QSC MX-1000a Bedienungsanleitung PDF herunterladen (Seite 7)

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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