was a multi-part series published by Radio & Television News in
the days when color TV was the domain of the more well-to-do folks on
the block. Needless to say, nobody I knew had color TV before abound
1968. One of my friend's father owned a fairly profitable gas station
and service garage, so they were the first to have one. For some inexplicable
reason, his mother never allowed more than one or two of us into the
house at a time, so we drew straws to see who got to witness that fabled
miracle of technology. I was about third in line. Insomuch as the 1960s
were a much more polite and private time than the present, peeking through
a living room window for a preview was
verboten. In fact, going into a friend's house for any reason was rare.
The privileged appointments were strictly adhered to and monitored by
my friend's mother. After a seeming eternity of days, it was finally
my turn to watch color television. The Flintstones were on that particular
afternoon. Discovering that Wilma had red hair was unexpected enough,
but a purple Dino the dinosaur nearly rocked me back on my 10-year-old
heels. Of what other essential details in the age of 'Living Color'
had I been deprived, I wondered?
Fundamentals of Color TV
Tri-Gun Receiver Circuits
By Milton S. Kiver
President, Television Communications Institute
Part 4. Tuner,
video i.f., video amplifier, and sound circuits of typical color TV
sets described in detail.
Comparing production model 3-gun, 15" color TV set with black-and-white
17" set. Color receiver uses over twice as many components.
In last month's article we examined in some detail the block diagram
of a color television receiver designed to operate with a tri-gun color
picture tube. Now we are ready to consider the actual circuits which
each of the blocks represented.
R.F. Tuner. The introduction
of color in no way alters or modifies the r.f. section of the television
receiver. Thus, the r.f. amplifier should still possess high gain and
low noise; the oscillator still provides a signal which, when mixed
with the incoming signal, will produce the desired difference or video
i.f. frequencies. For the reception of v.h.f. signals, either a turret
tuner or a continuous arrangement is employed. For u.h.f. reception,
continuous tuning is the most common method although there is also available
an 82-channel turret tuner.
A typical v.h.f. turret tuner circuit
is shown in Fig. 1. Cascode amplifiers are common in the r.f. stage,
although some manufacturers favor single high-frequency miniature pentodes.
The oscillator tube is invariably a triode, usually half of the mixer
tube. The latter may be another triode (i.e., 1/2 of a 6J6) or pentode
(1/2 of a 6U8). This arrangement requires only two tubes for the entire
In the tuner shown in Fig. 1, the cascode r.f.
amplifier uses a 6BZ7 duo-triode. One section of a 6J6 serves as the
mixer while the other section functions as the oscillator. Balanced
300-ohm and unbalanced 75-ohm (coaxial line) input impedances are provided
by a center-tapped primary winding, L101A
. All signals must
pass through a high-pass filter designed to attenuate all signals below
The secondary winding, L101B
, is tuned
by the input capacity (of the first triode unit) in series with alignment
Loading of L101B
provides the required bandpass, particularly on the lower v.h.f. channels.
The a.g.c. bias is applied to the first triode of V101
decoupling resistor R102
Direct coupling is used
between the first triode plate and the second triode cathode. This is
normal in cascode circuits. With cathode feed to the second triode,
is used to place the grid at r.f. ground potential.
Since the two triode sections of V101
are in series across
a common plate supply, the cathode of the second triode is positive
with respect to chassis ground. A divider across the "B+," consisting
, places the grid of the second
triode at a sufficiently positive potential (with respect to its cathode)
for proper operating bias.
The signal at the plate of the second
triode of V101
is inductively coupled into the grid circuit
of the mixer. At the same time, a voltage from the oscillator is similarly
brought into the mixer circuit. The mixer combines both signals to produce
the desired i.f. and then transfers this signal to the following i.f,
The oscillator is of the ultraudion variety with a front
panel fine-tuning control.
