Tektronix 494AP service manual Product see llst Dale2-22-88, Chg. Ref M63911

Product: see llst Dale:2-22-88 Chg. Ref: M63911

Thestandingcunenton thehorizontalamplifierMosletsisestablishedby a resistivedividerR4046,R3045, andR5038aslollows.Thebaseof Q5040is set3.5voltsnegativewilh respectto the+300voltsupplyThe. voltagedropacross85045isthen3 voltsandthe resultingemittercurrenlis 2.0ma.Currentfromthe-15VsourcethroughresistorR2041setsthe outputlevel.FeedbackresistorR2049setsthis outpullevelat approximalely142volts.DiodesCR2040,CR3040,andVR4030providetransientvoltageprotectionduring turnonandunderfaultconditionCapacitor.C2031withresistorR2035shapethephaseresponseof U2030.

Operationot theleftdrive(non-inverting)sectionis basicallythesameasthe rightdrive(inverting)section.

VerticalSection

VIDEOFILTEROUTfromtheVideoProcessor,andVERTCALSGNALfromlheDigilalStoragearerouled throughswitchU60554,undercontroloftheSTORAGEOFFsignalfromlhe DigitalStorageboard.Notethat thevideofilteroutsignalisbutferedby U7065to preventa changeinloadtransientsfromaftectingthesignal levelA. highontheSTORAGEOFFlineselectsthebufferedVIDEOFILTEROUTSIGNAL,anda lowselects theVERTICALSIGNAL.U6065invertstheselectedsignalandclampsitto ground.BoththeVIDEOFILTER OUTandtheVERTlCALSlGNALarespeciliedat0.SVtdivwith0Vforthebaselineandpositivevohagesabove thebaseline.

Thesignalis re-invertedandolfselbybuflerU6073socenlerscreenrepresents0V.BufferU6073supplies

asampleofthiscenteredsignallotherear-panelVERTOUTconnectorviapins1 and2 ol P7075andedge connectorpin46.Theoutputof u6073isalsoappliedthroughswitchu60558,whentheR/OOFFlineishigh, to theverticalshapercircuit.WhenR/O lineis low,theVERTICALR/Osignalis appliedto the shaper.

The VerticalsectionshaperR4062,R4065,R4077,R4069,R4063and CR4063,CR4064,plus the preamplifierV2062,operateslhe sameas the horizontalsection.04078 limitsposiliveexcursionsto approximatelyonedivisionabovethetopofthecrtscreento protecttheoutputstagesfrombeingoverdriven.

Theverticaloutputstagesaresimilarto the horizontalstages,withthe exceptionof highbiascurrentThe. outputstageproducesapproximatelySmA.Tocorrectforthisincreasedcurrent,resislorsR3108andR2086 arelowerinvaluethantheircounterpartsR5032andR5045in thehorizonlalseclion.

U6024compareslhesignallevelfromthebaselineclamp,U6065,wilha relerencelevelseibydividerR6020 andR6024This.producesthecLlP signallorthez-Axls interlacecircuit.when theVIDEOFILTEROUT

signalismorenegativethanthereferencelevel(approximately1 divisionabovebaseline),it pullstheCLIP linelow.R7035pullsthe CLIPLINEhighif the signalis morepositivethanlhe referencelevel.

VariableresistorsR5020and R5030can be usedto adjustthe relaliveverticalpositionof the character readoutsandependentlyof the vedicalsignalgain adjustmentRl066. 85020 is adjustedlor vertical displacementcharactersWilh.theadvanceol the reducedgraticulecrt,R5020andR5030canbe adjusted to positionthe readoutcharaClersoutsideol the graticulearea.

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