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

Maintenance- 494A/4g4ApServlceVot.1

5.Setting Sweep Range

a.Apply 5 ps time markersfrom the Time Mark Generatorto the RF tNpUT.Setthe FREOSPAN/DV to 500 kHz then back to 200 kHz to center the 2nd LO frequency.

b.Adjust the REF LEVEL to disptay the 200 kHz markers then center one of the markers with the CENTERFREOUENCYcontrot.

c.Adjust the 2nd LO Sweep R1O6Z,on the Span Attenuatorboard (Figure6241, so the comb lines on

oppositesides of the screen,are exactlyg majordivi_ sionsapart.

6.Check and Adjust Tune Linearity

a.With Frequency Corrections disabled (see part 2), apply 5 ps markers from a Time Mark Generatorto thE RF INPUT. SEtthE FREQUENCYtO20 MHZ.FREQ SPAN/DIVto 200 kHz and activate AUTO RESOLN. Adjust th€ REFLEVELso a comb of 200 kHz markersis displayed.

b.Turn the CENTERFREQUENCYcontrotcount€r- clockwise until the c€nter frequency stops tuning, decrease FREQ SPAN/DV to 50 kHz then tune tfre CENTERFREQUENCYup untit a markersignal is one major divisionfrom the left edge of the graticule. A comb line (or marker signal)shouldappearon or near the first major division in from the right sid€ of th€ scr€en.

c.lt the right marker is not exacily g divisionsfrom the left marker,note the €rror to th€ nearest0.5 minor division.

d.Tum the CENTER FREQUENCYcontrol ctock- wise, to increase center frequency, until the next markersignalis one divislonin from the lett edge and again note thE spacing between this marker and the markernearthe rightedge.

e.Continuethis process of tuningup in frequency untilthe centerfrequencystopstuning,notingthe slgnil spacingat eachcheckpoint.

f.lf the peak-to-peakenor is 2.5 minor divisions (25kHz) or l€ss, the lin€arityover the center2 MHz of swEepis satisfactory;if more,the shaperneedsadjust- mentor repair.

g.Switchthe FREQSPAN/DVto 200 kHz, tune to the low end of the sweep rangeand note the linearity over the centereight divisionsof span,thentuneto the highend of the 2nd LO rangeand againnotethe linear- ity. Peak-to-peakdeviation should not exceed 0.5 minordivision.

h.lf the shaperneedsadjustm€ntor repairproceed as follows:

6-42

(1)A shaperdiode or resistor may be defectiveif the comb line spacingis consistentfor part of the tuning range and 30 kHz or tnore off for the other parts of the sweep. To test the diodes for forward conduction,tune to the low end of the range and short R2049 (Figure6-20). The output of U1073A (pin 1) should equal about *3V. Ut0S9 diodes B throughG and U2059diodesA throughF shouldail havea 0.48V forward drop. Use a floatingor digital voltmeterto check the drop.

(21 Tum POWER off then temporarily replace ShaperresistorR2049with a 20 kO potentiometer.

Switch POWERon, and adjust the potentiometerto obtain the best overall linearity; decreasing resls- tancewill decreasethe spacingbetweencomblines in the upper portion of the tune range and spread the spacing for the lower portion. Increasingthe resistanceof R2049will reversethe effect. When the correct setting is found,turn POWERoff, meas- ure the resistance,and replacewith a fixedresistor of the sameor nearthe samevalue.

i.Checkthe tun€ sensitivityand sweeprangeof the 2nd LO. Repeatsteps4, 5, and 6 if necessary.

7.Conclusion

a.Replacethe housinglid with its 14 screws.

b.Tightenthe screwssequentially,startingfrom the centerof the lid and progressingtowardthe cornersto prevent gaps between the lid and the housing. Use care to not strip the screws as you tightenthem.

c.This completesthe 2182MHz PhaseLocked2nd LO calibration.Referto "Adjustcontrol system"in Sec- tion 5 for readjustingthe system.

Troubleshooting Aids for the 2182 MHz Phase Locked 2nd LO Assembly

lf the Phase Locked assembly is in the instrurnent,set the FREQsPAN/Dlv to 500 kHz or moreso the 2nd LO is not sweot.

The differencefrequencysignal (18MHz) from the 2200MHz Reference Mixer is amplified and fed to P224. Nominally,its amplitudeis -5dBm into 50O. P224 is convenient for monitoring the 16-20 MHz VCO. Whenphaselock is operating,the differencefre- quency exactly equals the frequencyof the vco. lf phase lock is not functioningproperly,the difference frequency signal will either disappear completelyor tuneto its rangelimitof -6 MHz or 30 MHz.

o o o o o o o

to

a o

to

O

o

I

o o o a a o o o a o a o o

t t

o o o o a

to

a o o a o a