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: INSTRUCTION MANUAL
For The
CG126 COLOR GENERATOR
INSTRUCTION MANUAL
for the
CG126 COLOR GENERATOR
Recognizing the need for a stable, low cost generator that produces
all necessary patterns for accurate convergence and trouble shooting in
the home, SENCORE has developed the CG126 Standard Color Bar Generator.
The CG126 is the smallest, lightest, and least expensive generator
of any on the market and yet it provides the same stable patterns of
generators costing twice as much. The CG126 provides the following
patterns:
Ten Standard Color Bars: The type and phase that is fast becoming the
standard of the industry. Crystal controlled keyed bars (RCA type with
30 degrees phase change between each) as explained in most service lit
erature, offer a complete gamut of colors for every color circuit test.
Adjustable Size White Dots: New stabilized dots, a must for convergence,
are created by new SENCORE counting circuits. The size of the dots are
adjustable from the rear of the unit.
Crosshatch Pattern: A basic requirement for fast CRT convergence, and,
also, for sweep linearity and "overscan" adjustments.
Vertical and Horizontal Bars: An added feature to speed up convergence,
not found on many other color generators.
SPECIFICATIONS
RF Output: Frequency is factory set to Channel 4. It can easily be
changed to Channel 3 or Channel 5, if Channel 4 is used in your
area.
Modulation: Any one of five patterns: Color Bars, Dots, Crosshatch,
Vertical Bars, or Horizontal Bars.
Tube Complement: 6-12AU7A, 2-12AZ7, 1-12BE6
Diode Complement: 2-1N295
Crystal Complement: 1-189KC t .005% for timer, 1-3563.795KC .001%
for color Lars.
Power Consumption: 35 watts at 117 VAC line.
Size: 11 inches wide by 8 inches high by 6 inches deep.
Net Weight: 9 1/2 pounds.
CONTROLS ON THE CG126
The CG126 is an extremely easy generator to set up and use. Only three
front panel controls are needed in normal use. These are the AC power switch,
the Pattern switch and the Color Output control. Additional adjustments for
the timing circuits, and for dot size, are available on the rear of the unit,
but these are all factory set and normally do not need to be changed.
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AC Power Switch: The AC power switch is used to turn the unit ON and OFF.
It also has a middle STANDBY position which is used whenever you wish to
disable the pattern and RF carrier, but still keep the tube filaments hot
for instant use.
Pattern Switch: The pattern switch selects one of the five patterns avail
able to modulate the RF carrier. Composite sync is fed to the modulator at
all times so that the patterns will not jump or fall out of sync when
switching from one pattern to another. The following patterns are available
on the CG126, each for a specific function.
1. Dots - There are 117 small size dots available, primarily for DC
convergence. A color TV set that is properly converged will have
white dots in the center of the picture tube (all three guns hit
ting the same spot on the CRT). Static (or DC) convergence is
controlled by the 3 small magnets spaced at 120 degrees around
the neck of the color CRT plus the blue lateral positioning magnet.
2. Crosshatch - In the crosshatch position, 9 vertical and 13 hori
zontal visible bars are generated. The crosshatch pattern is used
for dynamic convergence adjustments, overscan adjustments (height
and width) and for linearity adjustments.
3. Vertical Bars - Nine vertical bars are generated in the vertical
bar position. These are used primarily when adjusting the dynamic
vertical convergence controls.
4. Horizontal Bars - Thirteen horizontal bars are provided and are
used primarily when adjusting the dynamic horizontal convergence
controls.
5. Color Bars - Ten color bars are generated for color alignment and
trouble shooting in the color circuits of the TV receiver. The
color output is controlled with a separate control. The colors
which would be displayed on a normal color set are shown on the
upper right of the panel.
Color Output Control: The color output control changes the amount of color
signal that is fed to the modulator. It is used primarily to check the color
sync abilities of the receiver. A setting of 100 percent is normal. With
most receivers, the control can be turned to almost zero percent before the
set will lose color sync. This is indicated by diagonal bands of color
(barber pole effect) within each color bar.
