Audiomaster 1200 Powermixer - Shure

SYMPTOMINPUT OVERLOAD LED flashingNOTE: Occasional flashing is acceptableLoud clicks when certain microphones or cables are usedNo MONITOR output (program output normal)Sound quality poor (weak or thin)PROBABLE CAUSE AND CORRECTION1. Adjust INPUT ATTENUATION counterclockwise to reducechannel input level.2. Reduce input signal level at source.1. Make sure PHANTOM 24V switch is not on (when not needed).2. Make sure unbalanced low-impedance microphone or cableis not used when PHANTOM switch is on.3. Check for defective microphone cables.1. Make sure channel MONITOR and MONITOR MASTERcontrols are turned up.2. Make sure controls on external equipment connected toMonitor output jack are turned up.3. Check for defective cable from MONITOR output jack.1. Check for excessive treble boost or bass cut on equalizationcontrols.2. Check PROGRAM and MONITOR output jacks for signalquality (if acceptable, problem is in power amplifier section).3. Check for defective cables.4. Check impedance match (high- or low-impedance mics)at input.SERVICE INSTRUCTIONSEXTERNAL PARTSThe knobs and feet can be replaced without disassemblingthe 1200. All knobs are of the pull-off type, and are color-codedby function. The Equalization knobs are dual concentric types;all other knobs are single.DIAGNOSISA simple method of localizing problems without opening the1200 is as follows. Turn off the Power switch. Do not connect aspeaker or headphone load. Set all controls of the channelunder test only to full clockwise (other channel volume andmonitor controls full counterclockwise), except: Reverb Returnfull counterclockwise, and Eq controls centered. Turn the uniton and apply a 0.5 mV, 1 kHz test signal across pins 2 and 3 ofone of the Bal Lo Z input connectors. Normal voltmeter readingstaken at each output connector are given in Table 1. Similarly, a5 mV signal inserted in a HI Z input, 50 mV in an AUX input, or891 mV in the Power Amp and Monitor Inputs will yield similaroutput voltages. This method is useful for saving time in localizingproblem areas. Internal servicing should be performed onlyby qualified service personnel.TABLE 1. NOMINAL TEST VOLTAGESOUTPUTSINPUT Input Spkr Prgm Mix Mon Phones TapeLO z 0.5 mV 28 V 890 mV 1.78 V 6.31 V 178 mVHI Z 5 mV 22 V 708 mV 1.41 V 5.01 V 141 mVAUX 50 mV 28 V 890 mV 1.78 V 6.31 V 178 mVPA 891 mV 25 V --- --- 5.62 V ---WARNINGVoltages in this equipment are hazardous to life. Referservicing to qualified service personnel.EXTERNAL FUSE REPLACEMENTTo replace line fuse F1 (with no apparent problems in thePowermixer), disconnect the line cord from the ac source andremove the rear panel fuseholder cap by pushing it in andturning it counterclockwise (either a screwdriver or fingertipmay be used.) Replace the defective fuse only with a 5A 250VNormal Blow type.CAUTIONIf trouble symptoms (overheating, erratic operation, etc.)were apparent before the fuse blew, or if the replacementfuse blows, a qualified service person should troubleshootthe Powermixer carefully to find the source of theproblem. Do not continue to replace fuses; have thetrouble corrected.SERVICE ACCESSDisconnect the 1200 from its power source. To open the unitfor servicing, remove ten screws and lockwashers from the topand five from each side of the cover. The cover can then belifted off exposing the internal parts (see Figure 1).3