JBL LSR28P Technical & Service Manual

Also see for LSR28P: Owner's manual

10SERVICE PROTOCOLSThe maintenance of an electronic system can be divided into many sequential processes thathave to be explicitly followed in order to achieve a final result. Partial repair or incompleterepair will not suffice for the professional technician intent on complete customer satisfactionand will not adequately restore proper operation to a malfuntioning unit.Today’s audio requirements necessitate that the electronics engineer design amplifiercircuitry to operate within very tight standards or tolerances. Extended high and low frequencycircuit attributes demand that the components that are used within the circuitry be of excellentquality and are able operate within the designated parameters to produce the desired results. TheQ Point or operating point defines the class of operation of the specific amplifier. Both the lowand high frequency amplifiers operate in the range of class AB operation. This not only isefficient, but also avails the designer to achieve those desired results. In addition, electronicequipment will only properly operate if the circuit is operating at 100% efficiency. In order toachieve the efficiency the service technician needs to apply a sort of fault analysis, approachingeach breakdown from the standpoint of cause and effect analysis. Since electronictroubleshooting is synonymous with fault analysis, as applied to the parameters of the circuitunder observation, it must be methodically continued until successful completion or repair.The skill that is necessary to successfully troubleshooting electronics to the specific faultycomponent 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 eliminatethe electronic circuitry not associated with the fault and concentrate on the problem area. Testingfor 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. Furtherdiagnoses within that stage will usually reveal the root cause of the failure. Most often, the rootcause will reduce to a defective part, faulty trace or intermittent connection. Sometimes thereare several root causes that interact with one another causing erroneous measurements and,subsequently, inaccurate diagnoses of the malfunction. In these instances, the technician needspatience and persistence to accurately diagnose and then repair the problem to restore properoperation.Many times a faulty or intermittent connection at the leg of a component that is supposedlyattached to a specific trace will appear to be okay when in reality it is not. These "cold solderjoints" occur because of several different reasons. Either the joint did not get hot enough for thesolder to bind the trace and the component leg together. Sometimes, metal fatigue can break theconnection. Oxidation or contamination at the solder joints can occur especially if the unit isoperated within a very humid atmosphere. In any case, the technician needs to be aware thatphysics 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 anoscillation, 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 causeother parts to fail and, consequently, will require all defective components be replacedconcurrently to restore proper operation. This is especially true of modern amplifier design thatincludes the LSR Series. In order to attain the phase and distortion-less sound, the discreettopology of the main amplifier is capacitor-less. This design was intended and does enable the