The Power Supply

The best way to begin explaining the innards of the X-Y monitor is at its beginning or the inputs to the monitor. Ignoring the ground or common tie points for many of the components, which represents zero voltage, there is 30 volts AC going in pins seven and ten of P100 — the input jack. These voltages meet at DB100 which is a device that has four diodes in it. The 30 volts AC means the voltage and current alternate or jump up and down going positive and negative with zero voltage in between. DB100 and the capacitors immediately after it make up the power supply. Most of the circuits in the monitor can't use power that jumps up and down since your picture would do the same thing. DB100 chops up the wave form and capacitors C100 and C101 build up the power that DB100 chops up. The capacitors then leak it out so the power is smooth and not varying. If any component fails in the circuit, the usual result is blown fuses, burning in this area, or just less power. The power supply starts the whole ball rolling, but remember that other circuits build up voltages that can be tapped for those circuits that need more than this thirty plus thirty volts AC from the game transformer.

Let's go back to the input jack, P100, again. Along with the grounds and the two 30 volt AC inputs is the "X" and "Y" channel video information. The "X" input is a6out 10 volts AC and the "Y" input is about 7.5 volts AC. The "X" channel information represents parts of objects from LEFT to RIGHT on the screen. The "Y" channel information represents parts of objects from TOP to BOTTOM on the screen. To get complete objects, then, you MUST have both the "X" and "Y" inputs. If this is so, then why aren't the input voltages equal? Well, notice how a T.V. tube is shorter than it is wide? The up and down voltages ("Y" input = ± 7.5 volts AC) don't need as much as the side to side voltages ("X" input = ± 10 volts AC).

If we divide the picture into four quadrants, the responsibilities of the "X" and "V" amplifiers may be seen more clearly:

The upper left quadrant is represented by "-X" and

"+Y" information.

The upper right quadrant is represented by "+X"

and "+Y" information.

The lower left quadrant is represented by "-X" and

"-Y" information.

The lower right quadrant is represented by "+k"

and "-Y" information.

So let's say your monitor only has the right side of the picture and the left side is missing. The top and bottom right of the screen has "+X", "+Y", and "-Y" information. The left side has "-X", "+Y", and "-Y" information. But since the right side is O.K., obviously the only information missing is "-X". Therefore, there's got to be a problem somewhere in the "X" amplifier.

From P100, the "X" or "Y" signals each go through a resistor and the linearity control of their respective channels. The Wells Gardner monitor only has one linearity control per channel while the Electrohome monitor has two linearity controls per channel. These controls are supposed to be set at the factory. But sometimes they need additional adjusting. The best way to do this is to get a test pattern on the monitor screen, remove the glue holding the control adjustments in place, vary the controls until the size is right and the lines are nice and straight, and then re-glue the control adjustments so they can not move.

After the linearity controls, the rest of the circuitry just corrects the signal for the picture tube and amplifies it. The output power transistors (two for each channel) are heat-sinked on the bottom or the side of the monitor chassis. These feed the "X" and "Y" signals in the form of current to the yoke. The yoke then puts out two invisible electromagnetic fields or forces. These fields pull the stream Af electrons that is spit out of the neck of the picture tube to the various quadrants of the monitor screen where they will write or paint a picture. Just as you may use a magnet to pull nails across a table, so does the yokes magnetic field pull the electron beam all over the picture tube screen to write the picture. The "X" and "Y" information we talked about earlier is what tells the electron beam WHERE to write or paint the picture. When the electron beam hits the phosphor coating on the back side of the front of the picture tube or screen, the phosphor glows in proportion to the electron beam intensity. In other words, the more electrons in the beam, the brighter the light that comes from the screen of the picture tube where it is being hit by the electron beam. This varying beam intensity is the function of the "Z" amplifier.


At pin one of P100, the "Z" amplifier signal voltage is sent to the base of Q504 in the "Z" amplifier circuit. This circuit amplifies the AC "Z" signal and is then sent to the cathode of the picture tube. This varying "Z" signal voltage in turn varies the intensity of the electron beam producing at least eight different amounts of brightness or "eight gray scale steps" as the engineers would say.

In case the "X" and "Y" signals are missing, there is a 90 volt DC power failure — from the high voltage circuitry that feeds the "Z" amplifier, or if any other missing signal condition should occur, the "spot killer" circuitry comes on to effectively turn off the electron beam thus keeping the phosphor from being burned. At the same time, the light emitting diode turns on informing you of this. If the "spot killer" didn't come on when any of the above conditions exists, the electron beam wouldn't be moved around and the phosphor in the center of the screen would be burned from the intense electron beam that is hitting it without moving. Transistors Q500 through Q502 and their circuitry affect the voltages on Q503 to turn the beam current off. This DOES NOT mean you have automatic protection against CRT burns from too much brightness. In fact, it would probably be a good idea to keep the brightness and contrast controls TURNED DOWN to the point where the game looks good but not too bright. If the picture is way too bright, fine spider web-l i ke retrace I i nes wi 11 fol low the f i g u res wherever they move and you are headed for a burnt CRT. The brightness control affects the DC voltage between the cathode and G1 of the picture tube. The contrast control varies the amount of signal to the cathode. Both control picture intensity.


On the side of your monitor is a box-like cage with a wire that goes to the CRT. This is the EHT supply. It performs several functions, one of which is to supply the high voltage for the CRT.

