THEORY OF OPERATION: Wells Gardner "Quadrascan" Color X-Y Monitor CAUTION!!! LETHAL VOLTAGES ARE PRESENT IN THIS MONITOR, IF YOU HAVE ANY DOUBTS ABOUT YOUR ABILITY DO NOT ATTEMPT TO REPAIR YOUR MONITOR!! The "Quadrascan" color X-Y display was designed and built by Wells Gardner Electronics Corp. This display differs VERY LITTLE from the Electrohome G-05 Black and White X-Y monitor. The only difference is that this monitor has three Z amplifiers to control three color guns. Refer to the G-05 theory if there is any confusion on operating principles. PINOUTS 1 Red input 4.0V full on 1.0V black level 2 Green input 4.0V full on 1.0V black level 3 Blue input 4.0V full on 1.0 black level 4 Red GND 5 Green GND 6 Blue GND 7 X input 16V P/P 2.5Kohms 8 Y input 12V P/P 2.5Kohms 9 Not Used 10 X GND 11 Y GND 12 Power GND 13 25VRMS 14 Power GND 15 25VRMS LOW VOLTAGE POWER SUPPLY The input voltage of 48 VAC from the game power supply enters through fuses F100 and F101. Diodes D100 through D104 form a rectifier bridge that converts the AC input to an unfiltered DC (about 35V). Capacitors C100 and C101 form the first stage of filtering. Resistors R102 and R103 and capacitors C102 and C103 form two low-pass filters which help filter out AC ripple. Transistors Q100 through Q103 form and active filter that provides the stable, filtered DC voltages. The degaussing coil operates when power is first applied to the display, when the PTP(positive temperature coefficient) thermistor is cool. Diodes D106 and D107 form a protective barrier from any residual current that might enter the degaussing coil during normal game play. The output voltages from the low voltage power supply should be as follows: J101-2 ground J101-3 +25 volts J101-4 -25 volts The picture tube filament voltage is taken from the front end of the low-voltage power supply through D108 and R107. X AND Y AMPLIFIERS Both the X and Y amps are nearly identical. Only the Y amp is described. The Y deflection signal from the game board is applied to the base circuit of transistor Q600. Transistors Q600 and Q601 form a differential amplifier. Transistor Q602 is a constant current source providing current to the differential amplifier. Transistor Q603 is the driver transistor that provides current to the emitter-follower transistors Q605 and Q606. Transistor Q604 is a constant current source that provides current to the driver transistor Q603. Fuse F600 can open in case of circuit failure, protecting the deflection coil in the yoke from damage. Z AMPLIFIERS Since the red, green and blue amplifiers are identical, only the blue amplifier will be described. Transistor Q502 is a common emitter amplifier used to provide gain for the blue intensity signal. Resistor R509 is the blue drive pot which determines the amount of gain in the blue amplifier. Resistor R513 is the blue bias pot which determines the cut-off characteristics of the blud amplifier. The output of the Z amplifiers bias the cathodes of the three electron guns within the picture tube. Transistor Q503 is biased by a voltage from the spot killer. When transistor Q503 is cut off, the collector of Q503 rises to the zener voltage of ZD500 (+4.3V). This allows the emitter of transistor Q502 to rise in voltage, which disables the color signals to the picture tube. SPOT KILLER The purpose of the spot killer is to turn off the video intendity amplifiers when deflection is not occurring. Failure of the spot killer can cause the phosphor coating on the picture tube to become burned. The two signal input voltages to the spot killer are taken from resistors R610 and R710 in the X and Y deflection circuits. Diodes D801 through D804 and Capacitors C800 through C803 form two seperate voltage doublers. The output of the voltage doublers are applied to the bases of transistors Q801 and Q802. When either of the deflection amplifiers is not driving current through the deflection coils, then either transistor Q801 or Q802 becomes biased so that it conducts, which turns on transistor Q800 and the LED D800 in its collector circuit. When transistor Q800 is conducting, then transistor Q503 in the neck PCB is cut off, forcing the red, green and blue amplifiers to turn off their electron beams. HIGH VOLTAGE POWER SUPPLY Integrated circuit IC901 is a timer circuit that produces a 20KHz output which drives transistors Q904 and Q905. These transistors are current amplifiers that drive the primary winding of the Step down transformer T901. The output of T901 is used to turn on the main driver transistor Q906 that in turn drives the High Voltage Step-up transformer T900. The output of the secondary winding is applied to the picture tube at the focus and intensity grids and the 19.5KV anode. Diode D901 allows capacitor C910 to charge to +180V during the discharge of the primary's magnetic field. Transistors Q900 through Q903 are error amplifiers that regulate the +180V video B+. Pot R918 provides an adjustment to the video B+ and hence the high voltage. To adjust the high voltage, you will need a high voltage probe, a voltmeter, and an insulated screwdriver tipped adjustment tool. Turn off power to the display. Connect the high-voltage probe to to the voltmeter, and insert the tip of the probe under the high voltage anode rubber shield. Insert the insulated screwdriver tipped adjustment tool through the top of the high voltage cage, making contact with potentionmeter R918. Turn on the display and adjust the high voltage to 19.5KV.