Tektronix 494AP service manual A o o o o o a, AdJustnentProcedure- 494A/4g4ApServtceVot

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SpectnrmAnalyrer UnderTegt

Flgure5-25. Test equlpm€ntretup for adjustingllre phase Lock assembly.

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(3)Connect the counter to the coll€ctor of e1040 and not6the frequency. Frequencyshouldequalthe difference between pins N and K (e.g., 25.080MHz - 25.000MHz - 80 kHz). Disconnect the counterprobefrom the collectorof e1040.

(4)Connecta test oscilloscopeprobe to the collec- tor of Q1040and checkfor a signalwith a frequency of approximatelyB0 kHz, 507" duty cycle, and an amplitudeof approximatety5 V peak-to-peak.

d. Check Synthesizer

(1)Set the SPAN/DV to 200 kHz. phase tock shouldoccur.

{2) ChangeSPAN/DIVto 500kHz and connectthe counter to J500 on the Synthesizer board. Check for a readingof 50.00MHz.

(3)Connect the counter to Tp2O40(Figure 5_26a) andcheckfor a readingthat is near 25.0MHz.

(4)Connectthe test oscilloscopeto Tpl040 (Figure 5-26a)and check for positive pulses with an ampli- tude of approximately4 V peak-to-peak.

(5)Changethe SPAN/DIVto 200 kHz and observe thatthe signalon Tpl040, in part d(4)is stiilth€re.

G. Contolled Osc{llator- This part of the checkis only requirgd after repair or replac€mentof the Con- trolledOscillatorboard.

Bandpass filter adjustments c1041 and C1042are set at the factory becausethey requirea specialtest fixture. Theseadjust- mentsdo not needfurtheradjustm€nt.

lf adjustmentof C1013 and C2011is not sufncientto achievephase-lock,the board shouldbe replaced.

(1)Press <Blue-SHIFT> PULSESTRETCHERand select item #1 (FREQUENCYLOOPSCAL), then item #5 (PHASELOCK SYNTHESIZER)from the displayedmenu.

(2) Follow instructions'VER-

theuntilthe message, IFY LAST STEP'.Due to the interactionof adjust- ment capacitorsC1013 and C2011, the two steps will have to be repeated until the voltages are correct. Alternately press AUTO RESOLN and IDENTand adjustuntilthe two voltagereadingsare correct.

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5-26

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