Tektronix 494AP service manual TUNINGFAILURE- 2NDLO

Malntenance- 4tt0[l4g4Ap ServlceVol. I

Becausethe frequencycontrol loop may be unlockedwhen using the internal reference freguencyin a ucold'

instrument. no error messag€is displayedfor the internalrefer- ence unlocked condition. How€ver, l_U (lnternat-Untock)is disptayedat the REFER- ENCEOSCTLLATORpositaonfor the normal instrument rEadout. The proc€dure that deats with FREQUENCY REFERENCE UNLOCKEDmessageshoutdbe foilowedif l-U appearsafter the instrumentis warm.

The followingtroubleshootingproceduresare keyed to the brief error messages.Someproblemsmay pro- duce more than one error messagein which caje'the spectrurn analyzer will <lisplayonly th€ predominant error. A listingof all error m€ssageswill be displayedif you press <Blue-SHIFT> MAX HOLD. Combinations of error m€ssagesmay help determineand expeditethe proc€ss of findingthe problem.

. . Some of the proceduresuse firmwarediagnostics aid routineswhich can only be accessedby pressing >Blue-SHIFT> PULSE STRETCHERanO-ietectingmenuitem#3 (DIAGNOSTCADS).

Combination of Error Messages

The followingis a list of error m€ssagecombina- tionsand suggestionsas to theircause. lf the problem is not resolvedwith the followingsuggestionsor. if the combination of effor messages displayed is not covered,proceedto the listingof each error message andhow to troubleshootthe problem.

POWERSUPPLYOUTOF REGULATION (in comblnatlonwlth any otfrermeseage/s)

Amissingor inaccuratesupplyvoltageis probably causingthe othererrors. proceedto the POWERSUp- PLYOUTOF REGULATTONprocedure.

TUNINGFAILURE- lST LO and

TUNINGFAILURE- 2NDLO

The CF Controlboardis probablythe cause,partic- ularly if signalsdo not tune or do not tune smoothly. The problemis probablythe voltagereterenceor in the digitalcontrolsection.

Procedure Format

The format for thes€ proceduresis such that the problemis diagnosedin a dEscendingorder. The aim, to isolatea problemdown to one part of the system, usuailyan assembly(such as a moduleor boardl or a functionalsectionof the assembly. After the pr6btem

has been isolatedto the assemblyor circuitlevel,refer to the diagrams and circuit descr{ption,as suggested under General TroubleshootingTechnigues,for further isolation.

The proceduresare structuredas follows:

Error Message

TroubleshootingProcedure

1.

a.

b.

( 1 )

(2)

2.

Steps at the same level are either sequentialor alternativesteps, based on measurementor observa- tion. Proceedto the lower-levelsteps only if the condi- tions of the higher-levelstepsare met. lf the conditions are not met, proceedto the next step at the samelevel. An "(E)'at the end of a step, signifiesthis is as far as this procedur€can tak€ you to locatethe problem.

Severalof the troubleshootingproceduresrequire that frequenciesbe countedand comparedto eitheran expectedvalue,or the numbercountedby the spectrum analyzer'sinternalcounter. The frequenciescan differ by up to [(1x107)+ (counteraccuracy).

Theseprocedures,unlessspecifted,assumethe fre- quencyrangeis either0 - 1.8GHzor 1.7- 5.5GHz.

Somefailures,in the frequencycontrolsystem,may appear only at sp€cific oscillatorfrequencies.lf this occurs, in a higher frequencyrange, the fundamental frequencyof the appropriateoscillatorshouldbe deter- mined so it can be set to the same frequencyin the lowerbands. This can be doneby:

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