Video I.F. Section. The video i.f.
system follows the r.f. tuner. This will consist, usually, of four and
sometimes five separate stages. See Fig. 2. In the conventional black-and-white
television receiver, three i.f, stages was the number most frequently
used, although four stages were found in some sets. The increased number
of i.f, stages in a color receiver stems, in part, from the wider bandpass
required (4.2 mc.) and from the greater precautions that must be taken
to insure that the response curve will possess the right form.
The desired response curve for the video i.f. section is shown in
Fig. 3. Of particular interest is the care with which the low frequency
end of the curve must be shaped so that it provides the proper amplification
for the color subcarrier and its sidebands. Note that the curve is flat
down to approximately 41.65 mc, and then the "roll-off" is quite steep.
The steep decline is needed to prevent the sound carrier from receiving
too much amplification, producing a 920-kc. beat note at the video second
detector which would appear on the screen as an interference pattern.
Furthermore, too much sound voltage at the detector will produce a fine-grained
4.5-mc. pattern on the screen and/or sound bars. The latter effect,
of course, can occur in all television receivers, whether they be of
the black-and-white or color variety. The 920-kc. interference, however,
arises only when a color signal is being received.
Fig. 1. Typical r.f. tuner used
with color TV receiver. This is a turret-type unit for v.h.f. only,
however combination v.h.f., u.h.f, models are also used.
Fig. 2. The video i.f. circuits
of one color TV receiver. Four stages are used here 10 assure a wider
and more uniform bandpass than for black-and-white sets.
Video i.f. systems in color receivers follow the
same practice as for black-and-white receivers in so far as interstage
coupling is concerned. Most common types of coupling are bifilar coils
and/or single wound coils. For example, the circuit of Fig. 2 uses bifilar
coils predominantly (T201
), but two of the tuned circuits have single-wound
The interstage coils
are stagger-tuned, ranging from a low frequency of 41.4 mc. to a high
frequency of 45.5 mc. Also present are five shunt traps, three tuned
to the sound i.f. signal of 41.25 mc., one to the video carrier frequency
(39.75 mc.) of the adjacent higher channel, and one to the sound carrier
frequency (47.25 mc.) of the adjacent lower channel.
of sets resort to complex coupling circuits in one or more i.f. stages
in order to obtain the desired attenuation at certain trap frequencies,
such as the adjacent-channel video carrier, adjacent-channel sound carrier,
and the sound carrier of the channel being received.
Fig. 3. Video i.f. response curve
of a color TV receiver. Note the steep slope of the curve between 41.25
and 41.65 mc.
Fig. 4. Five stage video i.f. system
employed by RCA in its color TV sets.
Fig. 5. Sound i.f. and audio circuits
of a typical color television receiver.
In one RCA color receiver, a bridged-T circuit is inserted between the
tuner and the first video i.f. amplifier. See Fig. 4. The network contains
a trap tuned to the accompanying sound carrier, 41.25 mc. In order to
reduce interference from this source (i.e., cross modulation), the sound
carrier is attenuated as soon as possible in the i.f, amplifier. (The
signal is not removed completely, however, since enough must be available
for the sound system. The latter ties into the video system at a subsequent
A more elaborate bridged-T network, combined with an
m-derived bandpass circuit, is employed between the first and second
i.f. stages. This contains two rejection traps, one tuned to 39.75 mc.
(video carrier of adjacent higher channel), the other tuned to 47.25
mc. (sound carrier of adjacent lower channel). A second such complex
coupling network is found between the final i.f. stage and the video
second detector. This, too, contains two traps, one for the accompanying
sound carrier at 41.25 mc. and one for 47.25 mc.
It will be
noted from Fig. 4 that the sound take-off occurs in the plate circuit
of the final video i.f. amplifier. This does not necessarily denote
a split-sound type of receiver, as mentioned earlier, but stems from
a desire on the part of the set designer to avoid any interaction between
the color sub-carrier and the sound carrier that could produce (by mixing)
a 920 kc. beat note. The sound carrier is permitted to travel with the
video signal up to the plate of the final video i.f. amplifier and then
it is diverted to a germanium crystal where it mixes with the video
carrier to produce a 4.5 mc. signal. In the meantime, the monochrome
and color subcarrier signals proceed to the video second detector for
their demodulation. By this arrangement, the sound signal can be strongly
attenuated in the video detector thereby minimizing the development
of a 920 kc. beat signal.
