Star Tech Journal

24
STAR*TECH JOURNAL/JANUARY 1984
Troubleshooting Electrohome & Wells Gardner Monitors (Part 2) continued from page 23.
The frequency of the vertical circuit is a bit less than sixty cps
( cycles per second). It takes one-sixtieth (1/60) of a second to
scan one field. However, one complete frame takes one-thirtieth
( l /30) of a second. So actually there are thirty complete frames
or pictures per second.
Since we know how many frames per second there are, taking the
lines per second determines the frequency of the horizontal
oscillator. Five hundred and twenty-five lines time thirty frames
is equal to fifteen thousand, seven hundred and fifty. This is the
frequency of the horizontal oscillator.
Since the monitor's oscillators are set at the same frequency as
the logic board's sync signals, the logic color circuit's output
(RGB VIDEO) is displayed correctly on the monitor.
Blanking and Beam Limiter
At this point it would be good to mention briefly the purpose of
the blanking and beam limiter circuit. The blanking circuit
blanks or turns off the electron beams inside the CRT during
horizontal and vertical retrace time. As the beams are moving
from left to right across the screen, this is called trace or scanning
time. When scanning, the electron beams are on, allowing the
video information to be viewed on the screen. Retrace or
blanking is the time when the beams move from a right to left
direction. No video is wanted at this time, so the beams in the
CRT are cut off.
The blanking circuit consists usually of a transistor circuit,
which receives a horizontal and vertical sampling pulse from
their respective circuits. This blanking transistor is connected
directly to the color video output transistors on the neckboard.
There are three transistors, one to drive the red, another for green
and also one to drive the blue color guns ( cathodes) in the CRT.
During blanking time, either the horizontal or vertical pulse is
coupled to the blanking transistor. Then, its output turns or cuts
off the video output transistors. Therefore, the beams of retrace
time are not viewed.
The beam limiter also is connected to the video output transistors.
Without the beam limiter circuit, as the picture changes from a
dark to light, or light to dark screen, the beam current inside the
CRT would change. The wide change of beam current will cause
the picture to bloom out and have a blurry focus.
The beam limiter keeps the beam current within a given
operating range to prevent this from happening. If the cathodes
are made more positive, then less beam current flows and the
brightness is decreased. The beam limiter samples the second
anode current, then the CRT beam is increased or decreased as
needed.
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