The 200 percent setting of the control is used to force a defective
set to sync while trouble shooting.
OPERATING THE CG126
To use the CG126, plug the AC cord into a 117V AC outlet, turn the
unit on and let the generator warm up for approximately 10 minutes. Connect
the output cable to the antenna terminals of the TV set. Tune the TV set to
Channel 4, or to the same channel as the CG126 if the CG126 has been reset
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to some other channel.
Turn the pattern switch to the desired pattern.
Sharp, well defined patterns are necessary for convergence adjust
ments and for trouble shooting. Since the settings of the TV controls
will affect the quality and sharpness of the patterns produced by the
CG126, we will discuss briefly how these TV controls should be set. In
all of the following steps, the CG126 is on and the output cable is con
nected to the TV antenna terminals. The TV is set to Channel 4.
Fine Tuning - Turn the CG126 to color bars and adjust the TV fine tuning
control for sharp, clear bars with a minimum of smearing on the edges.
This is the best tuning point for the TV and the control should be left
at this setting for all black and white patterns, also.
Occasionally, however, a higher dot contrast range (dot brightness
versus background) can be achieved by mistuning the TV slightly to favor
the dot frequency.
Contrast and Brightness Controls - Since the CG126 has a minimum amount
of background "hash" on any of the patterns, the settings of the contrast
and brightness controls can be set to your liking. However, for conver
gence adjustments it is desirable to have a high contrast between the
pattern and the background level in which case the contrast control should
be turned to near maximum.
When viewing color bars it is sometimes desirable to eliminate the Y
signal completely by turning the contrast control to zero.
Vertical Hold - All color TV sets have vertical retrace blanking. If the
vertical hold is not adjusted properly, retrace lines can be seen in the
upper part of the picture. Adjust the vertical hold control for the mini
mum number of retrace lines consistent with good vertical hold.
Horizontal Hold - On some sets there may be a slight "fanning" at the top
of the picture of all vertical lines in the pattern if the horizontal hold
control is not adjusted properly. These can be virtually eliminated by
adjusting the horizontal hold control. A high contrast setting may also
cause this effect.
SETTING UP AND TROUBLE SHOOTING COLOR TV WITH THE CG126.
Setting up and trouble shooting color TV is easy with the CG126. In
addition to a good generator, however, an established procedure should be
followed so that time will not be wasted in following misleading conditions.
We like to think of color TV trouble shooting as a three step approach.
The first step is to be sure that the color CRT and associated circuits
are operating properly. This involves checking for purity and convergence.
The second step in the procedure is to get a good black and white picture.
If the trouble is not solved by the first two steps then it must be in the
color circuits which brings up the third step of trouble-shooting the color
section of the receiver
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Let's see how this works with the following example. If a color set
is tilted on its side or even tilted up as little as 30 degrees the purity
of the CRT will go off (the electron beams from each gun will hit all three
colors of phospher dots instead of just their own color). Under this
condition the black and white patterns from the CG126 (dots, crosshatch etc.)
will look quite presentable and you may not even be able to detect anything
as being wrong. However, if you switch to color bars, the colors will be
strange or completely gone and your first impression would be to start
analyzing the color circuits. Thus, you can see the importance of checking
for proper purity and convergence first.
Checking and Adjusting Purity. Tune the TV to an unused channel and ground
the blue and green grids on the CRT through 100K resistors. Slide the yoke
back and adjust the purity rings for a solid red spot that fills the center
of the screen. Slide the yoke forward until the screen becomes completely
red. At this point, connect the CG126 to the TV set, tune the set to
Channel 4, set the pattern switch to crosshatch, and check for proper over
scan adjustments. See Figure 1 for correct pattern.
Figure 1. Crosshatch pattern.
It is sometimes possible to get good purity only by pushing the yoke too
far forward, which, of course, reduces the width and height, such that the
set does not overscan properly. Refer to the manufacturer's service litera
ture for proper overscan.
