Blanking

Fig. 2. A block diagram of the monitor signal processing circuits, discharging the 4 7 uF capacitor in the collector circuit When the trigger pulse i$ finished, this capacitor begins to charge through the HORIZONTAL SIZE control, producing a voltage ramp that is amplified by U9. The output of U9 drives the complementary output transistors (Q4 and Q5), which have the deflection windings in their emitter circuit (pins 5 and 6 of the main board). Feedback to the inverting input of U9 via the 100k resistors helps to linearize the deflection waveform. The HORIZONTAL CENTERING control feeds a voltage to the inverting input which provides centering of the trace,

Vertical deflection (Fig, 6), The vertical circuits, with a few exceptions, function much like the horizontal circuits, with the vertical deflection windings connected to pins 3 and 4 of the main board, The differences involve the use of a much iarger discharge capacitor (2200 uF) to accommodate the 200-second sweep interval and the fact that the capacitor is cycled by a switch (the sweep switch) rather than a transistor. In the RUN position of the DPDT centeroff sweep switch, a reed relay (K1) on the main board is pulled in, causing its contact (K1A) to short out the capacitor and reset the sweep at the top of the screen. When cycled to the center-off position (RUN), the contacts of K1 open, permitting the capacitor to charge and initiating the sweep down

Fig. 3. Monitor video circuits. Unless otherwise noted in this and other schematics, all resistors are Va W, 5% carbon film, all decimal value capacitors are 50 or 100 V dipped mylar, all capacitors between 1 and 10 uFare 16 to 35 V tantalums, and all higher value capacitors are aluminum electrolytics rated at 16 V. All unmarked diodes are general-purpose switching types (1N457, 1N914, etc.). Circled numbers refer to board pinouts, while circled letters refer to interface points on other schematics.

Fig. 3. Monitor video circuits. Unless otherwise noted in this and other schematics, all resistors are Va W, 5% carbon film, all decimal value capacitors are 50 or 100 V dipped mylar, all capacitors between 1 and 10 uFare 16 to 35 V tantalums, and all higher value capacitors are aluminum electrolytics rated at 16 V. All unmarked diodes are general-purpose switching types (1N457, 1N914, etc.). Circled numbers refer to board pinouts, while circled letters refer to interface points on other schematics.

the screen. The other position of the switch (FOCUS) grounds the bases of Q6 and Q7( centering the trace on the screen. 1 his position is used to focus the CRT or the film camera and also serves as a standby position when the monitor is not displaying pictures. The tow-resistance vertical windings would cause one or the other of the deflec-

"-■ • . ' " - Vr ..J . ■ "- 1 . - -

tion transistors to draw excessive current if left for long periods in the RESET or RUN positions, in the FOCUS position, neither transistor is drawing current, and they will sit indefinitely without overheating.

Mainframe wiring (Fig. 7), Much of the mainframe wiring has been covered in previous circuit descriptions. In order to eliminate the chance that power transformers would cause unwanted deflection of the scanning beam, it is suggested that the power supply be remoted from the main cabinet. The mainframe does contain the + 5, +15, and -15 V regulator ICs, driven from the unregulated voltage buses from the remote sup-

ply, If the regulators were mounted on the supply itself, you would get unwanted voltage drop in the connecting cable. In addition, the mainframe contains the high-voltage module for generating the + 7-10 kV required for the CRT external anode. The high voltage module will be discussed in detail in the construction section. Regardless of the approach taken, however, it is desirable to power the module from its own regulators to minimize interaction with other circuit components.

Power supply (Fig. 8), This supply is one possibility for generating the required voltages, A low-voltage transformer (T3) provides the unregulated voltages via a bridge rectifier and filter capacitor network. The +350 volts required for the internal anode and focus grid of the CRT as well as the BRIGHTNESS network is obtained via a conventional power transformer (T2) and a full-wave rectifier assembly. T2 also provides the 6 3 V ac required by the CRT filament. The mainframe and power supply circuits are shown with Cinch Jones P-B08-AB and S-3D8-AB connectors, respectively. An 8-conductor cable with an S-308-CCT on one end and a P-308-CCT on the other is used to interconnect the monitor and its supply. The power supply is turned on via an ac lead actuated by an SPST switch (S3) on the mainframe.

Construction

Main Circuit Board

Fig. 9 shows the land layout for the main circuit board which carries most of the active circuits. If you do not have facilities for making boards, this particular board is available — a point that will be covered at the end of the article. The component layout is shown in Fig. 10 as viewed from the component side of the board. The main puzzle in wiring the board js the cluster of unused holes between the LM380 and the LM565. These holes mark the demise of an idea that was better on paper than it was in prac-

tig. 4. Monitor sync and phasing circuits.

Photo C. An IR frame from COES £ displayed on the prototype monitor Despite the small screen size, the picture is relatively clear, showing a major tropical storm off the west coast of Mexico. The IR format displays cold objects as white (space and high cloud topsI while warmer areas are darker, Note the warm surface temperatures in the southwestern US and northern Mexico in this frame. The use of a somewhat larger CRT would assure the full 800-line resolution in the APT pictures.

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