Star Tech Journal

23
STAR*TECH JOURNAL/JANUARY 1984
Troubleshooting Electrohome & Wells Gardner Monitors (Pan 2) continued from page 22.
Weak Emission Cures
For weak emission there are four possible ways to correct the
problem. Whether or not any step will restore the monitor
depends upon the condition of the picture tube itself.
Weak emission usually shows up as a washed out picture. The
color is not quite as vivid as it could be. One of the colors may be
weak or missing. The picture tube looks blurry or out of focus.
These conditions can be exhibiting a possible problem of weak
emission.
Checking the AC color signals at the collectors of the video
output transistors should be about eight volts peak-to-peak. This
could be checked with an oscilloscope. With the proper
amplitude of signal it may be that the CRT has weak emission.
1. Turn the horizontal sync off frequency, and adjust the
brightness and the three color screens (located on the
neckboard) all the way up. Let the monitor run like this for
about fifteen to twenty minutes. Then, go back and readjust
the controls to the normal positions. Sometimes this will clear
up the weak emission. If not, try the other solutions listed.
2. The only sure way to have a sharp, crisp picture is to replace
the CRT. Some technicians may use a CRT brightener, or
rejuvenate the tube. This may resolve the weak emission, but
brighteners and CRT restoring can shorten the life of the
CRT. The last resort would be a new picture tube.
3. Heater voltage to CRT low.
. A. Dim picture, check for four to five volts AC to heater pins
on the CRT socket (pins 9 and 10).
Raster Scan
In the last section, we have seen how the electron beams are
brought to the front of the screen. The CRT itself must be good
and the proper voltages need to be present to bias the tube. Then,
the front of the tube should light up.
The word "RASTER" means basically to rake. Its definition in
regard to picture tubes is the pattern of illuminated horizontal
scanning lines formed when no signal is applied. This is why,
without any of the logic board signals connected to a raster scan
monitor, a blank white screen can be obtained.
The X-Y monitor has a different way of putting up the picture
than a raster screen monitor. The X-Y system requires that both
the X and Y signals are present to the monitor inputs in order to
get a picture. Then, ofcourse, the Z amplifier(brightness) signal
determines the amount of intensity each line drawn on the screen
should be.
Well, let's get back to the raster scan monitor. Without the
horizontal and vertical deflection circuitry inside the monitor,
the electron beams would hit or strike only the center portion of
the screen. This will display a white dot in the middle of the
screen. The beams are attracted to the front of the screen, but
they're at a standstill. Being not able to view the picture makes
the game rather difficult to play.
The horizontal deflection scans the beams from the CRT' s left to
right side, while the vertical deflection moves the beams from the
top to bottom of the screen.
Loss of vertical deflection causes the picture to collapse from top
to bottom. The screen exhibits a thin line going down the middle
of the tube. It's displayed from one side to the other, through the
longest distance of the CRT.
Horizontal collapse causes the two side to pull into the middle of
the screen. Here the thin line appears, going through the shortest
distance of the tube.
No Vertical Deflection
No Horizontal Deflection
On the neck of the CRT, close to the bell, is where the yoke
assembly is located. It has two separate coils of copper windings
and is an electromagnetic device. One coil is for the vertical and
the other is for the horizontal deflection. The horizontal and
vertical output circuits feed into the deflection yoke .
Vertical deflection moves the electron beams from top to bottom
on the screen, while the horizontal is scanning from left to right.
When the beams reach the right side of the screen, horizontal
blanking occurs and the beams are returned to the left side.
Vertical blanking happens when the beams are returned from the
bottom to the top of the screen. This procedure is repeated over
and over again, when the monitor is on.
The time of blanking is also referred to as retrace. During
blanking, a pulse arrives from either the vertical or horizontal
circuit to the blanking transistor. The transistor cuts off and
prevents any video information to be displayed on the screen.
Blanking cancels all retrace lines from appearing on the tube.
Retrace lines are a few thin diagonal lines showing across the
screen For further information on this topic, refer to the section
on blanking and beam limiter.
The horizontal signal entering its yoke winding deflects the
beams from the left side of the screen to the right. Scanning is
when the beams are moved from the left to right. During a scan of
a horizontal line, the video information is displayed upon the
screen. As the beams reach the right side, they're cut off and
brought back to the left side of the screen.
The horizontal deflection scans the beams from left to right,
while the vertical moves the beams from top to bottom. In the
time it takes to move the beams from top to bottom, there are
262½ horizontal lines scanned on the screen. Because of this, the
horizontal frequency is much faster than the vertical.
The process of interface scanning is scanning the odd number of
lines first, then the even lines on the picture tube. There are a
total of 5 25 lines when one cycle of odd and even lines has been
completed There are 262½ odd and 262½ even lines on the
screen.
The odd lines are referred to as a field and so are the even. Two
fields ( one of odd and o~e even) are called a frame. A frame has a
total of 5 25 lines.
Continued on next page.