Tektronix 494AP service manual O o o o, Powersupplyoutof Regulation

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(1) Press <Btue-SHtFT>PULSESTRETCHEBand #0, then setect either the lst to reaooutlmenu item #1) or the lnd fg.readout (menuiteni *e1. I!9 _LO frequency wiil be displayecton the crt CENTERFREQUENCYreadoutpbsition.

(2[fter n-otingthe.freque^ncyof the oscillator,press <Blue-SHtFT> PULSESTRETCHERand #0, and selectcenter frequencyreadoutto returnto the nor- mal centerfrequencyreadoutmode.

Sincethe instrument'spower is usuallyswitchedon and off during troubleshooting,the ponJer-do*n set- tings, that are automaticallyLtored in register 0 of

memory,lhout6 be recaltedso

Paltery-backed-upthe instrumentsettlngs and operatingmode dupticatethose thatexistedwhenth€ errormessagewas ginerat"d.

. The following,describeseach error messageand the proceduresrecommendedto locatethe problem.

POWERSUPPLYOUTOF REGULATION

Any out-of-tolerancevoltage will cause this eror m.?.":"g.toe be displayed.A pow€r supptystatuscircuit withinthe power supplywill changetne itatus LED on the Z-Axisboard to red,whenany supptyexcept_17

-error'message v

changesby more than 25%. Anwill be also-be displayed. An apparent power supptyfailure canbe producedwheneitherthe supplyfailsor a circuit demandsexcessivecurrentand Ofowi L protectivefuse or producesa current limit condition. The following procedureshould determinethose voltagesthat are out of rangeand whetherthe failure is in th6 supplyor in a circuitoutsidethe supply.

Troubleshooting procedure

The spectrum analyzer uses a high eficiency pow€r supply, with the primJry ground potentialdifierentfrom chassis or €arth ground. An isolation transformer, with a turns ratio of 1:1 And a 5OOVA minimumrating, should be used between the power source and the spectrum analyzer power input receptacle, The transformermust havethree-wire input and output connectors with a through ground

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99!*99n input and output. S-tancor GlS1000 is an €xample of a suitable transformer.A iump€r shouldalso be con- n€ctedbetweenth€ primarygroundside to chassisground (emitterof e2061 and the groundt€rminatof the inputfitterFL301).

Malntenance_ 4g4Ll4g4ApServlceVol. 1

lf the power supply is separat€d from the instrument and operated on the bench, hazardouspotentialsexist withinthe supply for several seconds after power is discon_ nected. This is due to the slow discharge of capacitorsC6101and C61fi. DS5112 (next to 96111) tights when the potential exceeds80 V.

_ - 1. Verify that the power supply status LED, on the Z-Axisboard, is red. lf the LED is green,there is prob- ably a failurein the microprocessorlnterface.(E)

.2, Measurethe power supply voltagesat the test points on the Z-Axis board. To accessine test points, remov€ the hold down cover over the Sweep and Z-Axis boards.

Hazardousvoltages (900V and 100V) are presenton the Z-axis board.

The rangesfor each supplyare listedin Table6€. ThEseare tolerancelimits which are muchtighterthan the limitsused by the power supplysensingcircuit. A supply that exceedsthese limits may not trigger the error messageor causethe instrumentto malfunction. The +15 V supply is adjustableand affectsthe other supplies.Referto the Adjustmentproceduresectionof the manualfor adjustmentinformationif a supplyis iust out of tolerance.

. a. lf all suppliesare withinlimitsand the powersup_ ply status LED is r€d, the probtemis probablyin the power supply status circuit on the Z-Axis board. R1065may be misadjusted;adjustRl065 to see it the LED changesto green. lf it changes,set R1065at the centerof the 'green'range.

(E)

b.lf the +17 V or -17V supptyand any othersup- ply or suppliesare inaccurate,or, if both the +9 V and +5 V suppliesare inaccurate,the troubleis likelyin the PowerSupply.(E)

c.lf the voltage is high (in absolutevalue),the troubleis probablyin the PowerSupply.(E)

d.lf the voltagefrom a fused supplyis inaccurate, the troubleis probabtyin the power Suppty.{E)

e.lf the voltagefrom a fused supplyis absent,it indicatesthe fusecouldbe blown. To accessthe fuses. remove the cover at the top left hand corner of the Power Supply module (as viewed from the front of the instrument),A blown fuse generallyindicatesthat one of the circuitsthat this supply furnishesis defective; however,a fuse may open withoutan overcurrentcon- dition. Replacethe fuse and try again. lf the fuse opens,the troubleis definitelyin one of th€ circuitsthe supplyfurnishes.

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