Tektronix 494AP service manual EAUloNl, Main PowerSupplyModule Removal

Maintenance- 4g4A/4g4ApSeruiceVot.1

-3. Unplugthe CAL OUTcoaxialcabtefrom the grd Converter;then disconnectthe five crt cablesfrom the Z-Axis/RFInterface, High Voltage module, and DeflectionAmplifier.

4.Lookingat the top of the instrument,removethe one screw that holds th€ front panelto the side extru_ sion betweenthe crt and the right side of the instru_ ment. Removethe four screwsthat holdthe tront panel to the siderails.

5.Pullthe front panelup andoff the Motherboard.

Replacethe front panel by reversingth€ removal procedure.

Front-Panel Board Removal

AreplacementFront panel board comes with switches and controls for program- mablo diffErentversions of the spectrum analyzer. Before replacing an existing board,removethe switchesand controlson the new board that are not used on the par_ ticularversionof the instrument.

1.Removethe front panelassemblyas previously described,thenremoveall the knobs.

2.Placethe front panelon its face and removethe elevencircuit board screws plus the screw that heat_ sinks and holds U6090 on the board. Note that the screwnextto the connectorplughasa fiberwasher.

3.To preventtosingthe groundingrings or bush_ ings, betweenthe front panel controls and the front

panel casting,hold th€ circuit board againstthe front panelcastingwhileturningthe completeassemblyso it r€sts on the baseof the crt assembly.

4.Gently lift the casting from the circuit board. Ensurethat the groundingrings remainon the shaft of all controlsas the castingis removed.

Reversethe r€moval procedure,ensuringthat the fiber washeris on the boardscrewnextto the connec_ tor plug. ThiswaEherpreventsthe screwfrom shorting a circuitboardrun to the front panelcasting.

Replacing Front Panel pushbutton Switches

Removalof the front panelassembtyis not required to replaceany pushbuttonswitch.The procedurefol_ lows:

1.Remove the front panel knobs. Loosen and removenuts and washersfor the RF lNpUT, EXTER_ NALMIXER,and the 1st and2nd LO connectors.

6-24

2.Remove the screw under the CENTER FRE_ QUENCYtuning knob that holds the panelto the front panelcasting.

3.Loosen the black screws throughthe crt bezel so the panelcan be movedenoughto lift it off the cast- ing.

4.Unplugand replacethe desiredswitches.

Main PowerSupplyModule Removal

[eAUloNl

To avoid damageto the Mother boardcon- nector J5041 and lnterface connector J1034,during removalor installationof the

Power Supply module, use the following procedure.

1.Disconnectthe power cord and remove the instrumentcover.

2.On the circuit board side of the instrument. unplugthe coaxialcable connectorP620from the Log and Video Amplifierassembly. On the RF deck side disconnectth€ plug for the cableto the ReferenceLock assembly,at the lowerrightcomer of the PowerSupply module.

3.For programmableinstruments,removethe cable clampfor the GPIBinterconnectcableand unplugP560 to the GPIBExtenderboard.

4.Removethe three screws that hold the power moduleto the RF deck flange(bottomright side),then remove the four screws that hold the power supply moduleto the side rails.

5.With the instrumentRF deck on the near side, pull the left side of the power moduletrom its side-rail (no more than 1.5 inch). Now grasp both sides of the moduleand lift to separatethe modulefrom the Mother board.

BecauseC6111and C6101dischargevery slowly.hazardouspotentialsexist withinthe pow€r supply for severalminutesafter the power switch is turned off. A relaxation oscillator,formed by C5113, R5111,and DS5112, indicatethe presenceof voltages in the circuit until the potentialacross th€ filter capacitorsis belowB0V.

6.Loosenand removethe two screwsthat holdthe mounting bracket for P361. Lift the cover off the moduleand unptugP3045to the Fan Driveboard. The powersupplyshouldnow be accessible.

o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o