back into working order promptly. Plan B can be as simple as having a spare cable or amplifier for Just In Case (JIC) usage. When providing a system, my JIC collection will include spare cabling of every type used, a small mix console, and a small amp case with a stereo power amp plus an active crossover unit. In selecting JIC items, think of every possible item in the system and how you would manage if it failed. The response “go back and get another” should not be a possibility, nor should hoping some buddy will make a timely run for you with a replacement item.

The worst scenarios I can imagine in “B” or “C” rig sound systems would be the loss of a mains snake, or the mains console. If the snake is un-repairable in the field, one could abandon the FOH position and relocate to monitor world on the side of the stage. If the main console is out and no other console exists for monitors, then a spare console or mini-mixer should be part of the your JIC back in the gig rig. Back in my “Econoline” days, a dusty, knobless TAPCO six-channel mixer was located under the driver’s seat for such an occasion. No sound system provider keeps a complete spare system for JIC, but a few well thought-out spare items are worth it.

From my experience, here is a short list of common troubles and first-to-look-for fixes.

Forget to Patch Cable(s)—As obvious as it may seem, when it comes time to fire up the system and nothing comes out, usually it is because you skipped a step in patching the system. First, bang your head on something hard, and vow to stick with your method/routine of cabling-up next time. A variant of this is forgetting to plug things in to the power distribution system. You can claim that they are solar powered, but the stupidity is already showing.

Bad Speaker Cable Connection—The classic is the much-maligned TS phone plug/jack on wedges, and the tip-sleeve short that occurs when the plug is halfway out. Amplifiers show little signal output and possibly active limiter indications. Most of this goes away when using twist-lock connections.

Buzz or Hum in System—This is usually accompanied by my question, “Which of you musicians decided to plug into the wall, instead the stringer receptacles I provided?” Getting all the signal-critical cabling balanced and working on one power distribution system is the key. Having DI boxes with ground lift switches (like the Whirlwind IMP-2 for $40) is handy, but when all else fails, a couple of Sescom IL-19 XLR in-line isolation transformers is the last resort.

Effects No-Show—Especially with TRS connections, getting the sends and returns patched backwards at the mixer is a classic faux pas. On insert effects or dynamic processors that mute out when inserted, remember “ring” is “return,” “tip” is “send” on insert cables with rare exception.

Crunchy, Distorted Sound—Once you have double-checked for proper signal amplitudes, check for dirty or corroded cable connections. Make sure you have your DeoxIT or Pro-Gold cleaner handy. A couple of inserts/extracts may help rub the contacts clean as a temporary fix until the next break.

If you have the luxury of time or no other alternative, doing simple cable field repairs is a task to keep tools stashed for. Typically, a handy bag or box containing a pencil soldering iron, 60/40 rosin core solder, Suction-vise or Pana-vise, spare connectors, needle nose and diagonal pliers, shop rags, and WD-40 spray is a good start. The roadie’s secret weapon is WD-40, as it can clean some metals, and melt most tape residue off of cables, besides its automotive purposes. When mentioning tools, other handy special purpose tools for troubleshooting include a DeoxIT pen for connector contacts, an XLR-M phantom power detector (connector with a LED poking out of the boot), and a TRS phone plug or two with an intentionally shorted tip-ring link for getting past flaky TRS insert jacks.