SectionVII

breakdown of individual components contained in the Repair Kit and their respective parts kits. The printed circuit assembliesprovided in the repair kit may be substituted for assembliesin the instrument in order to isolate the trouble. A kit of microcircuit packagesfor each assemblyis provided so that once the trouble has been isolated to an assembly,the defectivemicrocircuits on that assemblymay be replaced.A systemcontroller and a printer are required for troubleshooting the I/O circuits. Thesemay be the controller and printer in the system in which the 3490A is used. A dc voltmeter is also required. The GPIB Troubleshooting Tree, Figure 7-13,checksmost of the 3490A I/O circuits.

7-57.ExternalTriggerCircuitGheck.

7-58. The external trigger circuits may be checked by the following procedure.

a. Place 3490L in local control by setting the RemoteControl Enableline (LREN) to HIGH.

b.Set front panelSAMPLERATE control to HOLD. Check logic level of External Trigger Flag (LETF) at Jl I pin 4. Measureto outguardgroundat pin 8 or 14 of Jl l, or on Outguard Data Assembly A33. LETF should be LOW until an externaltriggercommandis given.If not, checkA31Ql.

c.Momentarily connect Jl l pin 7 to outguard ground. 349OA should sample each time pin 7 is grounded.If not, check A3lQ2 and associatedcompo- nents,including pulsetransformerTl. If the instrument doesnot havethe Sample/Holdoption, ajumper should be installed in place of Tl. Make sure this jumper is. seatedproperly in the correct sockets.

7 . 59SAMPLE/HOLD.SERVICING .7-60.Accesto Sample/HoldGircuits.

7-61. The Sample/Hold Analog and Logic printed circuit assemblies,A27 and A28, arefastenedtogetherand, must be removed from their socketsas a unit. Printed circuit extender boards (Part Nos. 5060-6032 and 5060-5983, suppliedwith the 3490A Option 040/045) may be usedto mount the Sample/Holdassemblieshigh enough for accessto the circuits. The metal shield covering the Logic assemblymay be removedwithout separatingthe two assembliesDo. not separatethese assembliesunless it becomes necessaryin order to replacecomponents.

The Analog Assembly,A2T,especiallythose oreasin the vicinit)'of the teflon insulators,

must be kept cleon and free from ftnger- prints or other contamination, or Samplef Hold performsnce m6v be degraded.

Model3490A

742.lt it becomesnecessaryto separatethe Sample/ Hold printed circuit assembliesin order to replace certain components,it is preferablenot to disassemble more than necessaryFor. example,if a repair is to be made to the Analog Assembly,A21,the flexible shield should not be removedfrom A28. If the unit has been disassembled,use the following procedure and Figure 7-3 for reassembly.

a.Place Logic Assembly A28 (item 3 in Figure 7-3) component side down on work surface.Place4 spacers (item 4) over the four captivespacersin L28.

b.Carefully put flexible shield in place, aligning holesin shieldwith the spacers.

c.Secure shield to A28, using two nylon screws (item 6) in the two holes nearestthe bottom edge of A28.Thesescrewsmust be non-metallic.

d.Placetwo spacers(item 4) on top of the other two holesin flexible shield.

e.PlaceAnalog Assembly,A27, componentside up over the two spacersin stepd and securewith the screws (item 8).

f.Securealuminum shield(item 2) over component sideof A28, usingfour screws(item l).

g.Connectbrown, red and orangewiresfrom A28 to pins l, 2 and3, respectively,on A27.

7-63.0peratingthe3490AwithSample/HoldAssem- hliesRemoved.

7-64. lf a problem exists in the Sample/Holdcircuits which affectsoperationof the 3490A with Sample/Hold off, the S/H Analog and Logic Assemblies,A21 and A28, may be removed and replacedby jumper boards, which are suppliedwith the Sample/Holdoption. These boards complete the circuits necessaryto allow the 3490A to operate(seeFigure 4-27). The wire disconnected from the Analog assemblymust be connectedto the pin on the Analog Jumper Board for proper operation.Five pins are alsoprovidedat the left end of the Logic Jumper Board for terminating the wires disconnectedfrom the Logic assembly.Thesepins are not connectedto any circuitry.

7 . 65SAMPTE/HOLD.TROUBLESHOOTINGTREES .

l-66. Two troubleshootingtreesareprovidedto assistin isolating problems in instruments with Sample/Hold. The S/H GeneralTroubleshootingTree, Figure 7-14, is designedto determineif the trouble is in the Sample/ Hold circuits or in other circuits in the 3490A. If it is determined that the trouble is indeed in the Samplei Hold circuits,this tree will alsohelp determinewhether it is in the analogor logic circuits.The Logic Trouble- shootingTree,Figure7-15,providesfurther assistancein isolatingtrouble in the Sample/Holdlogic circuits.

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