Tektronix 494AP service manual O o a a o o o A o, Preselectordriver Diagram34

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the amplifier.This in turn, changesthe currentdriveto the oscillatorcoil.

Reference Supply

- .,Preamplifiere2052 plus amptifier lJ20S2and emitter followerQ2051, are the activecompon€ntsof th€ _1OV I-ej9fl"":.uppty. Bias for one sid'Eof 02052 is set by VR1055.The otherside.isset bythe _10 V Adj R1Og4. Any changein the supptyis amiltiRedby e20S2wnicn changesthe drive to the pass transistoieZOSt*nicf, compensatesfor the change. The diode network across the bas€-emitter junction limits the emitter currentto about 30 mA, protectingthe transistorfrom damage.

Mixer Bias Driver

The mixerbias drivercircuit,whichconsistsof quad IET switch U1016, amptifier Ul025A, and buffer O202VA1A28,plus associatedcircuitry,furnishesthe requiredbias current(up to 20 mA)to ttret st Mixercir- cuit. The bias voltagevariesfrom +1 V to _1 V for the internalmixer,and from +1 V to -2.25V for an external mixer. External mixer bias voltage range can be changedto -1to +2.25V by movingthe strap J2014 from+12V to the -12Vsuppty.

Mixer bias is selected,by the data out of U4017to
the quad FET switch U1016,and fed to the inverting
inputof u1025A. The outputof u1025A
			drivesthe base
9f^ 1_Rair	of complementarytransistors			e102g and
		-ti4ixer
Q2025whichprovidethe 1st		Biasvoltage. When
any of the D0 to D3 lines from u4017 go tow, the
respectiveswitch within U1016 closes anld connects
olg.9!11" Bias adjustmentpotentiometers
(the programmabte			or the output
pj.^U:_0-18		bias tine)to the input of
u1025A.

When the D3 output of U4017goes low, U1016 selectsthe ProgrammableBias line ls the 1st Mixer Bias source. This occurswhenExternalMixer modeis gelectgd. The programmableBias is set by the data loadedinto DAC U9022!y the microcomputeior by the front panel MANUAL pEAK controt. The MAhiUAL PEAKcontrolis connect€dto the inputof U201gwhen the DB4and DB7 inputsto U4OZ4go low at address7E and turnQ3019on. WhenMANUALpEAKis setected, the DACoutputis set for OV.

ProgrammableBias

When the microcomputersends address 7E to decoderU4034,pin 7 (output7) goeslow. At the end of deta:outputcycle, data is clockedinto either 1J4024or Ua032,dependingon which latchis enabledby DB6 or DB7. This data is then convertedto an analogcurrent

Theory of Operatlon- 4g4Al4g4ApServlce,Vol. 1

by U3022 which is thE current sourc€ for operational amplifierU2018. The resistancebetweenoutprrttermi- nals 16, 2, and 15 of U3022is th€ input resistancefor operationatamptifierU2019. R2022 is the feedback resistance.The outputof U20lg is a bias voltagethat is fed, via the ProgrammableBias line, to either the Presel€ctorDriver board where it is summedwith the drive voltage for the preselector; or, it is fed through U1016/U1025A,and e1028/e2025 to the 829Mfr2 Diplexer,then throughthe TransferSwitch on the RF deck to the ExternalMixer port.

PRESELECTORDRIVER (Diagram34)

The Preselector provides RF input selectivity between 7.7 and 21 GHz. This selectivity reducei spurious responsesover this frequencyrange. The PreselectorDriver supplies the drive iurrent to the Pr€selectorcoil (shown on Diagram 12) to tun6 the Preselector.lt also furnishesa voltagethat is propor_ t]o_B! t9 frequency change through the rear-panelACCESSORIESconnectorfor an externalpreselecior,if used. The circuit also operatesthe filter select relay that selects either the pr€selectoror Low-passFiltei.The majorcircuitsandtheirfunctionsare:

. The digital control circuit, which stores and decodesthe data from the microcomputerand con- trols the othercircuitswithinthe preselector

Driver. The digitatcontrotcircuitapptiesthe spAN voLTS signalto the oscillatorvoltageprocessorwhenFM coil spans are selected, selects the gain of the oscillatorvoltageprocessor,turns off the drivesig_ nal to lhe curr€ntdriverfor degausscyclesor when the preselectoris not in use, selectsthe lF offset voltages to be combined with the FINE TUNE VOLTSsignaf,adds noisefilteringat the driverout- put when th€ preselectoris not being swept,and controlsthe filterselectswitch.

rThe oscillatorvoltageprocessor,which attgnuates and offsets the input signal for applicationto the summingamplifi€r.

oThe lF offsetstage,which appliesan offsetvoltage to the summingamplifier..This offset is propor- tional to the 1st lF frequencyin use, includingthe effectsof fine tuningfrequencychangesof the 2nd LocalOscillator.

e The summing amplifier. which combines the effective oscillatorfrequency voltage and the lF Offsetvoltageto drive the trackingadjustmentcir- cu,ts.

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