JBL MPC - Manual (Page 20)

19
Troubleshooting
PROBLEM: Channel will not come out of muting.
• Reduce the gain to ∞ attenuation. If this releases the muting, raise the gain back up slowly
while you watch the SIGNAL and CLIP indicators. The problem may be an abnormal signal
(possibly with excessive ultrasonic energy, for example) that could otherwise damage your
speakers.
• If reducing the gain control to ∞ attenuation does not release muting, the channel is
defective or overheated (see “overheating” section).
PROBLEM: No Sound (MPS 300, 300T, 600 & 600T only)
• Is the PROTECT LED lit? If so, the channel may be muted (see below).
• Is the SIGNAL LED lit or flashing? If so, the speaker may be open or blown, there may be
an open circuit in the speaker wiring, or there may be an open circuit in the internal output
wiring of the amplifier.
• If the SIGNAL indicator is dark, there may not be enough signal, or none at all. Try turning
up the rear panel attenuators or boosting the signal level of the device that is sending the
signal to the amplifier.
• If the SIGNAL indicator shows little or no activity but the CLIP LED is lit or flashing,
there is probably a short circuit in the speaker wiring, especially if the PROTECT indicator
starts flashing. It is also possible, but less likely, that the channel’s output relay is defective and
will not open, thereby short-circuiting the channel output and producing the same symptoms.
PROBLEM: Hum in the audio
• Because of its grounded-collector output transistor configuration, which maximizes thermal
efficiency, the signal ground on an MPC amplifier cannot be lifted. The amplifier’s balanced
inputs are meant to reject hum, but if hum remains a problem, check the tightness of the two
mounting screws on the input panel (except MPC200 & 200T). If they are loose, the panel
itself might not be well grounded to the chassis. In some cases, such as when the audio signal
cables are routed near lighting dimmers that use triacs, you may need to use external input
transformers because of the extremely high noise field produced by the dimmer circuitry.
PROBLEM: Channel goes into muting, with PROTECT LED on (MPS300, 300T, 600
& 600T only)
• 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 “overheating” section).
• 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 caused by a DC fault or other amplifier failure (especially
if the SIGNAL or CLIP indicators are lit even with the attenuator turned all the way down).
PROBLEM: Overheating
• The thermal management system features a two-speed fan 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 impedance loads, the amplifier could overheat
even though the fan is running at full speed.
• MPC200 & 200T: At 85°C both channels mute. The POWER indicator remains lit, although
the CLIP indicator is dark. The amplifier remains this way until the temperature drops to a
safe level.
• MPC300, 300T, 600 & 600T: At 85°C, the channel’s output relay mutes the output. The
channel remains muted until the temperature drops to a safe level. Even while the channel is
muted, the SIGNAL and CLIP indicators function normally. If the CLIP indicator is flashing
or continuously lit, reduce the gain to hasten the cool-down and prevent repeated thermal
shutdowns.