Automatic gain control is applied
to the first two or three video i.f. stages in the same manner, and
for the same reason, that it is applied in monochrome receivers. The
r.f. amplifier also receives all or a portion of the same a.g.c. voltage.
Sound Channel. As indicated previously, the sound signal is
diverted from the video path in the plate circuit of the final video
i.f. amplifier. This signal and a portion of the video carrier are then
mixed in a germanium diode to produce the desired 4.5 mc. intercarrier
sound signal. See Fig. 5. This is followed by several 4.5-mc. i.f. amplifiers
and then the signal is applied to a ratio detector. Here the audio intelligence
is recovered from the FM signal. Further amplification by audio voltage
and power amplifiers raise the signal to the proper level for operating
a loudspeaker. Just how extensive this portion of the audio system is
will be governed by the price range of the receiver. If a high-fidelity
system is desired, then the audio stages can be elaborated, perhaps
by the addition of push-pull output, phase inversion, feedback networks,
etc. The system shown in Fig. 5 is commonly found in most TV receivers
where economy and good sound is desired.
The video signal is demodulated in the video detector (Fig. 7), providing
an output 0 to 4 mc. monochrome signal plus the I and Q color sidebands.
(The color subcarrier, it will be remembered, was deleted at the transmitter.)
The detector itself may be either a germanium diode (1N60 or its equivalent)
or one section of a vacuum tube. There appears to be a definite swing
toward the germanium crystal but vacuum tubes are still widely used.
Beyond the detector, both the monochrome and color sideband
signals are applied to at least one stage of amplification before they
are separated. In the circuit of Fig. 8, the output from the video second
detector is applied first to the triode section of a 6U8, then to the
pentode section. Both signals remain together only in the triode because
at the grid of the pentode, a portion of the signal is fed to the bandpass
amplifier, which is the input stage to the chrominance section of the
receiver. Hence, separation of the monochrome and color signals might
be said to occur at the output of the triode video amplifier.
The second video amplifier in Fig. 8 deals solely with the monochrome
portion of the total color signal. This fact is further accentuated
by the 3.58 mc. series trap which is present in the plate circuit of
this stage. The trap attenuates any 3.58 mc. color subcarrier voltage
which may be present here in order to prevent it from reaching the picture-tube
screen and producing a visible interference pattern. The presence of
the 3.58 mc. trap limits the response of the luminance or monochrome
channel to a somewhat lower value, usually 3.0 or 3.2 mc. Since most
present monochrome receivers operate within this bandwidth, both in
their i.f. and video amplifier systems, any loss of detail will be no
more apparent on color sets than on black-and-white sets.
this point the reader may wonder why a special 3.58 mc. trap is required
when, in fact, no 3.58 mc. color subcarrier is being sent with the signal
The answer rests in the fact that while it is true that at no time is
there any voltage at precisely the 3.58 mc. frequency, the phase excursions
of the color signal cause the carrier to move back and forth from frequencies
above 3.58 mc. to frequencies below 3.58 mc.
Fig. 6. Video amplifier circuit
using two pentodes and a 3.58 mc. trap in the first video plate circuit
for recovering the 3.58 mc. burst signal for color synchronizing.
Furthermore, most of the color energy is concentrated in
the sidebands around the 3.58 mc. frequency and if we remove the bulk
of this energy with a trap, we minimize any tendency of the color signal
to produce interference patterns on the screen.
to note is this: The frequency of the color subcarrier (and hence, the
frequency of its sidebands as well) was purposely chosen so that all
this energy would fall midway between the clusters of energy of the
monochrome signal. Any color signal reaching the screen of a monochrome
receiver will tend to at least partially cancel itself out on successive
frames so that its visibility is reduced. The same action occurs in
a color set when the color signal reaches the screen via the luminance
channel. Hence, the combination of the 3.58 mc. trap with the frequency
interlace principle act to reduce the visibility of any interference
pattern from this source to a considerable degree.
