Tektronix 494AP PHASELOCKFAILURE- 1STLO, Strobefreq, 016949MHZ, Enabledpresshelpto Disable

Malntenance- 494A1494ApServtce Vol. 1

5.Measurethe 2nd LO Tune Volts at Tp1044 on the Center FrequencyControl board.

a.lf the 2nd LO tuning vottage is within 200 mV of the DAC SEt value,measurethe 2nd LO frequency at the front-panet2ND LO Output connector.

(1)lf the measuredfrequencydoes not agree with the internallycounted roadout.the couhter board is probabtyat fautt. (E)

(2)lf the frequencyagrees with the Counted value,measur€the mixed down frequencyat the cablegoingto p513 on the Count€rboard. This frequencyshould equal the sum of th€ Desir€d Offset Freq and the Desired2nd LO Freq, less the measured2nd LO frequency.

(a)lf this freguency19present,measurethe 2182MHz oscillator tuning voltag€ on the

feedthrough capacitor CZaO} between the 16-20MHz phaseLock circuitandthe 2tg2 MHz MicrostripOsciltatorin the 21g2 MHz Phase Locked 2nd LO Assemblv. The nor- mal rangeof this voltageis 0V to -12.5V. With the phase locked loop unlocked,this vottage wiil probably be stighuyoutside one end of the range.

(0 tf the absolute value (magnitude)of the tuning voltage and the oscillatorfre_ quencyare ofi in the same directionfrom the centers of their respectiveranges [6

{-6) V and 2182MHz , the Microstrip Osciltatorhas probablyfaited.(E

(ii)tf th€ absotutevalue (magnitude)of

the tuning voltage and the oscillator fre- quency are off in the opposite direction from the centeror their respectiveranges 16(-6)V and 2i 82 MHz , someotherpart of the lock loop, besidesthe Microstrip Oscillator,has probablyfailed.(E)

(b)lf the mixed-downfrequencyis absent, either the 22OOMHz Reference, the 2182MHz Microstrip Oscillator or the 22AAMHz Reference Mixer probabty has faited.(E)

b.lf the tuningvottageis not within 200mV of the DAC Set value,the CenterFrequencyControlboard probablyhasfaited.(E)

PHASELOCKFAILURE- 1STLO

The followingprocedureassumesthat the oscillator is at the conect frequency,so the probtemmust be in the phase lock system.

The following crt displayof th€ lst LO phase lock DiagnosticAid displaysdata for troubteshootingth€ 1st LO phaselock loop.

lST LO PHASELOCKDIAGNOSTICAID

lst LO FREQ	2.A72000000cHZ
STROBEFREQ	5.016949MHZ
LOCKDISABLED	PRESS
	<HELP>
	TO
	ENABLE

PRESS'SHIFT'TOEXIT

Whilethe troubleshootinginformationis displayed, the lst Lo is repetitivelybeing stepped*750KH2. lf LOCK DISABLEDis display€d,the lock loop is open betweenthe outputof the phasegat€ and the inputto the FM coil. lf lock is enabled,the loop is closed,and the fourth line of thE display changes to LocK

ENABLEDPRESS"HELP"TO DISABLE.

The lst LO Freq readoutis the frequencythe oscil- lator should be at when locked. The frequencythat is measuredat the front-panel1ST LO Out connectorwill not checkexactlywith this valuebecausethe oscillator is unlockedand steppingin frequency.

The StrobeFreq is the frequencyat P502and P504 of the PhaseLock module.

This procedureshouldhelplocalizethe failureto the PhaseGateor to a sectionof the phaselock circuitry.

Troubleshooting procedure

Beforetroubleshootingdata on the phaselockloop is displayed,the FreqSpan/Divmust be in those spans that enablethe phase lock mode (200 kHz or l€ss for band1).

1.Press <BIue-SHIFT>PULSESTRETCHERand select#3 from the menuto bring up the DlAGNosTlc AIDS menu, then select #0 to display the 1st LO PHASELOCKdiagnosticaid information.

2.Withan oscilloscope,examinethe signalatP242 on the PhaseGate. Beat notes (burstsof signalat up to 500 kHz) at a 10 Hz rate should'bepresentas th€ oscillatoris stepped. Beat note amplitudeshould be about6 V peak-peak.The amplitudeof the positiveand negativepeaksshouldnot differby more than2Oh.

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