A VU meter measures the loudness of sound signals according to a decibel, or dB scale. The zero dB mark lies near the center of the gauge; a zero dB reading indicates moderately loud sound. Softer sounds indicate negative dB numbers and louder sounds will move the gauge into positive territory. They come in two main varieties: analog meters, with a physical needle that moves across a marked dial face, and LED or cold cathode fluorescent gauges that indicate loudness by lighting up different lengths of a horizontal or vertical bar. The bar indicator may have green LEDs indicating normal levels, yellow for loud levels and red for excessive levels.
An analog voice level meter may not be working because of a broken movement. The movement consists of a spring-loaded rotary mechanism driven by an electromagnet. Current flowing through the magnet twists the mechanism and moves the needle across the dial face. With no current, the spring keeps the needle positioned at the left side of the meter. A broken spring or foreign matter may keep the needle from turning. The dial face may have bent outward, causing the needle to stick at one point. The movement is too delicate to repair; replacing the meter is usually the best option.
Wires from the stereo mixer carry signals to the meter; if these break or corrode, the meter will always read zero. This generally happens to older equipment and mixers that have seen a great deal of handling and road use. If the meter itself is intact, a competent technician may be able to fix it by resoldering the broken connections inside the mixer.
Whether it is an analog or LED-type unit, the meter is not connected directly to the audio signal; a driver circuit takes the audio and provides the meter with the right amount of current. Problems with the driver will cause the meter to read incorrectly or not at all. Analog meters frequently use op-amp based drivers, and LED bar meters have their own driver ICs. If these ICs have failed, replacing them takes only a few minutes.
A voice level meter may be fine but appear to not work; the cause may be something unrelated to the meter itself. For example, if you have a battery-powered microphone and the battery is dead, the mixer and meter will receive no signal. Loose audio connectors and bad cabling will also result in a loss of signal. A problem with a mixer channel’s fader control or mute button will also prevent signals from reaching the meter. In these cases, a technician should inspect the mixer and all the equipment plugged into it.