Tektronix 494AP service manual A a o a o o o o o

Maintenance- 494A/{g4Ap ServtceVol. I

Troubleshooting and Galibrating the 16-20 MHz Phase Lock Section

Replacingoscillatorcomponentsin this sectionmay alter sweep linearity and the oscillator frequency. Thi followingchecks and calibrationaid in repairingand retuningthe assembly.

1. Preliminary

_ a. Test equipmentsetup is shown in Figure6-23. Removeand install the CenterFrequencyControl board on an extenderboard.

b.SwitchPOWERon and set the FREOSPAN/DV to 1 MHz.

2.Check Voltages

-a. Checkall inputvoltagesat the feedthroughcapa_ cJtorsin the housingwall. RefErto FigureOZb or ine data printed on the tid. The vottage LEVEL at the sweepand tuns input lines shouldbe 0 V +O.OSV with the FREQSPAN/DV)500 kHz.

b.Switchthe POWERoff. Removethe tid frornthe mu-metal housingassemblyto gain accessto internal circuitry.

c.SwitchPOWERon, then checkthe internalregu- latedvoltages;+12 V *0.4 V at C2201,-12V *O.i V at C2202, and +S.2V *0.2S V at TplO109 (see Figure6-16).Checkthe outputof the shaperat Tpl0gg for a levelbetween+0.9 V and -0.3V (0 V i0.g V).

3.Setting Center Frequency

a. Connecta frequencycounterto Tp2O35and note the frequency. tf the frequencyis within S0kHz of

18MHz no correctionis necessary;proceedto part 4 (SettingTune Sensitivityand nangay. tf outside the rangeproceedas follows:

(1)TurnPOWERoff. Unsotderand removeone end of ShaperOffsetresistorR1070. Unsolderthe wire strap betweenT2092and T1091,at the T1091end. andlift it free.

(2)Soldera flexiblewire jumperto the T2092end; then,by meansof a a plastictuningtool, attachthe free end to one of the three pads for T1091and notethe frequencyreadoutof the counter.

llthis flexible wire touches ground whil€ the circuit is operating, the supply regulators

can be damaged. The regulators are not protected against a short circuit.

6-40

(3)lf one of the pads provides a frequencythat is withinthe rangeof 17.5MHz *0.2S MHz,solderthe wire strap to this pad. lf the frequ€ncyis still out- side the range,movEthe jumperto the other padfor T2092 and repeat the procedure. Frequencymust equal17.5MHz +0.25MHz.

b.Turn POWEROFF. ReplaceR1070with a 10 turn 25 kO potentiometerin serieswith a S ko resistor.

c.Turn POWERon and with the counterconnected to TP2035, adjust th€ potentiometerfor a frequency readoutof 18 MHz *50 kHz.

d.Turn POWERoff, measure the total resistance valu6 and teplace R1070with a lyo resistor of thls measuredvalue. Switch POWERon and recheckthe frequency to ensure that it is 18 MHz *50 kHz. Disconnectthe counterfromTP2035.

4.Setting Tune Sensitivity and Range

a.Centerthe FineTune adjustmentR4040,on the CenterFrequencyControlboardand the 2nd LO Sweep adjustment Rl067, on the Span Attenuator board (Figure6-24).

b.Decreasethe FREQ SPAN/DVto 200kHz or less. The 2nd LO is now in the centerof its tunerange.

c.Press <Blue-SHIFT>10 dB/DlV to disabtefrs- quency corrections. Press <Blue-SHIFT> PULSE STRETCHERand select item #0 (ALTERNATEFRE- QUENCY DfSPLAY),then item #2 (znd LO FRE- OUENCYDISPLAY).Beadoutwill now indicatethe 2nd LO frequency.

d.Tune the 2nd LO to one end of its rangewhere

the frequencyreadoutno longer changes. Notethe frE- quency and measurethe voltageon the Tune Line at the input feed-throughcapacitor(Figure6-20).

e.Tune the 2nd LO to the other end of its range and againnotethe frequencyand the newvoltageread- ing.

f.Calculatethe frequencychange per vott (fre- quencyrangeversusvoltagerange). Frequencychange per volt should equal 128.0kHz *10o/o or range between115.2kHzand140.8kHz.

g.lf the frequency/voltchangeis low, decreaseihe vafueof R2072(Figure6-20).

h.Press <BIue-SHIFT>PULSESTRETCHERand menu item #1. then selectthe 2nd LO for calibration. Performthe procedurethat is called out for adjusting the Fine Tune RangeR4040and Fine Tune Sensitivity R3040to calibratethe 2ndLO tuningrange.

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