Tektronix 494AP O o o o a o O o o o o o o, 103, Theory of Operation- 4g4A/494ApServlce,Vol

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Theory of Operation- 4g4A/494ApServlce,Vol. 1

POWERSUppLy(Diagram10)

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The Main power Supplyfurnishesall the regulated
voltagesfor the spectrum
voltage		analyzer,exc€pt the crt high-
	The high-efficiency
:uppty.		desiin of the Main
Power Supply reduces		total weight ind conserves
en_ergy.

The power supplycircuitslre dividedinto th€ primarycircuits and the iecondary and fan drive cir- cuits.

primaryCircuits (Diagram46)

. .ft" power supplyprimarycircuitsconsistof the fol_ rowing:the rine input circuit,wnicn rectifiesand firters the incomingline voltage;andthe inverter,whichdrives the primaryof the powertransformer.

Line Input Circuits

_^ Poweris appliedthroughtinefilterFLg0l, lineFuse F301, and through FLgb2 (for additional normal

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mode/commonmode EMt fittering)to POWERswitch 5300. The power is then sentthro-ughlines€lectorcon- nector J1091. The line filter plevents power-lineinterferencefrom enteringthe powersupply,and it also preventsinternally-generated

signalsfrom out

ThermistorsRT209gand RT2097limitcurrentsurge at turn on. After the instrumentwarms up, the current demanddrops. The increasein temperaturedecreases the resistancevalue of the thermistorsso they have minimumaffecton the circuit.

ThermalcutoutswitchS2109opensif thEinteriorof the instrurnentreaches109"Cto preventoverheatingin casethe coolingfan fails.

E1094 and E2095 are surge voltage protectors. When the line selectorswitch is in the 115V position, only E1094is connectedacrossthe lineinput. it a peak voltage surge in €xcess of 230 V occuis across the input,or if the instrumentis accidentallyconnectedto a 230 V source, E1094 will break down and demand enoughcurrentto openthe line fuse. Whenthe instru- ment is operatedwith the line selectorat 230v, E1094 and E2095operatein seriesto protectthe inputagainst line surgesof approximately460 V peak.

,The voltage for the line trigger is taken across CR3096. This 48 Hz to 440 Hz vottagedrivesoptical isolatorU5043. The pulsating5 V outputis ac coupled, then sent both to the Sw€ep circuit to provideinstru, menttriggeringat ths linefrequenciosandto th€ Z-Axisboardfor the Power-FailDetectorcircuit.

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the powercord.

radiating

Inverter Circuit

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Line selectorswitch S302allowsinstrumentopera_ tion trom €ither a lSV nominalor 230V nominalline voltagesourcg. WithS3O2is in the 115V position,pins 1 and2 of p1091are connectedto the inputpower,and rectifi€rsCR3096 and.CR4094operate'inionjunction with energystoragefilter capacitorsC6101and C6111 as a full-wavedoubleqthus,the voltageacrossthe two

is the peak_to_peak

9lqacllol:vatueoJthe linevottage. WithS302in the 290V position,pins 2 and 3 of p1091 are connectedto the inputpowerandCR3096,CR4O95, CR3098,and CR4094operateas a bridgerectifier. As a-result,the output voltage applieclto the inverteris aboutthe samefor 1lS V or 23b-Voperation.

BecauseG6011and C610.tdischargevery slowly,hazardouspotentialsexist wiihintne power supply for severalminutesafter the POWERswitch is turned off. A relaxation oscillatorformed by C51lg, R5l11, and DS5112, indicatesthe presenceof voltages in the circuit until the potentialacrossihe filtorcapacitorsis belowg0 V.

The inverter consists of a multivibratorthat pro_ duces a rectangularshaped signal to drive the ramp generatorand th€ inverterlogic circuits. Therampgen- erator produces a low-level sawtooth ramp that is applied to the primary regulatorcircuit. The inverter logic circuitscontrolthe duty cycleof the inverterdriver and the inverter output stage. The primaryregulator circuit comparesthe +l 7 V supplyoutputwith a refer- ence voltag€,then gates the inverterlogic circuitsoff and on to control the inverter duty cycle and the effective primary voltage. The inverter driver stage amplifiesthe signal from the inverterlogie circuit and drives the output stage, The output stage consistsof two power switchingtransistorsthat drive the primary

of main power transformerT4021. The primaryovei- currentsenseand soft start circuitsadd protection.

Multivibrator. U6059,a low-powerS55tirner,is a multivibratorthat operatesat approximately66 kHz and 90-7-duty"- cycle. Oscillatorfrequencyis adjustedby R6061. The rectangutar-shapedoutputsignatis appliei throughR6052to the primaryof T6044in the rampgen- eratorand atsodirecttyto u6053,u6063A,u60638,and u6069.

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