ELECTRICAL EQUIPMENT—Q.8To separate the armature shaft from the driveend bracket, press the shaft out of the drive endbracket bearing. When fitting the new armature,support the inner journal of the ball bearing,using a mild steel tube of suitable diameter,whilst pressing the armature shaft firmly home.See also Para 4 (h)1.(f)Field CoilsMeasure the resistance of the field coils, withoutremoving them from the dynamo yoke, bymeans of an ohm meter connected between thefield terminal and the yoke. The resistance is6.0 ohms for the model C40A.If an ohm meter is not available, connect a 12volt d.c. supply between the field terminal anddynamo yoke with an ammeter in series. Theammeter reading should be approximately 2amperes. Zero reading on the ammeter or an"Infinity" ohm meter reading indicates an opencircuit in the field winding.If the current reading is much more than 2amperes, or the ohm meter reading much below6 ohms, it is an indication that the insulation ofone of the field coils has broken down.In either event, unless a substitute dynamo isavailable, the field coils must be replaced. Todo this, carry out the procedure out-Fig. Q.5.Tightening Pole Shoe Retaining Screws.lined below, first noting to which end of the coilthe field terminal is soldered.1.Drill out the rivet securing the field coil terminalassembly to the yoke and unsolder the field coilconnections.2.Remove the insulation piece which is providedto prevent the junction of the field coils fromcontacting with the yoke.3.Mark the yoke and pole shoes so that the lattercan be refitted in their original positions.4.Unscrew the two pole shoe retaining screws bymeans of a wheel operated screw driver.5.Draw the pole shoes and field coils out of theyoke and lift off the coils.6.Fit the new field coils over the pole shoes andplace them in position inside the yoke. Takecare to ensure that the taping of the field coils isnot trapped between the pole shoes and theyoke.7.Locate the pole shoes and field coils by lightlytightening the fixing screws.8.Fully tighten the screws by means of a wheeloperated screwdriver.9.Solder the original terminal and earthing eyeletto the appropriate coil ends.10.Refit the insulating sleeve and re-rivet theterminal assembly to the yoke.11.Refit the insulation piece behind the junction ofthe two coils.(g)BearingsBearings which are worn to such an extent thatthey will allow side movement of the armatureshaft must be replaced. To replace the bearingbush in a commutator end bracket proceed asfollows:—
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