Tektronix 494AP service manual O o o o o o O o o o o o a

Mafntenance- 494A1494ApServtceVot. 1

a.lf the calculatedand measuredfrequenciesare within 50 MHz of each other and the mbasuredfre_ quency agrees with the internallycount€d 1st LO Freg readoutor differsfrom it by a multipleof the AuxiliarySynthesizerFreq, r€turnto normalopera_ tion-(by pressing<Btue-SHtFT>). Now attemptto calibratethe CF Controlboardand the 1st LO Driver board by pressing <Btue-SHIFT> PULSE STRETCHERand select #1 from the menufor the FREQUENCYLOOPSCAL,and #1 againfor the CF Control board. or #2 for the lst LO Driver board. Exit from the CF Controlboard calibrationroutineby pressing<Blue-SHtFT>whenthe stepfor Ra040ii displayed. lf you are able to completethe calibra- tion routin€, check to gee if the error message is still present. lf it is, or if the calibrationroutines cannot be completed,continuetroubleshootingwith step 4. (E)

b.lf the calculat€dand measuredfrequenciesare within 50 MHz, but do not m€Etthe abovecondition in step 3a, measurethe AuxiliarySynthesizeroutput frequency at P1060 on the Auxiliary Synthesizer board.

0)lf the AuxitiarySynthesizeroutputfrequency is coffect,measurethe inputfrequencylrom the HarmonicMixerwith a spectrumanalyier,at the cabte connectionto p261 on ttre euiitiary Syn_ thesizer board. (A counter woutd probaOiygive an eroneous readingbecauseof the harmonic mixing process). The frequencymeasuredwith the speetrumanalyzershouldequalthe sum of the DesiredMixer Freq and the measured1st LO frequency,less the Desiredlst LO Freq, if the catculatedfrequencyis between10 MHz and 90 MHz. lf the calculat€dfrequencyis outside th€ 10 MHz to g0 MHz range, the lst Lo fre_ quencyis far from the d€siredvalue. Repeatthe previousstepsin this procedure.

(a) lf the HarmonicMixeroutputfrequencyis correct. measurethe frequencyat edge con_ nector 'l5, on the Auxillary synthesizer board, with a countgr. This should be 1i100th of the HarmonicMixer output fre_ quency.

(b)A correct frequencymeasurementindi_ catesthe Counterboard is defective.(E)

(c)An incorrect frequency measurement indicatesthe AuxiliarySynthesizeris defec- tive.(E)

(d)The HarmonicMixer is probablydefec_

tive if no signal is presentat the output or the signalfrequencyis incorrect.

{E)

(2) tf the output frequency, at p1060 is incorrect,measurefie 200_220 MHz VCo tune voltage between Tp1066 and Tp1074 on the AuxiliarySynthesizerboard. The range of the tuningvoltageis normally+S V to +12 V.

6-10

(a)lf the tune voltage is within th€ centerof its normal range and the output frequencyat P1060 is stable (varies no more than l- 2Hzl, the programmabledivider in the phase-lockedloop is probablydefective.(E)

(b)lf the tune voltageis in th€ centerportion of its normal range and the output frequency

at P1061 is unstable,the loop amplifierii probablydefective.(E)

(c)lf the tune voltage and oscillator fre- guencyare at the end or outsidetheirrange, in the same dir€ction(highor low),C1070tn the VCO may be misadjusted.lf adjustment of the capacitor do'esnot conect the prob- lem it is not in the VCO but somewhereelse in the loop.(E)

(d)lf th€ tuning voltage and the Auxitiary Synthesizerfrequencyare in oppositedirec- tions from the c€nter of their respective ranges(8.5Vand 210 MHz),the VCOis prob- ablydefective.(E)

{e) lf the calculatedand measuredfrequen- cies difier by morethan 50 MHz, removethe jumper plug P3043 on the 1st LO Driver

boardand measurethe oscillatorcurrent.

The oscillator coil has significant inductance. Interruptingthe oscilla- tor current will gen€rate hlgh vol- tage. Remove/replaceP3043 or connect/disconnecta current meter

after the power is off. fiypical vol- tages at P3043can rangeas high as

35 V.)

The coil currentshouldbe: 40 mA/y, where the voltag€ is the sense-resistorvoltageas previously measured across Rl040. The measuredcurrentshouldbe within1oloof this value.

(3)lf the measuredand calculatedcurrentsare within 17o.return to normalop€ration (by press- ing <Blue-SHIFT>) and determineif the fre- quency control error occurs with frequency span/divof 5 MHz or less, or above5 MHz/div, with the sam€ centerfr€qu€ncy. The frequency rangeshouldbe in eitherband1 or band2.

(a)lf the frequencycontrolerroroccursonly with frequencyspans of 5 MHz/divor less, one of the noise filter capacitors on the 1st LO Assemblyis probablydefective.(E)

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