JBL MPA275 - Manual (Page 20)

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Channel will not come out of muting:
A. If reducing Gain to zero does not release muting, the channel is defective or
overheated (see below).
B. If reducing Gain releases the muting, advance Gain slowly while watching the
LEVEL displays (in case there is an abnormal signal which could blow the
speakers).
No sound is heard:
A. Is the channel in muting? (PROTECT is bright, see below).
B. If the LEVEL displays are active, (1) the speaker is open (blown), (2) there is an
open circuit in the speaker wiring, or (3) there is an open circuit in the internal
output wiring of the amplifier. (Consult a JBL dealer for service.)
C. If there is no LEVEL (SIGNAL PRESENT, -20dB, -30dB) or CLIP displays, (1)
there is no input signal, or (2) the ribbon cable to the input module may not be
seated well.
D. If the CLIP display is bright or peaking with little or no LEVEL display, this
indicates shorted speaker line (especially if PROTECT starts flashing).
Channel goes into muting (temp-protect LED is on):
A. If flashing of the PROTECT display is visible, amplifier has muted due to
extreme overheating. Fan should be running at full speed, and unless ventila-
tion is blocked, operation should resume within one minute.
B. The amplifier will mute in response to extreme high frequency overdrive, and
output will not be restored until the frequency or level is lowered. Try turning
down the Gain control to release muting, and determine the source of abnormal
frequencies.
C. Muting which does not respond to either condition suggests DC shutdown or
other amplifier fault.
Hum Problems:
A. Ground lift of the signal is not available in these amplifiers due to the design
which utilizes grounded-collector transistor mounting to improve thermal effi-
ciency. Low-emission AC transformers are used, and balanced inputs afford
hum rejection. If hum persists despite the balanced inputs, check the tightness
of the rear panel screws which hold the input panel in place (two outer screws,
and two screws holding barrier strip). If the input wiring is near SCR (silicon
controlled rectifier) dimmers, transformer-isolated inputs may be necessary
because of the exceptionally high noise field voltages generated by SCRs.
Troubleshooting