QSC Audio MX3000a, MX2000a, MX 1500a, MX 1000a owner manual Troubleshooting

TROUBLESHOOTING

PROBLEM: Channel will not come out of muting

• If reducing the gain control to ∞ attenuation does not release muting, the channel is defective or overheated (see “Overheating,” below).

•If reducing the gain releases the muting, raise the gain back up slowly while you watch the “SIG” and “CLIP” indicators; the problem may be an abnormal signal (with excessive ultrasonic energy, for example) that could otherwise damage your speak- ers.

PROBLEM: No sound

•Is the channel muted? (If the “PROT” indica- tor is lit, the channel is muted; see below.)

•Is the “SIG” LED lit or flashing? (If so, the speaker is open or blown, there is an open circuit in the speaker wiring, or there is an open circuit in the internal output wiring of the amplifier.)

•If the “SIG” indicator is dark, there is prob- ably not enough signal, or even none at all. Try turning up the front panel attenuators or boosting the signal level at the input.

•If the “SIG” indicator shows little or no activity but the “CLIP” LED is lit or flashing, there is probably a short circuit in the speaker wiring. It is also possible, but less likely, that the channel’s output relay is defective and will not open, thereby short-circuiting the chan- nel output and producing the same symp- toms.

PROBLEM: Channel goes into muting, with “PROT” LED on

•If the fan is running full speed, the channel probably suffers from severe overheating. Unless there is a blockage in the flow of cooling air, the channel should return to normal within a minute or so (see “Overheat- ing,” below).

•If the fan is not running at full speed, or the channel does not reset to normal after a cool-down period, the muting is probably because of a DC fault or other amplifier failure, especially if the “SIG” or “CLIP” indi- cators are lit even with the attenuator turned all the way down.

PROBLEM: Hum in the audio

•Because of its grounded-collector output transistor configuration, which maximizes thermal efficiency, the signal ground on an MXa amplifier cannot be lifted. The amplifier’s balanced inputs are meant to reject hum, but if hum remains a problem, check the tight- ness of the two mounting screws on the standard input panel; if they are loose, the panel itself might not be well grounded to the chassis. If you have any problems with these screws, contact the QSC Customer Service Department. In some cases, such as when the audio signal cables are routed near lighting dimmers that use triacs, you may need to use input isolation transformers be- cause of the extremely high noise field pro- duced by the dimmer circuitry. See the para- graphs on Open Input Architecture for fur- ther information on input transformers.

In addition, be sure to check wiring to assure that shields are properly terminated and no ground loops have been created.

PROBLEM: Overheating

•The thermal management system on an MXa Series amplifier features either a two- speed fan (MX 1000a, MX 1500a, and MX 2000a) or a variable-speed fan (MX 3000a) that modulates the cooling air flow over the heatsinks in response to the cooling needs. If the air flow is blocked, however, or if the amplifier is overdriven into very low imped- ance loads, the amplifier could overheat even though the fan is running at full speed.

•At approximately 55° C, the fan runs at high speed.

•At approximately 80˚ C, the thermal limiter on the MX 3000a will start to reduce gain, up to 15 dB as needed, to reduce the amount of heat produced.

•At approximately 85˚ C, the channel’s output relay will mute the output. The channel will remain muted until the temperature drops to a safe level. Even while the channel is muted, the “SIG” and “CLIP” indicators will function normally. If the “CLIP” indicator is flashing or continuously lit, reduce the gain to hasten the cool-down and prevent repeated ther- mal shutdowns.

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