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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.