Tektronix 494AP service manual O o A o O o o o O o a o, HTGH,VOLTAGESUPPLYDAGRAM29

Theory of Operation- 4g4[l4g4Ap Servtce,Vot. 1

TransistorsQ1017 and Q1015 provide curr€nt for the trace rotation coil. Trace Rotation adjustment Rl021 sets the currentso the displayedtraceis aligned with the graticule.

Power-Fail Detector

This circuitdetectsan instrumentpowerfailureand transmitsth€ informationto the processorand Memory boards. The LINE TRIGGERsignat from the powei Supplyboardthroughedgeconnectorpin 60 is supplied to Q2011. Q2011buffersthe signatand apptieslt to the anputof retriggerabteon€-shot U20g4A. U2034A per- forms as a missing-pulse detector to generate a power-failsignalthroughO30l l to notify the processor and Memoryboards if more than two 60 Hz cycles are dropp€d. To avoidan undefinedstate,the outputfrom U2034Ais latchedtow by U2051. Undernormatoperat- ing conditions,the POWER-FAILsignalfrom e3011 is high.

Power-Supply Monitor

This circuitdetectsif one or more of the instrument power supplieshavefailed. Eachvoltagesupplyin the instrumentis fed into thick film resistornetworkRgOSl, which balancesth€ currents to provide a null output (approximately1 Vdc). Any line changeof more than *,25ohdrives the input to window comparatorU3OSI beyondits *200 mVthresholdandgeneratesa low out- put. 02059 and Q2067 drive the duat tight emitting diode DS1062to provide visual indicationof power- supplystatus(grEenindicatesnormaloperationand red indicatesa faultcondition).The outputof U3051is also fed to tri-state bufferU3052. After instrumentpower up or if a failureis detected,the microprocessorwill poll address CF to determinepower-supplystatus over the databus.

Options Switch

Switch S1010 works with switch S1050 on the Memoryboard(A54)to configurethe firmwarefor use in variousinstrumentsand options. Settingsare notedon the schematicdiagram. U3052placesthe switch data on the instrumentdata bus at pow€r-upas described abovefor th€ Power-SupplyMonitor.

Timer

An electromechanicaltimer,Ml019,is calibratedfor

aduration of 5000 op€rating hours. The current throughR1015andthe timercausesthe copperbandto progressalongthe scale.

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HTGH,VOLTAGESUPPLY(DAGRAM29)

The High'VoltageSupplyfurnlshesthe -3860V crt bias and 6.3 Vac filam€nt voltage to the crt cathode, and providesdc restorationfor the Z-AXISDRIVEsig- nal. Th€ supplyconsistsof four maincircuits:

1.The high-voltageoscillatorcircuit producesthe crt filament voltage and the 2@ vac that is stepped up and appliedto the voltagedoublercircuit.

2.The voltage doubler circuit rectifies and filters the high voltagefor applicationto the crt cathode.

3.The high-voltageregulatorcircuit samplesthe high voltage and regulates the op€ration of the high-voltageoscillator.

4.The Z-Axis clipper and rectifiercircuits couple the z-Axls DRIVEsignalto the crt controlgrid.

High-Voltage Oscillator

This circuit consists of transistor 01073, transformerT2065. and associatedcomponents.The approximately200Vac, oscillator output is coupled acrossT2065,where it is steppedup for applicationto the voltagedoubler,and steppeddown for application to the crt filament.

Voltage Doubler

The voltage doublerconsistsof CR4041,CR4035, C4027,C5021, C4024,R3038,and R1039. The output of the doubler is taken ofi the anode of CR4035and appliedto the crt cathodethroughthe filterconsistingof R3038,R1039, and C4024. Referencevoltagefor the regulatoris also taken off the end of R1039. R1039 keepsthe filamentat the samepotentialas the cathode.

High-Voltage Regulator

This circuit consists of amplifier U4083 and sur- rounding components. The high voltage is applied througha voltagedividerthat consistsof R1017Band R1017C. This voltage divider is connectedthrough R1042to +15 V. The sampleof the high voltageat pin

Uis appliedthroughR4075to the input of comparator U4083. The correctionsignal,in the form of dc drive,is appliedas bias to Ql073 to set the oscillatorcurrent.

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