Kristel 1428-30k series, Руководство по обслуживанию

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1428-30k series, AUTO BIAS
AUTOMATIC BIASING VIDEO SYSTEM
PURPOSE
The purpose of the system is to establish and maintain a constant and correct black level (cutoff level) on
the CRT screen, without the need for manually adjusting the biasing of the individual video amplifiers.
This has the advantage of greatly simplifying the initial setup, plus maintaining the biasing conditions
throughout the life of the CRT as it ages. It also eliminates the need for individual red, green, and blue
bias potentiometers, and even a brightness control. The Automatic Biasing System is integrated into the
video amplifier system.
OVERVIEW OF OPERATION
The system works by sending a DC correction to each of the video amplifiers, in response to cathode
current changes resulting from a fixed step in G1 voltage. During a setup and sampling period after ver-
tical retrace, a fixed step is applied to the G1 grid, while the three cathodes are held steady. When bias
conditions are correct, this voltage step results in a small step in cathode current for each gun. This step
produces a faint horizontal line at the top of the screen, visible if the vertical size is set low enough.
OPERATION DESCRIPTION
Video System:
The basic video system consists of a three-channel video processor IC201, and three
video output stages (Q205, Q202, Q206, and Q207 comprise the Red Video Output stage). The Auto-
matic Biasing System adds a Bias Control Processor, IC202, plus Beam Current Buffers IC203B-D, a G1
control circuit IC203A, and clamping circuits. IC201 contains three video preamplifiers with DC-
controlled gain and DC clamping. IC202 contains timing circuits, sample-and-hold circuits, and buffers.
The three video channels are identical, so only the Red channel will be described in detail. The Red video
input signal is terminated by R201, and is delivered to the input of IC201 at pin 4 through R274 and AC
coupling capacitor C201. Pin 11 supplies a DC bias level for the video input signal through R204.
IC201 increases the 0.7V p-p nominal video input level to about 2.7V p-p at pin 25, which then drives
the base of Q202. The exact amount of gain is determined by the DC level on pin 12, the Contrast con-
trol input. This DC level controls the gain of all three video channels simultaneously.
Q202 and Q205 operate as a cascode video output amplifier, with a gain of about 15. The video swing
at Q205's collector is about 40V p-p. Q206 and Q207 operate as a buffer, driving the red CRT cathode
through R242.
Q201 is driven by a horizontal retrace signal, and supplies a negative-going clamping pulse to IC201 pin
14. This provides DC restoration for the video signal by forcing the output of the video processor,
IC201 pin 25, to the same DC voltage as on pin 24, during horizontal retrace time.