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7
AMPLIFIER APPLICATIONS
AFTER INSTALLATION
The key to finding the problem in a misbehaving sound system is to
isolate parts of that system in a logical fashion to track down the fault.
Description of the Diagnostic system built into all MB QUART
amplifiers:
The diagnostic system will shut down the amplifier, until reset by
turning the head unit off, and back on. This state of affairs will be
indicated by the front panel PROTECT LED lighting up under the fol-
lowing conditions:
1 - A sort circuit on the loudspeaker leads.
2 - An internal amplifier fault that causes a DC offset on the
loudspeaker output.
Should the amplifier go into diagnostic mode, simply disconnect all
RCA and speaker leads, while k12 volt, power ground and
remote leads connected. Now turn the amplifier back on, and if the
diagnostic LED lights, the amplifier has an internal fault.
If not, plug the RCA cables back, and reset the amplifier. If it goes into diag-
nostic now, the fault lies in the input, either with bad cables or source unit.
If the amplifier seems fine with RCA cables plugged in, connect the
speakers, one at a time, and if one of the speakers or its wiring is
faulty, it will activate the diagnostic system.
Amplifier heatsink overheating:
The amplifiers will shut down when the heatsink temperature reaches
80 degrees centigrade, and turn back on once the unit has cooled
down below that point.
Causes of overheating:
1 - Inadequate cooling - relocate or remount to provide better
natural airflow over the fins.
2 - Driving high power levels into low impedances - back off on the
volume control, and/or make sure you are not loading the ampli-
fier with less than the recommended loudspeaker impedance.
Low output power:
1 - Check that level controls have been set up properly.
2 - Make sure that the battery voltage, as measured at the amplifier’s
+12 volt and ground terminals, is 11 volts or more.
3 - Check all +12 volt and ground connections.
Fuses blowing:
1 - The use of loudspeaker impedances below the recommended mi-
nimums will draw more current - check.
2 - A short on the main +12 volt cable from the battery to the vehicle
chassis will cause the main fuse to blow.
3 - If an amplifier fuse blows continually, with only +12 volt, ground
and remote leads connected, the amplifier may be faulty.
System does not turn on:
1 - Check all fuses.
2 - Check all connections.
3 - Measure the +12 volt and remote turn on voltages at the am-
plifier terminals. If these are non existent or low, take voltage
measurements at fuse holders, distribution blocks, the head unit’s
+12 volt and remote leads to localize the problem.
Noise problems:
System noise can be divided into two categories,
hiss, and electrical interference.
En
gl
is
h
4-channel full range stereo /
1-channel mono
POWER
PROTECT
INPUT
CH1
CH3
CH6
CH2
CH4
L
CH5
R
CH 5 & 6
LP HP FULL
X-OVER
LO-MED-HI
LEVEL
CH 3 & 4
LP HP FULL
X-OVER
LO-MED-HI
LEVEL
CH 1 & 2
LP HP FULL
X-OVER
LO-MED-HI
LEVEL
FULL RANGE STEREO
LINE INPUTS FROM
RADIO / CD PLAYER
CHANNELS 1 - 4 CROSSOVER SWITCHES
IN “FULL” (FULL RANGE)
CHANNELS 5 - 6 CROSSOVER SWITCH
IN “LP” (LOW PASS)
GND REM BATT +12V
+ ch1 - + ch2 - - ch5 +
+ ch3 - + ch4 - + ch6 -
BRIDGED
SPEAKER OUTPUT
POWER
BRIDGED
BRIDGED
+
-
+
-
+
-
+
-
REMOTE TURN-ON
CHASSIS GROUND
B 12 VOLTS
FULL RANGE STEREO SPEAKERS
4-Ohms Or 2-Ohms
FULL RANGE STEREO SPEAKERS
4-OHM OR 2-OHM
40 Amp Fuse
DESIGNATED MONO (SUBWOOFER)
INPUTS FROM RADIO / CD PLAYER
FULL RANGE STEREO LINE INPUTS
FROM RADIO / CD PLAYER REQUIRE
A “Y-ADAPTOR” AS SHOWN
Y-ADAPTOR
MONO SUBWOOFER
8-Ohms or 4-Ohms
+
-
4 gauge
4 gauge
18 gauge
NOTE: This amp is capable of 4-Ohms and 2-Ohms wired in the
“stereo” mode & 4-Ohms when wired “mono bridged”. It is not
capable of 1-Ohm stereo or 2-Ohms mono bridged.
This diagram illustrates wiring 4 full range speakers wired stereo and a subwoofer wired mono bridged.
Please note position of X-Over switches.
Troubleshooting a system
Hiss, or white noise:
1 - High levels of white noise usually occurs when amplifier level
controls are turned up too high - readjust according to the
procedures in section ”Setting up systems after installation for
best performance”
2 - Another major problem that can cause excessive hiss, is a noisy
head unit - unplug the amplifier input RCA cables, and if the
hiss level reduces, the source unit is at fault.
Electrical interference:
The inside of an automobile is a very hostile electrical environment.
The multitude of electrical systems, such as the ignition system, al-
ternator, fuel pumps, air conditioners, to mention just a few, create
radiated electrical fields, as well as noise on the +12 volt supply and
ground. Remember to isolate the problem - first unplug amplifier
input RCA cables, if the noise is still present, check the speaker leads,
if not, plug the RCA’s back, and investigate the source driving the
amplifier, one component at a time.
A ticking or whine that changes with engine RPM:
1 - This problem could be caused by radiation pickup of RCA cables
too near to a fuel pump or a distributor, for instance, - relocate
cables.
2 - Check that the head unit ground is connected straight to the
vehicle chassis, and does not use factory wiring for ground.
3 - Try to supply the head unit with a clean +12 volt supply directly
from the b, instead of using a supply from the in dash
wiring/fusebox.
A constant whine:
This type of noise can be more difficult to pinpoint, but is usually
caused by some kind of instability, causing oscillations in the sys-
tem.
1 - Check all connections, especially for good grounds.
2 - Make sure that no speaker leads are shorting to exposed metal
on the vehicle chassis.
3 - RCA cables are notorious for their problematic nature, so check
that these are good, in particular the shield connections.
