Filter
Top Products
Theory of Operation
3–32
1780R-Series Service Manual
Diagram 11 Horizontal Magnifier & Output Amplifiers
The Magnifier Amplifier provides selection of fixed voltage gains for the Line
and Field Ramp Generator sweep ramps. This provides the time per division
variation on the horizontal axis of the CRT display. In addition, the Magnifier
Amplifier provides variable amplification for the sweep ramps, external
horizontal input (ICPM mode), and external RGB staircase. The Magnifier
Amplifier drives the Horizontal Output Amplifier which drives the CRT
deflection plates. The input to the Horizontal Output Amplifier is switched
between the sweep ramp and the Microprocessor-generated readout signal to
provide display messages and menus.
In the magnified sweep modes, only a small portion of the line or field is
displayed on the CRT. The remainder of the time the beam is off to the left or
right of the screen. Without blanking during this off-screen time the CRT would
appear to be backlit.
Magnifier Amp. The Magnifier Amplifier is an inverting, feedback amplifier with
its summing junction at its input (pin 2 of U580D). The amplifier voltage output
is at the collector of Q573. Amplifier gain is controlled by the combination of
the feedback resistors (R569 and R478), the attenuator resistors (R472-477), and
the input resistance. The input resistance for the sweep ramps is on Diagram 10
(R581). The other input resistances are R378 and R576 for the horizontal
positioning voltage, R683 for the external horizontal input, R682 and R765 for
the RGB staircase input, and R678 for the slow sweep ramp. CR579, CR581,
and CR582 are back biased when the sweep ramp is selected and individually
turned on when their input is selected for display.
The attenuator resistors (R472, R572, R477, R470 and R471, R473 and R474),
along with R476, when switched on alter the feedback resistance. When one or
more of the transistors, in series with the attenuator resistors, is saturated the
Overview
Circuit Theory