Tektronix 494AP O a o a o, Malntenance- 494A/494ApServlceVol, TPl034 GroundRefer€nce

Malntenance- 494A/494ApServlceVol. 1

Table 6-3

POWER SUPPLYRANGES

Supply	Range	Test Polnt	Circult Protecton
+300 v	280V to 310V	TPl052	Fuse(F1033)
+100v	9 5 V t o 1 0 5 V	TPl048	Fuse(F1035)
+17V	10.8V to 18.6V	TPl047	Fuse(F2013)
1 5 V	14.85V to 15.15V	TPr046	Currentlimit
+ 9 V	8.5V to 10.5V	TPl011	Fuse{F1014
+ 5 V	4.8V to 5.2V	TPl044	Curent limit
- 5 V	-4.8 V to -5.2 V	TPl036	Gurr€ntlimit
- 7 V
	-7 V to -8.5 V	TPl037	Fuse(Fl013)
- 1 5 V	-14.85V to -15.15V	TPt035	Currentlimit
- 1 7 V 8			Fuse (F3038)
Gnd		TPl034	GroundRefer€nce

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f.lf the fuse does not open and the vottageis still absent,it indicatesthe troubleis the powerSupply.(E)

g.lf the voltage from a currentlimited supply is absentor low, the problemcould be the supply,or cir- cuits the supply furnishesmay be drawing excessive curr€nt. Turn the POWER ofr, then disconnect the suspect assembliesor modulesfrom the supply and re-measurethe voltage; or, remove the power Suppty from the instrumentand measurEthe unloadedvoltagei on the PowerSupplyconnector.

(1)lf the supplyvoltageis correctwith assembtyor module removed, or when the voltage with th€ power supply removed is normal,the circuits this supplyfurnishesare causingthe problem.(E)

(2)lf the voltagefor the unloadedsupply voltage is still inaccurate,the powersupplyis defective.(g

TUNINGFAILURE- 1STLO

The 1st LO is set by a combination hardware/software loop. There are two distinct hardwareblocks to the loop: the block that measures the oscillatorfrequency and the block that sets the oscillator to frequency. The microprocessorsystem closesthe loop by det€rmininghow muchthe oscillator rnust b€ tuned to set the desired frequency. The microprocessorindirectlycountsthe 1st LO, tunesit as needed,andcountsagain.

The l st LO Tuning Failure error message is displayedwh€n the lst LO has not been set correcuy after a numberof iterations. The numberof timesthe 1st LO is countedand tunedvarieswith instrumentset- tings.

The l st LO Control DiagnosticAid displaysdata on the crt screenwhich can be used to determinewhich part of the loop has failed. To displaythis data,press <BIUE.SHIFT> PULSE STRETCHERand #3. th6n select#1.

The first two lines list the voltageto b€ exp€ctedat the output of the lst LO s€ction of the Center Fre- quencyControl and the voltag€ across the senseresis- tor of the l st LO Driver. The nominalvaluesare based on the Desiredlst LO Freq andthe nominaltuningsen- sitivityof the oscillator.

The DAGSet valuesare basedon the settingof the lst LO tuning DACs. The DAC Set values can differ from the Nominalvaluesbecausethe systemcannotbe exactlycalibrated,tbe tuningsensitivityof the oscillator is possiblynot its nominalvalue,and the DACSwill be movedin an attemptto set the oscillator.

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aThe -17 V..:upPty is nol monitored. by trre Power supply status cirouit nor does it have a iest point on the z-axis board, It this suppty rails, the cooling fan wilt not tun. The fsn will atso not run it tre im5ient tempe.ature is tow. The -17 v suipty witt probably affect other suppliei ai weil.

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