JBL A6000GTi, Service Manual, Page: 30 / 62

137159-1 Rev. 1 A6000GTi Service Manual

2003 Crown Audio, Inc.
Maintenance 4-11
HF: The amplifier senses high frequency oscillations with frequencies outside of the audio
bandwidth. The most likely cause for this fault is that the diodes D801 & D901 on the
output filter board are bad. If this does not correct the problem, replace the front-end
boards.
Low Energy: The audio section has lost the low voltage supply. Check the voltage on the
power supply at L1 and L4. You should have +/-24 volts, respectively. If the voltage is
wrong on the power supply, disconnect both cables from the power supply to the audio
section and recheck the voltages. The negative rail should be around
–50 volts when
disconnected from the amplifier. If the voltages are ok, the problem is on the audio section.
If not, refer to the power supply trouble-shooting guide. If the problem is on the audio
section, plug the ribbon cable back in from the power supply, but leave the 6-conductor
cable disconnected. Check for shorts on regulators on the main board under the heat sink.
If any are faulty, replace them and recheck the voltages. If the regulators are ok, remove
the front-end boards and recheck the voltage. If either front-end board is causing the drop
in voltage, replace the faulty boa
rd. If the problem still persists, check for IC’s for heating or
shorted pins.
VCC : Loss of VCC rail voltage. Check the voltage on the outer pins of the 6-conductor
cable while plugged into the audio section. It should be greater then 280 volts dc. Remove
the cable from the audio section and check the voltage again. If the voltage is still low,
verify that at least 12 volts is present on the power input to the amplifier. Measure the
voltage on the secondary of the power transformer; each should be at least 140 volts ac. If
these voltages are ok, check the rectifier for shorts or improper operation.
If the problem is not on the power supply, check the shorted rails on the audio section. The
most likely cause of shorted rails is a bad mosfet or diode in the switching stage of the
audio section.
DC/LF : The DC/LF fault will cause the amplifier to crowbar and shut off. The REM voltage
must be cycled to turn the unit back on. Check for shorts between the rails and the
outputs. If a short is present the most likely cause is a bad mosfet in the switching stage of
the audio section. Measure the gate to drain impedance as an indication of a bad part.
Replace the blown components and the gate drive module. Often, when a mosfet blows, it
damages the gate drive, and failure to replace the gate drive will result in more blown
mosfets. If there is no short present, check for the proper voltages on the regulators on
the pip and input boards. If either of those boards don’t have the proper differential voltage,
they will drive the amp to a DC output.
High Average Current:
This mode is most evident when the amplifier is shorted. The
channels are held off for a few seconds then turned back on. In this situation all the signal
lights on the display board flash, then the channels cycle off then on. The most likely
cause is a component on the output filter board shorted to chassis. This can be measured
as a short on the output channel. Disassemble to audio section and look for shorts under
the boards.