Tektronix 494AP service manual O a o o O o o, Axisand RF Interface DTAGRAM28

Theory of Operation- 494A/494ApServlce,Vol. 1

ing sid€ ar€ throughthe parallelcombinationof resis- tors R4049and R4048and capacitorC4057. The series connectionof resistor R4049 and variable capacitor C4057provideshigh-frequencyrespons€compensation. CapacitorC2A7 controlshigh-frequencyfeedback.

Input to the non-invertingside is through resistor R5029to the base of Q4025A. R4019and RS035set the dc level for the feedback loop to the base ol 040258. VariablecapacitorC5021providesadjustment to set transientgain. High-frequencyfeedbackis con- trolledby capacitorC3021.

Gain of each amplifiersectionis approximately20. (Horizontaldeflection sensitivity of the crt is approxi- matefy 21.3Vldiv per side.) Each section is singte- ended and incorporatesa gain- degenerateddual pNp transistorat the input side (for temperaturecompensa- tion)connectedas a differentialamplifier.For example,

Q40388 of the right deflectionamplifierdrives emitter follower Q4047.

Signalswith a low rate of changedrive the output transistorthroughR5037and p3033. As the rate of rise increases,the drop acrossR5037increasesand when it reaches0.6 V, eitherQ4035or Q4042are biasedon. Thesetransistorsprovidethe highcunentdrivefor the output transistors. when the signalrate of changeis low, Q1043drives the crt d€flectionplate and e1o4g provides bias current for the amplifier. As the rate of rise increases,C3039couplesthe signalto the baseof Q1049. Q1049 provides the positive drive to the deflectionplate,and 01043 providesthe negativedrive. Eachoutputtransistorcan providea 200V excursionin approximately1 ps.

The horizontalamplifiersoperatewith approximately 1 mA ol bias currentin the outputstage,as set by the current through resistor R3031,R1052,and R1049 at the base and emitterof Q1049. Cunentthroughresis- tor R3031also providesthe currentfor the input stage, 04038A/040388. Emitter follower e4042, operates at approximately2.5 mA. ResistorsR104Sand R1034.in the emittercircuit of Q1049and el043, degeneratethe output stage for fast steps. Currentfrom the -15V sourcethroughresistorR4033,setsthe output operat- ing level.FeedbackresistorR3045setsthis outputlevel at approximately142V.

Operation of the right-hand(inverting)section is basicallythe same as th€ teft-hand(non-inverting)sec- tion.

Vertical Section

VIDEOFILTEROUT,from the Videoprocessor,and VERTICALSIGNAL,from the Digitalstorage,are routed through switch lC U6055A, under control of the STORAGEOFF signal from the DigitatStorageboard.

7-48

Note that the vlDEo FILTERouT signal ls bufieredby lC U7065to preventa changein load transientsfrom aff€ctingth€ signallevel.A high on the STORAGEOFF line selectsthe bufferedVIDEOFILTEROUTsignal,and

alow selects the VERTICALSIGNAL. U6065inverts the selectedsignaland clampsit to ground. Both the VIDEO FILTEROUT ANdthE VERTICALS]GNALArE specifiedat 0.5 V/divwith 0 V for the baselineandposi- tive voltagesabovethe baseline.

The signal is re-invertedand offset by bufier U6073 so centerscreenrepresents0 V. BufferU6073supplies

asampleof this cent€red signalto the rear-panelVERT OUT connectorvia edge connectorpin 46. The output of U6073is also appliedthroughswitch U60558,when the R/O OFF line is high, to the verticalshapercircuit. WhenR/O OFF line is low, the VERTICALR/O signalis appliedto th€ shaper.

The verticalsectionshaper(R4062,R4065,R4067, R4069, R4064, and CR4063, CR4064, plus the preamplifierU20621operatesthe sameas the horizontal section. Q4078 limits positiveExcursionsto approxi- matelyone divisionabovethe top of the scr€ento pro- tect the outputstagesfrom beingoverdriven.

The verticaloutputstagesare similarto the horizon- tal stages, with the €xceptionof higher bias current. Current flow of approximately1 mA, through resistors R3089and R3098,producesapproximately5 mA in the output stag€s. To correct for the increasedcurrent in the dual input stage transistors,Q408il and Q4101, resistorsR5081and R5099are lower valuethan their counterparts R5041 and R5027 in the horizontal amplifier.

U6024comparesthe signallevel from the baseline clamp, U6065, with a referencel€vel set by divider R7032/R7034.This producesthe CLIP signal for the Z-Axisinterfac€circuits. Whenthe VIDEOFILTEROUT signal is rnore negative than the reference level (approximately1 division above baseline),it pulls the CLIP line low. R7021pullsthe CLIP line highif the sig- nal is more positivethanth€ referencelevel.

Z-AXISAND RF INTERFACE (DTAGRAM28)

The Z-Axisand RF Interfaceboard containsthe RF interfacecircuits,crt Z-axis drive circuits,power moni- tor circuits, and a timer that measures operational hours. This board providesbeam intensity(nominally from the front panel),baselineclipping.and unblanking logic for the signalsor readoutdata. Unblankinglogic comes from the Sweep board, the Crt Readout,the DeflectionAmplifiers,and the Digital Storage.The RF Interfacecircuits receivedata from the microcomputer that controlsthe RF Attenuation,transferswitch,and lF selection.A powerfail circuiton the boarddetectsany

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