Star Tech Journal

22
STAR*TECH JOURNAL/JANUARY 1984
BALLY MIDWAY
TROUBLESHOOTING ELECTROHOME & WELLS GARDNER MONITORS (PART 2)
We continue our 4-part series with "Lighting the Screen•: Part
1 covered the power supply section. Parts 3 and 4 will appear in
our February and March issues consecutively.
The outside of the tube is coated and connected to ground. The
inside has a high voltage potential. Both sides are separated from
each other by glass ( an insulator). Because of this, the CRT acts
as a very large capacitor.
LIGHTING THE SCREEN
CRT Construction and Operation
The picture or CRT is an output device. In other words, the end
result of the circuits working inside the monitor is displayed on
the screen. If the video information and sync signals are good
feeding to the monitor's input, then the display should be normal
(providing the monitor and wiring are good).
The physical structure of the CRT is a large glass envelope
which is under a vacuum ( inside, when made, the air has been
removed.) If air enters into the tube, the heater will not light.
Never carry the CRT by its neck. This is the most fragile part.
Being that no air is inside the tube, if broken, the air outside could
rush inside the CRT and cause an implosion.
Elements inside the picture tube consist of the heaters. The
heaters or filaments are used to heat up the cathodes. As the
cathodes heat up, they begin to boil off electrons. The cathodes
also is where the video information is brought onto the tube to be
displayed. An open cathode can cause the screen to be missing a
color. Same as with an open filament. A shorted cathode would
normally give the screen a dominate color with retrace lines.
If ever the high voltage anode lead is to be removed, discharge
the second anode to ground while the monitor is off. Use a
jumper lead connected to the metal blade of a screwdriver.
Attach the other end of the jumper lead to ground (chassis).
Holding the insulated handle, place the metal blade of the
screwdriver under the rubber cup to the second anode lead. A
loud pop may be heard. Disconnect the high voltage lead from
the tube.
Normally, as long as the proper voltages are present to the tube,
the electron beams reach the front of the screen. This should light
the front of the CRT. If a dark screen is obtained, try turning up
the brightness. Still no lit screen and proper voltages are present
- then it may be a defective CRT.
Brightness Problems
Brl~htness too high
1. Screen or black level control adjusted up too high.
A. Electrohome - Rl 1 screen control
B. Wells Gardner - VR201 black level control
The next grid approaching toward the front of the screen is the
control grid Very little can be said about this grid. It is connected
to ground and is to control the amount of electrons being drawn
to the front of the screen.
Moving on, we come to the next two grids, These are the first
anodes: accelerator or velocity grids. They are more commonly
known as the screen or G2 grid and are used to draw or attract
the electron beams to the front of the screen where the focus
voltage on the-grid narrows the beams to give a sharp, focused
picture.
The electrons are first boiled off the cathodes by the heaters.
Then, the more positive voltage potential at the screen grid
attracts the beam of electrons down the center of the neck
towards the front of the screen. The focus makes the beams as
fine as possible for good picture definition.
Finally, the electron beams arrive in the area of the bell of the
CRT. As they approach the front of the tube, the beams come up
to the shadow mask.
The shadow mask is a large metal sheet which is parallel to the
inside front of the tube. Tiny slots in the mask allow the electron
beams to pass through to strike the screen.
The mask is made and positioned inside the tube, so that the
electrons strike its proper color phosphor. If the mask is loosened
and shifts, the screen will exhibit a very bad purity problem
which cannot be corrected except by tube replacement.
After the shadow mask is the front of the tube. Here on the glass,
color phosphors are struck by the electron beams and colored
light is illuminated on the screen.
There is a conductive coating on the inside of the bell of the tube.
This coated material is connected to the second anode. The
second anode is where the high voltage is brought onto the CRT.
The electron reaches its final destination when it has reached the
second anode.
2. Possible crack or open connection on neckboard.
3. Three color screen controls on neckboard set too high.
A. Electrohome controls (Rl 13, Rl 14, Rl 15)
B. Wells Gardner (VR403, 404, 405)
4. Low DC voltage to collectors of color output transistors on
neck board:
A. Electrohome voltage on collectors ofXlOl, Xl02, and
X103 should be about 160 to 170 volts DC.
B. Wells Gardner measures normally around 95 to 115
volts DC (TR401, 402, 403).
5. On Electrohome monitor, check for shorted diode, D503
(located near the flyback transformer).
6. Beam limiter or blanking transistor leaky or shorted.
A. Electrohome - X303, X304
B. Wells Gardner - TR201, TR202
7. Defective CRT (internal short)
Brightness too low
1. Screen or black level control adjusted too low.
A. Electrohome - Rl l, screen control
B. Wells Gardner - VR201, black level control (located on
interface board).
2. G2, screen grid bias voltage too low.
A. Check pin 7 of CRT socket for approximately 450-500
volts DC. If the voltage cannot be adjusted to this range,
troubleshoot screen control circuitry.
Continued next page.