If purity cannot be obtained by normal adjustments it is probable that
the CRT screen mask has become magnetized, in which case a degaussing coil
is needed. Move the coil around the picture tube screen for about one minute.
Then remove the coil about 6 feet from the set and turn it at right angles to
the CRT before turning it off.
Checking for Static and Dynamic Convergence. Connect the CG126 to the TV,
tune the TV to Channel 4, and set the pattern switch to dots. Observe the
dots near the center of the screen. The electron beams from all three guns
should hit the CRT at the same point. If they do not, adjust the beam posi
tioning magnets and the blue lateral magnet until they do. This is known
as DC or static convergence. Note that only the dots in the center of the
screen are used in this check.
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For dynamic convergence the crosshatch, vertical bar and horizontal
bar patterns are used. Dynamic convergence procedures are slightly dif
ferent for each color set and to make these adjustments to specifications,
the service literature for the set should be followed.
The second part of the procedure is to check all of the circuits which
will affect the black and white picture. These circuits in color TV sets
do not differ appreciably from the circuits found in black and white sets
except for an additional group of controls used for black and white track
ing adjustments. This group consists of separate controls for the screen
voltage on each CRT gun plus a B gain, G gain and a CRT bias control. In
older sets "background" controls took the place of the B gain, G gain and
CRT bias controls.
To adjust these controls you should refer to service literature for
the set, but basically the screen controls are adjusted for equal intensity
of each color whereas the B gain, G gain and CRT bias controls are adjusted
for the best black and white picture, throughout the normal range of the TV
brightness and contrast controls.
Other slight differences in color TV circuits are a regulated high
voltage supply to reduce blooming and loss of convergence, a separate focus
voltage supply and a wider IF response to pass the color information.
If the CRT is functioning properly and a good black and white picture
can be obtained, then the third step on a defective color set is to check
the color circuits. The color circuits are not difficult to service if you
understand the various functions that they perform. They consist of the
following:
Band Pass Amplifier - The band pass amplifier is used to separate
and amplify, from the video signal, the band of frequencies from
3 to 4.1 megacycles. All of the transmitted color information is
within this band.
3.58 Megacycle Reference Oscillator - The reference oscillator
is used to restore the color carrier in the demodulator circuits
of the color TV set. It is kept in phase with the original car
rier at the transmitter by controlling it with the color bursts
which are on the horizontal back porch. The oscillator provides
two signals separated by a 90 degree phase shift for the demodu
lators .
Demodulators - The demodulators beat the reference oscillator
signals against the chroma signal from the band pass amplifier
to produce color information signals for the CRT grids. In Zenith
sets the color gun grids are driven directly from the demodulator
plates. In other sets, the color signals are amplified in R-Y,
G-Y and B-Y amplifiers before being applied to the grids.
Burst Amplifier - The burst amplifier separates and amplifies the
burst pulse from the video signal to control the 3.58 megacycle
reference oscillator.
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Color Killer - The color killer circuit produces a negative bias
in the absence of a color signal to cut off the.band pass ampli
fier. It is controlled by the output of the phase detector.
Phase Detector - The phase detector compares the burst signal
from the burst amplifier with the reference oscillator signal.
It controls the phase of the reference oscillator signal and,
also, controls the color killer.
To troubleshoot these circuits, you need the CG126 set to the color
bar pattern and a good wide band service oscilloscope with a low capacity
probe such as the SENCORE Model PS120.
Let's start with the band pass amplifier. Zenith uses a two stage
amplifier, See Figure 2, whereas RCA uses a single stage amplifier, See
Figure 3. Waveforms (Wl, W2 etc.) that you should get with the CG126 are
shown at each input and output of the amplifiers.
1 V PP 4V PP 8V PP
Fig. 2. T w o - s t a g e ba nd pa ss a m p l if i e r in Zenith Chassis 2 7 K C 2 0 receives input from v id eo detector.
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50V PP