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

Maintenance- 494A1494ApServiceVot. 1

_ Checkthe freguencyby notingthe readingon the frequencycounter. lf above 729.900MHz, the stub must be lengthened. Solder a bridge across the cut r.e"!99\frequency. Nominalfrequencyfor an uncut

"q

stubis 710MHz.

f.Shorten the line so the frequency is near 723.500MHz. For example:The frequencydifference betweenthe desired and thE actuatdivideciOyZ Unz, equalsthe numberof minor divisionsfrom thE line end

for the new cut. Make a cut acrossthe line and check that the new frequencyis between723.100MHz and 723.900MHz.Repeatas necessary.

g.Coverthe 719 MHz osciltatorcavitywith the g29 MHz Converter cover, press down to ensure good shielding, and note the frequency readout of the counter. Frequencyshouldfall within723.600MHz and 724.400MHz.

h.Reconnectp2S4(100MHz) andp237(2i82 MHz) and confirmthat phase lock is operatingby notingthai the voltageon Tp1011 is betweenO.7SVand 7.5V. This completesthe adjustmentof the 71gMHz Lo.

Replacethe cover and reinstallthe g2g MHz converter assembly.

4.829lr/lHzCoaxial Band-passFitterAdjustment

This procedureis necessaryif the position of one of the variablecapacitorloops(tabs) has been altered, changingthe bandpass characteristicaof the filter.

a. Testequipmentrequired:

Spectrumanalyzerwith tracking generator(such as

aTEKTRONIX49X-Series SpectrumAnalyzerwith TR 503 TrackingGenerator,or 7L,t4with i TR 502 TrackingGenerator);FrequencyCounter(suchas a TEKTRONIXDC 510 Counterwith a Dp 501 pres_ caler);and a Beturn Loss Bridge(suchas a Wiltron Model62BF50.)

b.Unsolderand reconnectthe jumper,on the g2g MHzAmplifierboard,to the test peltolajack J1029(see Figure6-13).

c.Connectthe spectrumanalyzer,trackinggenera- tor, and frequencycounter togetheras a system,with the frequencycounter connectedto the Auxiliary- RF Outputof the trackinggenerator(seeFigure6-14).

6-30

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Figure6-13.829MHzampllfiertestiackandiumper.

d.Connectthe spectrum analyzerltrackinggenera-

tor systemthroughthe returnloss bridgeto the Peltola

jack (J1029) on the 829MHz amplifierboard {see Figure6-14). ReconnectP235(100MHz referencesig- naf)and P2g7(2182MHz input)to the LO sectionof the converter.

Terminatethe 110 MHz Output (J2 21 connector with 50o, using a bnc-toSealectroadapterand 20 dB bnc attenuator.Pull the lF SELEGTline highby switch- ingto band2 (1.7-5.5GHz).

e.Set the test spectrum analyzerReferenceLevel to -20dBm, VerticalDisplaymodeto 2 dBldiv,Resolu- tion Bandwidth to 300 kHz, and Freq Span/Divto 20 MHz. Tune the CenterFrequencyfor a readoutof 829.00MHz on the frequencycounter.

f.Adjust the 114wavelengthlines in the filter in sequence,startingwith the resonatorat the 8291 MHz input(see Figure6-15), Adjustmentis madeby shorting the adjacentresonatorto groundwith a low inductance conductor, such as a broad blade screwdriver,then bendthe loop or tab of the respectivestub with a non- metallictuningtool to changethe seriescapacitanceof the resonator.

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