Tektronix 494AP service manual O o o o o A o o o o

AdiustmentProcedure- 494A/tg4Ap ServiceVol. 1

(3)Connect the counter to Tp2o11 on th€ con- trolled Oscillator board (Figure 5-26a) and atter- natelypress AUTO RESOLNand IDENTand check for a count reading of either 25.0g4gMHz or 25.0328MHz.

f.Chec-kOpera0onof Strobe Drlver

The *PhaseLock Synthesiz€r"test is still used for this test. lf aborted, press <Blue-SHtFT> PULSE STRETCHERto return to the Synthesizerroutine. Any step in the routinewillwork.

(1)Connectthe test osciltoscopeto Tp2015on the Strobe Driver board (Figure 5..26b)and eheck for a square wave response with a Time/Div setting of

.05 ps. Amplitudeshould be ==5V peak-to-peak.

(21 Connect the test osciiloscope to Tp2Og7and check for a sinusoidal waveform of approximatety 5 v p p .

(3)lf the amplitudeof the strob€ signatis low and noisy,chang€the valuesof selectcapacitorsCl016, C1018,C1032,and C1034for maximumamptitude and minimumnoiseat TP2087. The rangeof values for thesecapacitorsis 3.3 pF-27 pF.

(4)Connectth€ counter to TP2087and check for a count of either5.018868or 5.00642 MHz.

(5)Gonnectthe test oscilloscopeto JS04and check for5Vlogiclevels .

(6)Press<Blue-SHIFT>to abortthe test.

g.ErrorAmpllfier- This proceduresets loop gain whichis requiredwheneitherthe PhaseLockassembly, 1st LO,PhaseDetector,or ErrorAmplifieris replaced.

(1)Set SPAN/DIVto 200 kHz then press <Btue- SHIFT> PULSE STRETCHER.The DTAGNOSTTC

FUNCTIONSmenu will now be disptayed. Setect menuitem 3 (DIAGNOSTCADS)and setectt st LO PHASE LOCK (sub-menu item 0). phase tock shouldbs disabled.

(2)Connectthe test osciiloscopeto Tp203B(Figure

5-26b) and set the test osciltoscopeTime/Div to

20 ms. Check for a waveform with an amplitude that is approximately6 V peak-to-peak.

(3)PresslOdBiDlVto enablephaselock and note that the messageindicatesLOCKENABLED.Con- nect the test oscilloscopeto Tp 3081(Figure5-26b) and vary R3082from stop to stop and note that the beat note signalvaries in amplitude.Press <Blue- SHIFT>to returnto normalop€ration.

(4)Set th6 TIME/DIV to AUTO, FREOUENCY RANGEto 1.7-5.5 (Band 2), and SpAN/DV to 50 kHz.

5-28

(5)RemoveP3057(Figure5-26b). This turns on the strobe to the Phase Gate. Set Loop Gain R3082 fullycounterclockwiseMove.P2035to pins2 and3.

(6)Monitor TP3081 with the test oscllloscope. Triggerthe test oscilloscopeexternallywith the sig- nal at TP2037 (U204&6) shown Figure 5F26b. Set the test oscilloscopeTime/Divto 5 ms andVolts/Div to 0.5 v. Note the beat notes. Beat notes are pro-

duced by the differencebetw€en strobes from the phaselock (one every5 MHz)andth€ particularfre- quencythe lst LO is tunedto.

O)Vary R3082 clockwise slowly and make a note of th€ amplitudeof the beat notesprior to lock The beatnot€swill dasappsarwhenlock is achieved.

(8)Set R3082fully clockwise. ResetSPAN/DIVto MAX, RESOLUTTONBANDWIDTHto 100Hz, and TIME/olv to AUTO. D€activatevlEW A and VIEW B .

(9)As the sweep scans across the span, note the position of the smallest beat note. Tune the CENTERFREQUENCYto positionthe fr€quencydot at this location, then reduce the sPAN/Dlv to 100MHz. Set TIME/DIVto 1 s and activateVIEWA.

(10)Adjust R3082to set the amplitudeof the beat note to 1.5x the amplitudenoted in sub-part7 of part g.

(11)Reset the TIME/DIVto MNL, and deactivate VIEWA. Set SPAN/DIVto 50 kHz, then increaseit to 100kHz. Center th€ crt beam with MANUAL scAN control. set the oENTER/MARKERFRE- OUENCYcontrollor a nullof the displayon the test oscilloscope.

(12)Positionthe crt beamwith the MANUALSCAN control4 divisionsfrom centerscreen(400kHz from c€nterscreen).

(13)Monitor TP1031on the Error Amplifierboard withthe test oscilloscope.Externallytriggerthe test oscilloscopewith the signalat TP1031. Set R1061 to midrange.

(14)Vary Rl061 clockwiseuntil the oscilloscope displayjust startsto breakup.

(15)Use the MANUALSCANcontrolto positionthe beam 4 divisions on the oppositeside of center screen. As the beam crosses center screen,the displayon the test oscilloscopeshouldgo througha null. lf no nufl occurs as the beam reachescenter scre€n, reset the CENTER/MARKERFREQUENCY controlfor a null of the displayon the test oscillo- scope.

(16)Adjust R1061 such that break points are 400kHz on eithersideof centerscreen.

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