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Circuit Theory
1740A/1750A/1760 – Series Service Manual
3–19
Diagram
12
Front Panel
There are 28 momentary contact switches arranged in a matrix. When U7, a
serial in/parallel out shift register, is loaded, shifted, and read, PB8 – PB10 are
pulled low along with the CS (chip select and SH/LD (shift/load) for U6 and U8.
When one or more of the push-button switches is closed a low state is loaded
into one of the U6 parallel inputs. The levels on the inputs are clocked into and
through the serial output by the BSCK (buffered serial clock). The serial output
is put onto the MOSI (serial interface bus) to be read by the Microprocessor.
Low levels to complete the LED circuits are loaded into the serial in/parallel out
shift registers (U2, U3, U4, and U5) from BMOSI (buffered serial interface bus).
Levels are then shifted into the parallel register by the LED CS and clocked out
by the LED EN, which is delayed by U10A, a D-type flip-flop.
Diagram
13
DACs & Digital Control
The reference signal is ac-coupled through a tuned circuit, C102 and L11, to
drive the Chroma Amplifier, Q33 and Q34. Luminance is removed and in
normal operation the chrominance is amplified by about three times.
U12, Q13, and Q14 form a chrominance switch that selects either chrominance
from the Vertical Amplifier (Diagram 4) or the SCH Signal (1750A/1760
[Option SC] –Series) to drive the chrominance input to the Demodulators
(Diagram 16). Q13 and Q14 clamp the input side of U12A or U12D to ground
when the switch is open.
U18 is a series of serial input DACs. Serial data is loaded from BMOS (buffered
serial bus) when LD goes low. Output levels, from the internal sample and
holds, are clocked out by the buffered serial clock (BSCK) from the Micropro-
cessor.
U19 is a serial in/parallel out shift register used to decode the vector/SCH-related
instructions from the serial bus. It outputs chip selects and enables for SCH
board identification, to control the DACs, and to enable the Vector Locked, SCH
Locked, and Burst Present outputs to the serial interface bus.
U22 is a serial in/parallel out shift register to output asynchronous switching
signals for the vector and SCH modes.
Switching
LED Drivers
Chrominance Amplifier
Chroma Switch
DAC
Serial Address DeMUX
8-Bit Serial Output
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Содержание 1740A Series
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