Toshiba SB-420 Service Data

SB—420 |POWER ON-OFF SHOCK NOISE MUTING CIRCUITThiscircuitmutestheamplifierwhenthepowerswitchisturnedONandprotectsthespeakerfromclicknoise.CapacitorC1startscharging bythecurrentflowingthroughresistorR4immediatelyafterthepowerswitchisturnedON,andtransistorsTR3andTR4areturnedONbythecurrentpassingthroughresistorR3.Inputsignal(Click noise)isdividedbyinternalresistance ofR1andTR4,andofR2andTR3tolowertheoutputlevel.TheinternalresistanceoftransistorwhichisturnedONistooweakascomparedwithR1 andR2toobtainoutputsignal.WhenthevoltageatbothterminalsofC1comestoacertainlevelafterafewseconds,TR2isturnedONandthevoltageofnegativepowerwhichconsists ofD2,R7andC2isappliedtothe bases ofTR3 andTR4toturnOFFtheTR3andTR4,therebyincreasingtheinternalresistanceofTR3andTR4,andconsequentlyoutputsignalcan beobtainedwithoutreduction.Anyclicknoisewhichenters theinputsignalforthesesecondsdoesnot presentinoutputsignal.WhenpowerswitchisturnedOFF,thenegativepowerreducestoOVprecedingto+B,emitterpotentialofTR1isincreasedbypotentialofC1,TR1isturnedONandC1isdischargedpromptly.ThenTR2isturnedOFFand+BvoltageisappliedtoTR3andTR4throughTR3.As+Bvoltagereduces slowlyevenafterthepoweristurnedOFF,TR3andTR4canbekepttobeONforacertaintimeduringwhichmutingcircuitcanbe activatedandclicknoise thatoccurswhenpoweristurnedOFFcan be eliminated.Output+BFigure 4