Tektronix 494AP service manual

Malntenance- tl94A/494ApSeMce Vot. 1

Schematic diagrams list the Tektronix part No. (670-xxxx-)for the assembty or board atong with the assemblynumber (e.9. AS0) and name. The tast two digits or sufftx of the part numberare not indicatedon the diagram, however, they are list€d in the El€ctrical Parts section. These two digits rsflect changes or modifications to the assembly or board. When a change is made to the assemblythe suffx rolls one digit. The diagramindicatesthesechangeswith a grey tint drawing of the original circuit or if a component changes value the symbol is enclosedwith a grey tint box. When a major modificationis madeto an assem- bly or board and it is no longercompatiblewith eadier instrumentsa nsw part number ls asslgned and a separate schematic with associated illustrations are added. all diagrams indicate the new part number and the instrumentserlal numberbreak. lf the assemblyis compatiblewith earlier lnstrumentsand the changeis significantenoughto requirea separateschematic,this will also be identified.

Correctionsto the manualand instrument modificationsare documentedby ad<ling conection pages behind a tabbed page, labeled Change Information,at the rear of the manual. Checkthis ChangeInformation section for changesto the manualor thE instrument.

Circult Board lllustadons and ComponentLoeator Gharts - Electricalcomponents,connectors,and test points are id€ntifisd on circuit board illustrations that are locatedon the insidgfold of the correspondingcir- cuit diagramor the back of the precedingdiagram. A grid on the circuit board illustrationand the circuit sch€rnatic,plus a look-uptabl€. providethe meansto quicklylocatecomponentson eitherthe diagramor the circuitboard.

ln most cases,circuitnumbersare assignedaccord- ing to the physicallocation of the componenton the board or assembly. The first digit designatesthe row of

agrid, the second the column, with the last two reservedas an expander. Three digit numbersdesig- natechassismountedcompon€nts.

Diagnostcs - The spectrum analyzer contains firmwarethat will assist in locatingtrouble in the fre- quency control system and the power supply. This diagnosticinformationis part of this section.

General Troubleshooting Techniques

Before using test equipment,to measureacross static-sensitivecomponentsor assemblies,be certain that voltag€sor currentsuppliedby the test equipment does not exceed th€ limits of the componentsto be tested.

Try to isolat€ the problem to a componentthrough signalanalysis. Determinethat circuitvoltageswill not damagethe replacement.

Semlconductor Checks - Semiconductorfailures account tor the majority of electronic equipment failures. All semiconductorsare solderedto the boards to reduce pin contact problems. The followingguide- lines shouldbe observedif you substituteany of these components.

1.Tum the poweroff beforeremovingan assembly or board.

2.Use a de-solderingtool and 25 W or lesssolder- ing iron to removethe components.

3.Use only good componentsfor substitutionBe. sure the new componentis insertedinto the board properly before soldering. Refer to the manufacturer'sdata sheet for integratedcircuitand transistorlead configuration.

lf a substituteis not available,check the transistoror MOS FET with a dynamictes- ter such as the TEKTRONIX Type 576 Curve Tracer. Static type testers, such as an ohmmeter.can be used to check the resistanceratio across some semiconduc- tor junctionsif no othermethodis available, however,DO NOTMEASURERESISTANCE ACROSS A MOS FET to avoid damage from static charges. Use the high resis- tance ranges (R x 1k or higher) so the externaltest currentis limitedto less than

6 mA. lf uncertain,measure the external test current with an ammeter. Resistance ratios across base-to-emitter or base-to- collector junctions usually run 100:1 or higher. The ratio is measuredby connect- ing the meter leads across the terminals. note the reading, then reverse the leads and notethe secondreading.

Dlode Checks - Most diodes can be checkedin the circuit by taking measurementsacross the diode and comparing these with voltages listed on the

diagram. Forward-to-backresistanceratios can usually be taken by referring to the schematicand pulling

appropriatetransistorsand pin connectorsto remove low resistanceloopsaroundthe diode.

Do not use an ohmmeterscalewith a high external current to check diode junctions. Do not check the forward-to-backresis- tanceratiosof mixerdiodes.

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