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

Mafntenance- 494A1494ApService Vot. 1

Somecircuitboardsand assembliesmust be placed on extendersto access test points or adjustments. Beforeremovingthese boards and assembli6s,the air bafie attached to the left siderail must also be r€moved.

Turn the power off before removingan assembly.

Removing and Installing the GplB Board

The GPIB board connectsto the GplB port on the back panel,througha GptB Extend€rboard (A56A1),a ribbon cable (W560), and a GplB Interface board (A30A54 in the Power Suppty modute. The GptB Extenderboard edge connector is clamp€d to th€ con_ nector on the GPIB board by meansof a lockingkey that extendsthroughthe connector. When the fly ii turned, so it faces inward,the connectoris clamped. To releasethe connector,so the GplB board can be removed,proceedas follows:

1.Unscrew the mounting screws that hold the metal shield over the GPIB, processor. and Digital Storageboardsand removethe shield.

2.Lift th€ key to the GptB Extenderboard connec- tor up so it iust clearsthe boardand turn it gOdegrees, so it facesthe rear of the instrument.This will spread the connectorso the GPIB board can now be pull from the connectoron the Motherboard.

3.Use a board puller to pull the GplB board free from the Mother board.

lnstallthe boardas tollows:

1.With the key tor the GplB Extenderboard con- nectorturnedso the connectoris spread(top of the key facing to the rear of the instrument),slide the GplB board throughthe guides and onto the Mother board connector.Ensurethat the boardis well seated.

2.Turn the key 90 degreesto lock the connectors of the GPIB Extender board and the GplB board together.Pushthe keydownto its rest position.

3.Re-installthe shieldover the GplB, processor, and DigitalStorageboards.

Removing or Replacing Semi-rigid

Coaxial Cables

Performanceof the instrumentis easilydegradedif theseconnectorsare loose,dirty,or damaged.The fol- lowingprocedurewill help ensurethat the connectionis goodenoughto maintainproperperformance.

1.Use a 5/16 inch open-endwrenchto loosenor tighten the connectors. lt is good practice to use a

secondwrenchto hold the rigid (receptacle)portionof the connectorto preventbendingor twistingthe cable.

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2.Ensurethat the plug and receptacleare clean and free of any foreign matter.

3.Insertthe plugconnectorfullyintothe receptacte beforescrewingth€ nuton. Tightenth€ connectionto g in-lbs to ensure that the connectionis tight. Do not overtighten('l5to 20 in-lbs)becausethis can damage the connector.

Replacing lhe Dual Diode Assembly in the 1st Mixer

The diode subassemblythat houses the Schottky mixer diodes permits easy field replacementof the diodes. The subassemblyis securedin placewith four

0-80 screws. An 8-32threadedholeis providedto facil- itate insertionand removalof the subassembly.There are threecontactpointslocatedon the substratesideof the subassembly.Use care to ensureproperfit when mounting and orientingthese contacts in the mixer assembly. Insertionand removalof the subassembly more than twice is not recommendeddue to th€ gold- ribbonattachingtechniqueusedin fabrication.

'A tuning screw is adjustedto null a start spur on Band 1. This tuningscrewis mountedthroughthe top of the diode asssembly,adjacentto the 8-32 hole. lf adjustmentof this screw is warranted.care shouldbe taken to not force the tuning screw after it bottoms out on the surfaceof the quartz-suspendedsubstrate.

The diode assemblyis packagedin a static-free package. Keepthe diodesubassemblyin this package until ready to install. The following should be used when replacingthis assembly.

The diodes are beam-lead devices, mountedon a quartz-suspendedsubstrate. These diodes are extremely sensitive to static electricitydischarge. Refer to the caution note on static discharge at the beginningof this section. Do not expose the diodeassemblyto RF fields.

1.Loosenand disconnectthe three coaxial cable connectionsat the 1st converterassembly.

2.Removethe two mountingscrews,and remove the assemblyfrom the instrument.

3.Removethe four 0-80screwsthat holdthe diode subassemblyin the 1st Converter,and insert a 8-32 screwsinto the threadedhole providedin the centerof the diodeassembly.

4.Lift the diode assemblyout of the mixer assem- bly by m€ansof the 8-32screw,thenremovethe screw.

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