JBL LSR28P, Техническое и сервисное руководство

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INITIAL SYSTEM TROUBLESHOOTING
The majority of problems in use will be with electro-mechanical components; DIP Switches
SW1, switches SW2 and SW3, potentiometers R1, R4 and R56. Before dis-assembling a faulty
LSR28P system check the action of these electro-mechanical elements as follows:
•
System Fails to Power Up
1. First check the fuse is intact. In addition to observing that the Green LED does not
illuminate, listen closely to the loudspeaker drivers at switch on. If a barely audible
thump occurs a couple of seconds after switching the power on and some low level noise
can be heard at these speakers the problem is not the power switching, check that the
Neutrik Combo connector is not damaged and try applying signal to each connector style
in turn (XLR and Jack). If no noises can be heard operate the power switch SW3 several
times with no power cord attached and then retry powering up. If operation is restored
replace the switch. Similarly exercise the voltage selector switch SW2 and replace if
indicated by the test.
2. If the fuse blows the fault is most likely in the main board. Refer to the guide “Amp
Assembly Troubleshooting LSR28P” .
3. If neither of the above checks leads to a fix, proceed to dis-assemble the input board
assembly and carry out appropriate checks below.
•
No output at either loudspeaker
Exercise trimmer R1 by turning from end to end a few times. Select a lower system gain
(-4dB or -8dB at DIP switches SW1B or SW1C). If either of these actions restores signal
replace trimmer R1.
•
No output at high frequency loudspeaker
Exercise trimmer R4 by turning from end to end a few times (remove the hole plug
nearer to the DIP switches to gain access to this trimmer). If this action restores signal
trimmer R4.
A) Initial Setup
Connect board under test to a +/- 15v power supply at connector P4 pin 3 to +15v, pin 4
to -15v and pin 2 ground. Connect a sinewave signal generator (or a function generator)
to the XLR input connector with output level set to +4dBu (3.5 volts peak to peak). Turn
generator off.