Microvitec Series 3 Colour Displays Monitors Service Manual

							LINE OUTPUT - DESCRIPTION AND OPERATION A.      LineOutput1.       L202. L203 and T202 primary, are tuned during the flyback period by C222. This lasts for 11.8/11. 1uS,on 14/20 monitors.2.        Line output transistor TR2C2, is driven directly from  the secondary winding of T201. ON current iscontrolled by R227, turn off dissipation is minimised by L204.3.        Line linearity correction is provided by L203, which is damped by C217, R230 S correction is providedby C218.4.      Field timebase +25V (IC301), is achieved by rectifying a negative going flyback voltage from a secondarywinding on line output transformer. A fusable resistor provides CRT protection under possible faultconditions.LINE SYNC with 10 to 1 scope probe 
						

							TR202 COLLECTOR with 100 to 1 scope probeB.      EHT Supply1.       23.5kV required for CRT is generated by a tripler module driven from  a 7.5kV, overwind on T203.Inductance of the transform er (between prim ary and overwind), is tuned to the 7thharmonic of the flyback frequency by tripler input capacitance and self capacitance of the overwind.2.       The breathing performance of the display is further improved by deriving a high focuspotential from  a resistive thick film/substrate potential divider from  the EHT. giving rise to a constantbleed current from  EHT, thus lowering output impedance of the EHT circuit.3.      An extra input diode within the tripler has its anode connected to the tube base ground and via a beamcurrent sensing circuit to 0V. C223 and a network of resistors provide a load for the diode and effectivelydamp out ringing which may occur during scan. The resulting 1000V which occurs across C223 is usedto generate A1 potential across CRT.C.       HT Supply1.      Derived from the main secondary winding of the switch mode power supply via R231. R231 is chosen tooptimise picture breathing performance and offer protection to TR202, during CRT flashover. 
						

							FIELD TIMEBASE* - DESCRIPTION AND OPERATIONThe field timebase -..:• ;^   s carried out by  ;-.:2C*    2>:-.-: - --_ -. ^~~:.   s determ ined by VR307, R308 and C303.produ; ~: j  2C.S   : .:c .: -- 3:  at pin 12   - •? -  ; • .-.-;;:-   s  :?r
ved from  potential across C305 and C306 as •--••.   :-ange towards-25Vsupp .   .-j - 11-   R305 R306 and R301. W hen field scan is completed the   COuS pulse dischargesC305 and C306  ready for charging cycle to be restarted.1.       FieldLinearityA.       Sawtooth output on pin ? of IC301 via R?13 and ???? field linearity control.IC301 PIN  1     with 10 to 1 scope probe2.      Field FlybackA.        In order to achieve a short fieid flyback time, a supply voltage larger than required duringscan, must be applied to field deflection coils during flyback period. Madepossibleby using a separatefield flyback generator, within IC301.B.      Main HT supply for IC301 is supplied to pin5 via D302. During flyback thegenerator doublesthe supplyon pin 5, the potential on pin 3 is switched from 0V during scan to +25V during flyback The change involtage occurs on pin 5 via C304 causing potentialto double duringflyback.C.      D302 isolates pin 5 from +25V supply. W hen deflection coil field has collapsedand potentialacross fieldscan coils has fallen below  +25V pin 3 is switched back to0V and scan cycleresum ed.D.       Synchronisation of IC301 is achieved by feeding a positive going) field sync pulse on pin 8 of IC301 
						

							!C301 PIN 8 with 10 to 1 scope probe3.      HT Supply for IC301A.       Derived from  a scan rectified rail from the line output stage. C305, C306 are fed via VR306 (heightcontrol) from  scan rectified supply and 12V rail.  Proportions of current and associated time constantsR303, C301 and D301 are used to minimise picture bounce thus maintaining accurate tracking of thefield scan with line scan amplitude during CRT beam  current variations, therefore reducing picturebreathing effects.4.       Field OutputA.       Sawtooth output on pin 1 is applied to output stage (within IC301) and scan output isavailable from  pin 4 to field deflection coils. Current within coils is sampled by R323, then fed back viaR31 7 to the virtual earth input pin 10 of IC301.B.       Gain of output amplifier is set by the ratio of R314 and R31 7. DC operating point by R318 and R316. 
						

							IC301 PIN 4 with 10 to 1 scope probe5.       Picture GeometryA.     CRT E-W pincushion distortion is corrected by modulation of line deflection current intransductor (T202) actively driven by TR301. which is then fed from the parabolic waveform at the top ofthe S correction capacitor C311. AC gam from  the amplifying driver is used to control amount ofcorrection applied to CRT.TR301 COLLECTOR with 10 to 1 scope probe 
						

							CRT TUBE BASE PANEL - DESCRIPTION AND OPERATION1.       CRT Tube Base Panel (Refer to fig. 1)2.        Circuit DescriptionA.      All CRT electrodes are protected by a resistor, capacitor and spark gap.(1)     Scark gaps on all electrodes (except focus) are form ed by a 1-2kV ring trap gap,positioned within CRT base socket assembly. High focus voltage has a separate 10kV spark gapcontained within tube base socket.(2)     CRT cathodes are stood off from video outputs by 220 ohm resistors, the grid 100k and A1s 820k.(3)     Decoupling of grid and A1s is achieved by C910 and C911.(4)      Focus voltage is provided by a potential divider located within tripler module, providing anadjustable voltage of 5-8KV.(5)     A1 voltage is adjusted by VR932, offering a range of 350-820 volts.(6)     CRT heaters may be disconnected by removing TL901, in order to make videoadjustments.(7)     CRT cathodes are directly driven from video output stages mounted on CRT panel.NOTE: The component values given in this section refer to a standard 14 inch model. For equivalentvalues, refer to the parts listing.3.       Video Output Stages - Circuit Description and OperationNOTE: Red, Green and Blue video outputs are identical, the following text refers to the red output stage.A.      TR902 forms a class A amplifier, AC gain is derived from the ratio of R935 to R902. VR903 and DC gainby a DC offset current from R905 and VR906.B.       R904 forms video output load and TR902 represents a low impedance drive source to CRT inputcapacitance during its conduction.C.      During turn off of TR902. the source im pedance of the load R904 is considerably reduced by TR901,ensuring a good pull-up performance.D.       Video compensation is achieved by split capacitances, C902, C903 to help maintain a constantamplifier response curve over the full range of VR903.E.      The emitters of TR902. TR904 and TR906 are connected together with a DC reference of approximately7.5V, used to set video black level voltage.F.       TR907 performs line and field blanking of video information.(1)     TR907 is driven by negative (going) mixed blanking pulses from TR102. TR907 conductsproviding a 7.5V black level reference.(2)      During line and field flyback TR907 is turned off, forcing video outputs off.(3)     Beam current inform ation is sensed on tube base panel resistively, across line output ground to 0Vline by R937, D117, D118 on main PCB.