The input to the EHT supply is at pin eight of P900 where 40 volts AC is fed through a large resistor, R900. Actually, this is a VERY important resistor because it limits the current to the oscillator, keeping it from taking off on its own and increasing the high voltage to the point where X-rays are emitted from the CRT, which is DEFINITELY NOT GOOD.

Did we mention an oscillator? What's an oscillator? Well, in this case, it is made up of: transistor Q903, the primary winding of the "flyback" transformer, and a few other components that toss the voltage back and forth (oscillate) 25,000 times each second. By doing this, it electromagnetically induces a bigger voltage in the "flyback" transformers secondary winding since it is bigger. This voltage is rectified (chopped up) by diode D904 to get 12,000 volts DC in Electrohome monitors and 14,500 volts DC in Wells Gardner monitors. This voltage is used to light up the CRT (picture tube). The other transistors, from Q900 to Q902 and their circuit components keep the power to the oscillator steady or regulated, as they say in engineering. There is an adjustment control, R905, to make certain the oscillator is fed the proper power.

The "flyback" transformer also has an additional secondary winding which generates more voltage to power other circuits. At pin three of P900 there is about 400 volts DC for focus voltage to the CRT. This can be adjusted with R909, the focus control. From pin five at the other side of the "flyback" transformer secondary winding, there is 90 volts DC for the "Z" amplifier circuit. In between pins three and five of P900 there are two diodes and capacitors that change the AC from the "flyback" secondary winding to DC just like the power supply. In fact, that's just what it is, a "mini power supply'..

The CRT has already been described indirectly. However, to make a picture or turn the CRT on, certain voltages are needed. Otherwise it won't work. These are: about 6 volts AC (note that's AC) is needed for the heater filament in the tube neck to light up; the electron beams intensity must be controlled by the "Z" amplifiers signal which is applied to the CRT's cathode; there must be voltage at G1 of the CRT for brightness; there should be about 400 volts DC at G2; there should be focus voltage which varies but can go as high as 400 volts DC; and there should be high voltage at the anode of the CRT which runs into the thousands of volts (this voltage can jump almost one inch - so BE CAREFUL!!)

Always remember that a monitor can bite like a snake. Even when it is turned off, capacitors hold voltage and will discharge it to you should you be touching chassis ground. The CRT or picture tube, itself, is a giant capacitor, so avoid the flyback anode plug hole. With the monitor on, the power supply circuit and/or the flyback, which puts out at least 12,000 volts, CAN BE KILLERS!! Avoid handling power transistors (usually output transistors), yoke terminals, and other high power components when the monitor is on.

WARNING: That picture tube is a bomb!

When it breaks, first it implodes, then it explodes. Large pieces of glass have been known to fly in excess of 20 feet in all directions. DO NOT carry it by the long, thin neck. Discharge its voltage to ground by shorting the anode hole to ground.

Use a plastic handled screwdriver, connect one end of a wire with an alligator clip at each end to chassis ground and the other end to the metal shaft of the screwdriver. Using ONE HAND ONLY (put the other in your pocket) and touching ONLY the plastic handle of the screwdriver (DO NOT TOUCH THE METAL SHAFT) stick the blade of the screwdriver into the anode hole. Be prepared for a fairly loud pop and a flash. The longer the monitor has been turned off, the smaller the pop and dimmer the flash. But BE CAREFUL, picture tubes will hold a very healthy charge for at least a week if not longer. Even after you've discharged it once, it may still carry a residual charge. It's better to be too careful than dead, which is why electronic equipment always carries stickers referring servicing to qualified personnel. Handle the side with the viewing screen against your chest when changing it. ALWAYS wear safety goggles when handling the picture tube.


If you are totally confused about where to begin to hunt for a problem, and can't find the problem in the "SYMPTOM DIAGNOSIS" subsection, there may be another way to proceed.

Take a VOLTMETER and (if possible) an oscilloscope and begin probing the jacks. You can start with the input jack to the monitor. Using the oscilloscope, make sure both the "X" and "Y" information is present (which it isn't during the "SOUND" test).

NOTE: It is advisable to use one of the games test patterns (obtained when you put the game into the Self-Test mode) when using the oscilloscope. The simple diamond one is a good choice. This way the "X" and "Y" information at the above jack isn't changing and a recognizable wave form is easy to see if it's there. The DC voltages tend to jump around like crazy when the game is being played or is running through its ATTRACT mode, so, using the test pattern tends to keep them still.

Next, use the volt meter to make sure the other voltages are present at each pin. Similarly, you proceed to P500 on the deflection board, and P900 on the EHT unit to make sure all the correct voltages are present. Use the schematic to determine what the correct voltages should be.

Check the pins on the CRT to be sure the voltages are getting this far. If everything looks good to this point, perhaps the CRT is bad. DO NOT check the anode voltage unless you have a special high voltage probe or you may wind up repairing X-Y monitors in heaven.

DO NOT BE FOOLED by the silent operation of the monitor. Regular T.V. sets and monitors buzz and crackle a lot when they're operating — this is normal for them. BUT, Vectorbeam monitors are noiseless unless something is wrong.

Whatever you do, ALWAYS read the literature that comes with any test equipment you use so that you will not damage the equipment, the monitor, and most of all YOURSELF.

Was this article helpful?

0 0

Post a comment