JBL LSR28P, Техническое и сервисное руководство

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SERVICE PROTOCOLS
The maintenance of an electronic system can be divided into many sequential processes that
have to be explicitly followed in order to achieve a final result. Partial repair or incomplete
repair will not suffice for the professional technician intent on complete customer satisfaction
and will not adequately restore proper operation to a malfuntioning unit.
Today’s audio requirements necessitate that the electronics engineer design amplifier
circuitry to operate within very tight standards or tolerances. Extended high and low frequency
circuit attributes demand that the components that are used within the circuitry be of excellent
quality and are able operate within the designated parameters to produce the desired results. The
Q Point or operating point defines the class of operation of the specific amplifier. Both the low
and high frequency amplifiers operate in the range of class AB operation. This not only is
efficient, but also avails the designer to achieve those desired results. In addition, electronic
equipment will only properly operate if the circuit is operating at 100% efficiency. In order to
achieve the efficiency the service technician needs to apply a sort of fault analysis, approaching
each breakdown from the standpoint of cause and effect analysis. Since electronic
troubleshooting is synonymous with fault analysis, as applied to the parameters of the circuit
under observation, it must be methodically continued until successful completion or repair.
The skill that is necessary to successfully troubleshooting electronics to the specific faulty
component requires that the technician understand how the equipment correctly operates.
Obtaining an accurate, serial number-coordinated schematic diagram is also an essential tool.
Then, by determining what is operating correctly and what is not, the technician can eliminate
the electronic circuitry not associated with the fault and concentrate on the problem area. Testing
for typical voltages and oscilloscope waveforms at various test points and, as a last resort,
initiating methodical signal tracing can usually isolate the fault to a particular stage. Further
diagnoses within that stage will usually reveal the root cause of the failure. Most often, the root
cause will reduce to a defective part, faulty trace or intermittent connection. Sometimes there
are several root causes that interact with one another causing erroneous measurements and,
subsequently, inaccurate diagnoses of the malfunction. In these instances, the technician needs
patience and persistence to accurately diagnose and then repair the problem to restore proper
operation.
Many times a faulty or intermittent connection at the leg of a component that is supposedly
attached to a specific trace will appear to be okay when in reality it is not. These "cold solder
joints" occur because of several different reasons. Either the joint did not get hot enough for the
solder to bind the trace and the component leg together. Sometimes, metal fatigue can break the
connection. Oxidation or contamination at the solder joints can occur especially if the unit is
operated within a very humid atmosphere. In any case, the technician needs to be aware that
physics is constantly at work and can cause variety of acute symptoms to occur.
Unfortunately, several scenarios are not as straightforward. If the observed behavior is an
oscillation, a very high total harmonic distortion or simply a DC voltage seen on the outputs,
these abnormalities will be seen throughout the amplifier. Additionally, a single part can cause
other parts to fail and, consequently, will require all defective components be replaced
concurrently to restore proper operation. This is especially true of modern amplifier design that
includes the LSR Series. In order to attain the phase and distortion-less sound, the discreet
topology of the main amplifier is capacitor-less. This design was intended and does enable the