Fig. 7. Two types of video second detectors found in color
TV sets. (A) Germanium diode; (B) triode vacuum tube with grid
and plate connected to form a diode.
|EDITOR'S NOTE: Part 1 of this series, which
appeared in the March, 1954 issue, explained color mixing and
its application in color TV. Part 2, appearing in the April
issue, described the NTSC color signal. The block diagram of
a typical color TV receiver was described in the May issue.
This and forthcoming articles will describe and analyze the
various circuits used in present color TV sets.
of the many requests received, RADIO & TELEVISION NEWS will
publish this series in reprint form. The first three parts are
in a single unit (50 cents), the balance will be reprinted in
individual parts at 20 cents each. For quantities of 50 or more,
write for quotations. Address your inquiries to RADIO &
TELEVISION NEWS Reprint Editor, 866 Madison Ave., N.Y. 17, N.Y.
Returning to the circuit of Fig. 8, the luminance signal is finally
applied to the matrix section where it combines with suitable I and
Q signals to provide the original red, green, and blue voltages.
Two additional representative video amplifier systems are shown
in Figs. 6 and 9. The circuit in Fig. 6 is taken from an RCA schematic
and employs a 1N60 crystal diode as the video second detector. The output
of this stage is fed to a 6CL6 video amplifier. Here both chroma and
monochrome signals are amplified. The monochrome signal is then transferred
to a second video amplifier and from this stage to the matrix network.
The chroma signal is taken from the cathode circuit of the 1st video
amplifier and transferred to the bandpass amplifier which stands at
the head of the chrominance section.
Fig. 8. Video amplifier circuit
using a triode-pentode tube.
There are a
number of things to note about Fig. 6. A 3.58 mc. resonant circuit in
the plate circuit of the 1st video amplifier transfers the 3.58 mc.
signal to a burst amplifier for use in the color sync section of the
receiver. The same arrangement also attenuates the amount of 3.58 mc.
voltage reaching the second video amplifier. The response of this latter
amplifier extends to approximately 3.2 mc., enabling it to impose additional
attenuation on the color subcarrier.
Fig. 9. Cathode follower video amplifier
circuit for color TV.
Connection to the
sync and .g.c. circuits is made at the plate of the 1st video amplifier.
Also, a 1.0 microsecond delay line is inserted in the path of the luminance
signal between the 1st and 2nd video amplifiers. The delay line is terminated
in a 1500-ohm potentiometer which serves as a contrast control for the
luminance signal. A contrast control for the chrominance portion of
the signal is mechanically ganged to the luminance contrast control,
thereby insuring that both signals will be varied in equal amounts.
This is required to maintain the proper voltage relationship between
the two signals.
A 4.5 mc. trap in the cathode leg of the 1st
video amplifier attenuates any 4.5 mc. voltage that may develop , in
the video detector through the beating of the video and sound carriers.
For the color TV video amplifier circuit shown in Fig. 9, the
detector stage is formed by using one-half of a 6BK7 duo-triode. The
grid and plate are tied together so the triode function as a diode.
The second triode section of the 6BK7 is operated as a cathode follower,
thereby permitting a number of circuits to obtain their signals from
the detector without imposing any capacitive loading on this stage.
The plate circuit of the cathode follower provides signal voltages
for the sync separator, a.g.c., and burst amplifiers. The cathode of
the same tube contains a 500-ohm potentiometer which provides the signal
for both a luminance amplifier and a bandpass amplifier and controls
the contrast for both channels simultaneously.
or luminance signal is amplified by a single triode stage and then passed
through a 1.0 microsecond delay line that is terminated in the matrix
network. There are no special traps in this circuit, but response falls
off rapidly beyond 3.2 mc. attenuating any color subcarrier and 4.5-mc.
voltages that might be present.
Posted September 1, 2013