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JL Audio MHD900 Manual

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20 | JL Audio - MHD900/5 Owner’s Manual
21
“My amplifier shuts off once in a while, usually at higher volumes”
Check your voltage source and grounding point. The power supply
of the MHD900/5 will operate with charging system voltages
down to 9V. Shutdown problems at higher volume levels
can occur when the charging system voltage drops below
9V. These dips can be of very short duration making them
extremely difficult to detect with a common DC voltmeter.
To ensure proper voltage, inspect all wiring and termination
points. It may also be necessary to upgrade the ground wire
connecting the battery to the vessel’s chassis and the power
wire connecting the alternator to the battery. Many vessels
employ small (10 AWG - 6 AWG) wire to ground the battery to
the vessel’s chassis and to connect the alternator to the battery.
To prevent voltage drops, these wires should be upgraded
to 4 AWG when installing amplifier systems with main fuse
ratings above 60A. Grounding problems are the leading
cause of misdiagnosed amplifier “failures.”
“My amplifier turns on, but there is no output”
Check the input signal using an AC voltmeter to measure the
voltage from the source unit while an appropriate test tone is
played through the source unit (disconnect the input cables
from the amplifier prior to this test). The frequency used
should be in the range that is to be amplified by the amplifier
(example: 50 Hz for a sub bass application or 1 kHz for a full
range / high-pass application). A steady, sufficient voltage
(between 200mV and 8.0-volts) should be present at the
output of the signal cables.
Check the output of the amplifier. Using the procedure explained
in the previous check item (after plugging the input cables
back into the amplifier) test for output at the speaker outputs
of the amplifier. Unless you enjoy test tones at high levels,
it is a good idea to remove the Speaker Connector Plugs
from the amplifier while doing this. Turn the volume up
approximately half way. 5V or more should be measured
at the speaker outputs. This output level can vary greatly
between amplifiers but it should not be in the millivolt
range with the source unit at half volume. If you are reading
sufficient voltage, check your speaker connections as
explained below.
Check to ensure that the speaker wires are making a good
connection with the metal inside the Speaker Connector
Plugs. The speaker wire connectors are designed to accept up
to 12 AWG wire. Make sure to strip the wire to allow for a
sufficient connection with the metal inside each terminal.
“How do I properly set the input sensitivity on my amplifier”
Please refer to Appendix C (pages 18, 19) to set the input sensitivity
for maximum, low-distortion output.
“My amplifier doesn’t turn on”
Check the fuse(s), not just visually, but with a continuity meter. It
is possible for a fuse to have poor internal connections that
cannot be found by visual inspection. It is best to take the
fuse out of the holder for testing. If no problem is found with
the fuse, inspect the fuse-holder.
Check the integrity of the connections made to each of the
“+12VDC”, “Ground”, and “Remote” terminals. Ensure
that no wire insulation is pinched by the terminal set screw
and that each connection is tight.
Check to make sure there is +12V at the “Remote” connection of the
amplifier. In some cases, the turn-on lead from the source unit
is insufficient to turn on multiple devices and the use of a relay
is required. To test for this problem, jump the “+12VDC” wire
to the “Remote” terminal to see if the amplifier turns on.
“I get a distorted / attenuated sound coming out of the speaker(s)”
Check the speaker wires for a possible short, either between the
positive and negative leads or between either speaker lead and
the vessel’s chassis ground. If a short is present, you
will experience distorted and/or attenuated output. The
“Status” LED will turn amber (yellow) in this situation. It may
be helpful to disconnect the speaker wires from the amplifier
and use a different set of wires connected to a test speaker.
Check the nominal load impedance to verify that each channel
of the amplifier is driving a load equal to or greater than
1.5 ohms in stereo mode (3 ohms bridged).
Check the input signal and input signal cables to make sure signal
is present at the “Amplifier Inputs” and the cables are not
pinched or loose. It may be helpful to try a different set of
cables and/or a different signal source to be sure.
Reset the amplifier by removing the power plug, then wait ten
seconds and reconnect the power plug.
“My amplifier’s output fluctuates when I tap on it or hit a bump”
Check the connections to the amplifier. Make sure that the
insulation for all wires has been stripped back far enough to
allow a good contact area inside the connector plugs.
Check the input connectors to ensure that they all are making good
contact with the input jacks on the amplifier.
APPENDIX D: TROUBLESHOOTING
… 11


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