Tektronix 494AP service manual HhFr $ f, Malntenance- 494A/494ApServlce Vol, F t

Malntenance- 494A/494ApServlce Vol. 1

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(7)ActivateView A, View B, and MAXHOLD. Note the responseas th€ oscillatorsw€eps.

(8)Activate SAVE A and deactivate MAX HOLD, then selectCW Markeron the SweepOscillatorand r_noyethe signal to the upper end of band S (19_ 21 GHz).

(9)Msualizean imaginarytinethroughthe midpoint of th€ SAVE A display and select a point on the saved display (betweent9 and 20 GHz)that inter_ sectsthe imaginaryline.

(10)Switch VIEW A on and off as requiredwhite rnovingthe CW Marker to the s€lectedpoint, then switch MEW A ofi.

(11)Adjust R1070 until the CW Markeris at the 6 divisionlevelor maximum,whichEveroccursfirst.

(12)DeactivateSAVEA and repeatstep I parts 6 through11 untilthe best overallflatnessis achieved.

10 MHr Reference Oscillator Accuracv (Agingrate is 1 x 10-e)

The 10 MHz ReferenceOscillatoraccuracyis not a performancerequirement;however,it must be checked so th€ centerfrequencyaccuracycan be verified. Since the Calibratoris tocked to the 10 MHz Osciilatorthis procedureverifiesaccuracyby countingthe frequency of the calibratorsignal.

a.connect the cAL ouT signal to the frequency counter. (counters with a frequency range above 20i) MHz may requirea 150 MHz low passfilterto €nsurea stabletriggeron the 100 MHz CAL OUTsignat).

The Tektronix DC 510 must be modifiedto accept an external oscillator reference. Refer to the TM500ffM5000 Series Rear Interface Data Book, Part No. 070-2088-04 for modificationinstructions.

b.Connectthe frequencystandardto the Extemal FrequencyStandardlnputof the frequencycounter.

c.Removethe protectivescrewfrom the oscillator.

lf the 10 MHz CrystalOscillator(th€ instru- ment) has not been powered-up for an extended period, additionalwarm-up time (in excess of the recommendedwarm-up time tor making adjustments) may be necessary before a final adjustment is made. ln that case, several frequency checks must be made before the final adjustmentis made.

d.Adjustthe FrequencyAdj on the 10 MHz Crystal Oscillatorslowlywith a smallscrewdriveror adjustment tool until the frequency counter indicates 100MHz

*1 0 H z .

e.Replacethe protectiveserew in the oscillator, and disconnectthe count€rfrom the CAL OUTconnec- tor.

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6-48

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