Hal Communications CWR6850, Instruction Manual

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CHAPTER 5 PAGE 40
5.8 Using a Television Set or External Monitor
Although the internal cathode ray tube (CRT) of the CWR6850 produces an excellent display, you
may wish to use an external TV Monitor or TV set to display the text; a larger monitor tube will
give a larger character size display. Any standard composite video monochrome TV monitor may
be connected to the VIDEO OUT connector on the CWR6850 rear panel. The video output is 1 V
PP
,
composite video, 75 ohm impedance. Connection of an external monitor does not affect the dis-
play on the internal CRT of the CWR6850.
Some good quality black and white TV sets can be easily modified to serve as video monitors. The
change does not affect normal operation of the set except that the video cable from the CWR6850
must be removed from the TV when the set is to be used to receive television programs. It is im-
portant that the selected TV have adjustments available for both horizontal and vertical size. Nor-
mal TV alignment results in "over-scan" of the viewing screen, preventing margin lines but also los-
ing parts of the picture at the edges and corners. To avoid losing characters of the CWR6850 dis-
play along the corners and edges, it may be necessary to "shrink" the size of the TV raster with
the width and height controls. These adjustments will be made after the TV set is modified and
when you can look at the CWR6850 video output.
*** CAUTION ***
DO NOT attempt to use as a monitor any television receiver in which one side of the AC line is con-
nected to the chassis or circuit ground of the set unless you supply AC power to the set through
an isolation transformer.
The modification is simply a matter of capacitively coupling the external video signal to the input
of the first video amplifier stage in the TV set. Figure 6 shows a typical transistor TV video circuit.
Although the component values and the biasing scheme may be slightly different in your set, the
circuit should be essentially as shown. The CWR6850 video signal is injected at point A.
The modified circuit is shown in Figure 7. Connect the negative end of a 100 µF, 16 V electrolytic
capacitor to the base of the first video amplifier transistor. Mount a UHF (or BNC) video connector
on the cabinet as close as possible to the capacitor connection. Connect the center pin of the coax
connector to the positive end of the electrolytic capacitor. Use hook-up wire if the connection is
less than 6 inches; use a small coaxial cable such as RG174 for longer connections.