Tektronix 494AP service manual O o a o e, Shift Pulsestretcher

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a. lf.b€at notesare present,press HELPto enable the lock. Check the- Error'Amplifiei output at T?2O3T,on th€ Error AmplifierOolrJln the prrase Lock module. gutput signat amptituOe'sfroufO

approximately 6 V peak-peakaho its Oe frequency should be 10 Hz. fhe up anO Oown-oui.t_r"ng"

:igngj:, on edge connectors g and 10 oi tn" error. Amplifierboard, shoutdbe togglingOetween0 V and +5 V.

(1)..1fthereis a signatat Tp2O37but oneor both or tne out-of-rangelines is not toggling,the out- of-range comparator on the frior- Amptifier board, or the sensing circuit on the phase Lock Controt board, has probabty failed. firis coutO cause problemsin maintaininglock but not in acquiring lock. lf the instrument does not acquire lock. note the out-of_rangeproblem and continuetroubteshootingwith steFgj.

(2)lf there is no signal at Tp20g7, the Error Amplifierhas probabtyfailed.(E)

(3)ff there is a signal atTp2A}Z, th€ switching circuit that connects the output of the Error Amplifierto th€ FM coil of tne i st LO has prob_ ably failed.(E)

P lf b€at notes arg present,but their amplitudeis

::::_^T"l !:9 v (peak-peak),or the amptitude qrnerence

of the positiveand negativesxcursionsis more than 2AVo,the phase GatJ is probably defec- tive. (E)

c. lf there are no beat notes, mEasurethe strobe frequency,at p504 on the phase l_ocf moOuie.

(1) lf the strobe frequencyis the same as the readouton the diagnosticaiOdisplay,it is possi_ ble, but not probable,that the r it [o systemis miscalibratedand that th€ lst LO is near tne wrong harmonic of the Auxiliary Synthesizer. Press <Blue-SHIFT>to returnto'normatopera- tion and look at the calibratorlinethat is ctosest in.frequency to the frequency (in Band 1) at which the error occurs. lf the irequencyindi_ catedfor the calibratorline is correci(a multiple 9t..tlO MHz), the phase cate has proOa-Utyfailed.

lf the frequencyindicatedis incorrect,attemptto
calibratethe 1st LO systemby pressing<Blue-
SHIFT> PULSESTRETCHER-
	and sete-cting
		#1
(FREQUENCYLOOPS CAL), and tben #O
(OVERALLSYSTEM)from the rnenu. Exit from
the calibration routine when the display for
R4040, on the CF.	board	appears by
_C*ontrol

pressing <Blue-SHIFT>.lf the calibrationcan not b€ completed,or it does not rEsultin the correct frequencyindicationfor the calibrator rine,troubleshootthe lst LO system using the procedure under TUNTNGFAIiURE _rst LO errormessagestep3b. (E)

Malntenance- 4g4Ll4g4ApServiceVol. 1

(2) lf there is no strobesignal,checkfor a sig- nal on feedthroughM, on the Strobe Driver boardin the PhaseLockmodule.

(a) lf thereis a signat,the StrobeDriverhas probablyfaited.(E)

(b) lf thereis no signat,the ControiledOscit- latorhas probablyfaited.(E)

(3) lf the frequency of the strobe signal is effoneous.but is stable(within1_2 Hz),in the normal strobe range of 5.006477MHz to 5.018868MHz, the programmabte

dividerin the

Synthesizerhas probabtyfaited.(E)

(4)lf the listed Strobe Freq is betow5.OOZ10O MHz and the actual strobe frequencyis slightly aboveth€ desiredfrequency;orabove 5.01g240 MHz and the actualstrobe frequencyis slightly below the desired frequency,attemptto cali_ brate the Phase Lock Synthesizer. press <BIue-SHIFT>PULSESTRETCHERand select #1, then #5. lf you are able to completethe calibration,checkto see if the error messageis still present. lf it is stiltdisplayed,or the calibra_ tion routinecould not be completed,proceedto th€ next step as if th€ strobe frequencywas not withinthe aboverange.(E)

(5)lf the listedStrobeFreqis outsidethe range

in th.epr_ejgdingstep, measurethe tune voltage for the VCO, at feedthroughH on the Controll€d Oscillatorboard in the phaseLockmodule.The normalrangeis from 5.9 V to 11.3V. Withthe loop unlocked,the voltagewiil probablybe near or beyondone end of the range.

(a)lf the voltageis aroundthe centerof the range, the loop filter and amplifier,on the Error Amplifierboard, are probablyat fault.

(E)

(b)lf the tuning vottageand the strobefre- quencyare displacedfrom the centerof their range (8.6V and 5.013MHz) in the same direction,the VCO is good and something elsewithinthe loop hasfaited.(E)

(c)lf the tuning voltageand the strobefr€- quencyare displacedin oppositedirections from the center of their range,th€ VCOhas probablyfaited.(E)

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