Microvitec Series 3 Colour Displays Monitors Service Manual

TYRAP POSITIONS ON CABINET BACK COMPONENTSNote: All tyraps tobe fully tight 

___________MODIFICATION FOR IBM/APPLE INTERFACES_________(1)     Remove TR1 (BC337) and replace with TIP 120 as shown.(2)     Remove R1 (6 K8) and replace with 8K2 1/4 watt 5%.(3)      Race a 10 microfarad 35V electrolytic as follows: (i) negative side, to top of existingC4 position (ii)  positive side,  mechanically wrapped around and soldered to left sideof R3. 

SWITCH MODE POWER SUPPLY - DESCRIPTION AND OPERATIONThe power supply is a variable frequency, self oscillating, switching flyback convertor type, providing mainsisolation and three stabilised voltages, of 18V, 124V and 200V.1.       Control Circuit - DescriptionA.       Operating Normally In A Ready State(1)     TR2 turns on a step voltage whose amplitude depends on the instantaneous value of rectified mamsacross T2 primary.(2)     Current in winding and TR2 collector increases in a linear fashion from...

TR2 BASE START-UP Fuse out(10)  HT stabilisation(a) HT stabilisation is achieved by controlling the duty cycle of the switching transistor.(b) Increasing load = increased duty cycle and peak collector current.(c) Increasing mains supply * increased operating frequency.(d) HT adjustment is made by VR4.(e) Extra damping for C14. R6 and D7. are required to limit Vce of TR2. 015 and D16 providenegative off drive and base current tracking, L2 optimises storage time of TR2 forminimum  switching losses.(11) Max...

TR2 COLLECTOR Fuse in3.       OverVoltage ProtectionA.      Controlled by a second feedback loop attached to TY1, consisting of:(1)    A zener diode which senses the reference rail voltage and HT voltage proportionally.(2)     If this reference exceeds zener voltage. 020 conducts, fires TY1 and terminates drive to TR2.During which time enough volts are developed across TY2 gate-cathode to causeconduction, and latch on.(3)     Drive to TR2 is stopped until C23 is fully discharged (10ms).(4)     Power...

VIDEO INPUT INTERFACE CIRCUIT - DESCRIPTION AND OPERATIONINPUT: Connections are made at PL101-red, green and blue video, sync options 1, 2, 3 and TTL videonormal/invert. All inputs are flashover protected by, resistors and diodes. R,G,B drives are split  in two ways:1.       Test selectable links TL103, R,G,B.2.      To IC101Select Position  1: The input stage is in the linear mode, the video is buffered and level shifted by emitter outputstages. These provide temperature tracking with TR103, 104 and 105...

TR102 COLLECTOR with 10 to 1 scope probeIC101 is also used to provide the following sync options:Composite negative sync: Fed in on PL101, pin 7, ensure TL102 in non-active position, allowing pin 2 of IC101, to bepulled high where upon IC101 performs a sync inversion and provides an attenuated positive sync waveform for drivingsync separators of IC201, via R201 and TL106.Composite positive sync: Fed in on PL101, pin 7 with TL102 in its grounded position. IC101, now provides an output inphase with input...

LINE TIMEBASE - DESCRIPTION AND OPERATION1.        Line TimebaseA.       Line oscillator function is based on IC201, providing three outputs:(1)      Horizontal drive pulses for control of line output stage.(2)      Vertical sync pulses compatible with synchronisation of IC301 field output IC.(3)      A sandcastle pulse providing burst gate and clamping information.NOTE: This facility is only used with IC TDA3301, currently required by 1 volt 75 ohm linear/PALinput  monitors.2.      Sync SeparatorA....

4,      Line Driver StageA.       Horizontal drive pulses from pin 3 (IC201), are D.C. coupled to TR2Q1 and used to control drivertransformer (T201), providing the impedance conversion necessary to provide 600mA forward basecurrent, for saturation of line output transistor (TR202). Ringing is damped by R225 and C214 at TR201turn off, thus limiting its Vce to a safe value. HT supply to the linedriver comes from main HT supply rail, prior to R231 and HT scaninterlock (PL201 pins 5/6; allowing itsoperation...

5.      Vertical Sync OutputA.       Output of field sync separator is used to drive vertical sync output stage on pin 10 (IC201).B.      In addition, this pulse is used internally to inhibit the first phase detector during the field sync period, thuspreventing top flutter as a result of equalising pulses.6.       Sandcastle PulseA.       Sandcastle pulse is on pin 7 (IC201), used on models with linear interface PCB assembly consistingof two sections.(1)      Upper portion, suitable for burst gate and...