Tektronix 494AP service manual O o o o a a o, Freqspan/Dv Max, FREQSPAN/DIV MAX Autoresoln on

Maintenance - 4g4A/4g4Ap Servlce Vol. I

Coarse Tune Range R1032

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3_Z.B EtiasI 5.4-18.Bias2

Mirer BiasTptoll

15-21Bias 3 R1026

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Flgure 5-27.1st LO Ddyer board adjustment and test point locatlons.

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Baseline Leveling (Video processor)

a.Connectthe test equipmentas shown in Figure 6-28.Set the ALC switch on the RF ptug-into the MTR position. Set the power Levef to approximately -10 dBm thsn set the Gainon the Sweepbscittatorfoi stableoperation(outputstopsoscillating).

b.S€t the SpectrumAnalyzercontrolsas follows:

FREOUENCY 0.00GHz

FREQSPAN/DV MAX

AUTORESOLN on

REFLEVEL -20 dBm

MINRF ATTEN OdB VERTCALDTSPLAY 2 dB/Dtv

PEAK/AVERAGE FuilyCounterctokwise

c.Set the SweepOscillatorto the AutomaticInter- nal Sweep(MarkerSweep),sweeptime to 100s, and se! lhg SweepOscillatorso it sweepsfrom 10 MHz to

1.8GHz.

d.Activate MAX HoLD, VIEW A, and VIEW B. Selecta TIME/DIVso there ar€ no breaksin the stored display(Figure6-29a).

e.ReactivateMAX HoLD and SAVEA. Trace and record the responseof Band1. Note the numberof divisionsfrom the baselineto the lowest point in the first5 divisionsof the disptay.

f.set the CENTERFREQUENCYto ptacethe dot markerdirectlyover the towest point within the last o divisionsof the display.

-9.Set the SweepOscillatorCW MarkercontrolOn, ancladjustit so a signalis at the cent€rof the crt.

h.AdjustR1O12(Band1 Stope)on the Videopro- cessorboard(Figure6€0) to set the top of the signalto the levelnotedin part e.

i.Repeat until the frequency responseis within

1.5dB of the mid-pointbetweenth€ h^/oextremes.

j.Resetthe SpectrumAnalyeercontrolsas follows:

FREQUENCYRANGE 5.4-18.0 GHz

FREQUENCY 10 GHz

FREQSPAN/DIV MAX

AUTORESOLN ON

REFLEVEL-10dBm

VERTTCALDTSPLAY 10 dB/DlV

TIME/DIV10 ms

The UNCALindicatorwill light.

k.AdjustmentresistorR3030 is set at the factory and usually does not require adjustment. Remove P3035and replaceit. lf the baselineremainsstraightor breaks up after the plug is replaced.compensationis required. Adjustmentprocedureis as follows:

(1)Activate wlDE vtDEo FTLTERand change TIME/DIVto 50 ms.

(2)Set the REF LEVELso the basetineis nearthe top of the graticule.Resetthe VERTICALDtspLAy to 2 clB/Dlv,and set the REF LEVELsuch that the displayis at mid-screen.

(3)Vary R3030counterclockwiseuntil the display breaksup towardsthe right side of the display(Fig- ure 6-31a).

(4)Vary R3030clockwise1/8th tum past the point where the display broke up. Store the displayin RegisterA.

(5)Alternatelyset Rl013 throughRl061 fullyclock- wise and fullycounterclockwise,so that everyother potentiometeris fully clockwise and the adiacent potentiometeris fullycounterclockwiseThe.display shouldnowlook likea triangularwaveform.

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