Fundamentals of Color TV
Tri-Gun Receiver Circuits
June 1954 Radio & Television News
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
4. Tuner, video i.f., video amplifier, and sound circuits of typical
color TV sets described in detail.
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.
Comparing production model 3-gun, 15" color TV set with
black-and-white 17" set. Color receiver uses over twice
as many components.
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.
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 tuner
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 channel 2.
The secondary winding, L101B,
is tuned by the input capacity (of the first triode unit) in series
with alignment trimmer C105 Loading of L101B
by R101 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 through 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, C103 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 of R103
and R111, places the grid of the second triode at a sufficiently
positive potential (with respect to its cathode) for proper operating
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.
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, T202,
T203 and T204), but two of the tuned circuits
have single-wound coils (L108 and L201)
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.
A number 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 point.)
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.
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.
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.
Luminance Channel. 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.
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
At 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.
Another fact 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
In view 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
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.
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.
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.
The brightness 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.
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