This 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
expressly 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.
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
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 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 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, stages.
The oscillator is of the ultraudion variety with a front panel fine-tuning
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, 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.
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.
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.
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.
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 sets.
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,
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
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
Fig. 9. - Cathode follower video amplifier circuit for
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.
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.
Color Television Articles
Posted September 1, 2013