7
TROUBLESHOOTING
PROBLEM: Channel will not come out of muting
•If reducing the gain control to ∞attenuation
does not release muting, the channel is
defectiveoroverheated(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
•Isthechannelmuted?(Ifthe“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.
•Ifthe“SIG”indicatorshowslittleornoactivity
but the “CLIP”LED is lit or flashing, there is
probablyashortcircuitinthespeakerwiring.
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
normalwithinaminuteorso(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-
catorsarelitevenwiththeattenuatorturned
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
MXaamplifiercannotbelifted.Theamplifier’s
balancedinputsaremeanttorejecthum,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
panelitselfmightnotbewellgroundedtothe
chassis.Ifyouhaveanyproblemswiththese
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-
ducedbythedimmercircuitry.Seethepara-
graphs on Open Input Architecture for fur-
ther information on input transformers.
Inaddition,besuretocheckwiringtoassure
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
ontheMX3000awillstarttoreducegain,up
to15dBasneeded,toreducetheamountof
heat produced.
•Atapproximately85˚C,thechannel’soutput
relay will mute the output. The channel will
remainmuteduntilthetemperaturedropsto
asafelevel.Evenwhilethechannelismuted,
the “SIG”and “CLIP”indicators will function
normally.Ifthe“CLIP”indicatorisflashingor
continuously lit, reduce the gain to hasten
the cool-down and prevent repeated ther-
mal shutdowns.
