The PLANET (PRANDONI) MOD. 671 CHASSIS AL-2-73 MF-1-73 is A simple on steel chassis developed on 2 main sides.
Left side all signal parts, and right side power supply and EHT.
Even with some PCB parts the chassis has some handwired sections.
Tuner unit is completely keyboard "preomat" developed.
The tuning circuits has a large knob potentiometers tuning system which use voltage controlled capacitances such as varactor diodes as the frequency determining elements.
How AFC Circuit Works in B/W Analog Television Receiver:
Push-Button
tuning on u.h.f. while being very convenient often leaves a margin of
mistuning, especially after some wear and tear has occurred on the
mechanism. Even dial tuning can lead to errors due to the difficulty
many people experience in judging the correct point. Oscillator drift
due to temperature changes can also cause mistuning. Automatic frequency
control (a.f.c.) will correct all these faults. The vision carrier when
the set is correctly tuned on u.h.f. is at 39.5MHz as it passes down
the i.f. strip. Thus if at the end of the i.f. strip a discriminator
tuned circuit is incorporated centred on 39.5MHz the discriminator
output will be zero at 39.5MHz and will move positively' one side of
39.5MHz and negatively the other as the tuning drifts. This response is
shown in Fig. 1.
If the tuning is not correct then the discriminator output is not zero and if this output is applied to change the reverse bias on a tuning diode mounted in the oscillator section of the u.h.f. tuner it will correct most of the error. Tuning, varicap or varactor diodes-to give them a few of their names-are junction diodes normally operated with reverse bias but not sufficient to bias them into the breakdown region in which zener diodes operate. The greater the reverse bias the lower their capacitance: a typical curve, for the PHILIPS BB105 or STC BA141 tuning diode, is shown in Fig. 2. All diodes e
xhibit
this basic type of characteristic but special diodes have to be used
for u.h.f. because they must not introduce any excessive loss into the
tuned circuits they control. In other words, just as a coil has to have a
good Q so does a varicap diode. Normally, we don't worry about the Q of
a capacitor as it is usually very good. However, a tuning diode is not a
true capacitor. It has, for example, leakage current so the Q of the
diode is a factor which has to be considered. The diode manufacturer
however will have considered these points and if you buy a diode
specified for u.h.f. use you will have no trouble. These points have
been mentioned to clear up any misunderstandings and to show why any old
diode won't do.
Basic AFC System
To return to our TV set, if the oscillator frequency is too high then the vision carrier frequency will also be too high and in the simple arrangement shown in Fig. 3 the discriminator will give a negative signal to decrease the bias on the tuning diode thus increasing its
capacitance
and in turn reducing the oscillator frequency and correcting the error.
Note that in this diagram the reverse bias on the diode is applied to
its cathode. It is therefore positive with respect to ground so that a
negative signal from the discriminator will reduce the positive voltage
on the diode thus reducing its bias and increasing its capacitance. In
this arrangement the diode is biased somewhere near the mid point of its
characteristic by the positive d.c. bias fed into one side of the
discriminator. The discriminator thus adds to or subtracts from this
d.c. bias.
AFC Loop Gain:
The amount by which the error is reduced depends on the gain of the circuit. An estimate of the gain required must first be made by guessing how much error is likely to be given by your push -buttons or hand tuning: 1MHz would be an outside figure as a tuning error of that magnitude would produce a very bad picture of low definition in one direction and badly broken up in the other. This error should be reduced to
about
100kHz to be really unnoticeable, indicating a required gain of ten. In
fitting a.f.c. to an existing set some measure- ments should be done as
an experiment before finally deciding on the circuit gain. The first
thing to do is to add the suggested discriminator to the i.f. strip. As
the circuit (Fig. 4) shows a Foster -Seeley type discriminator is used
and with the coils specified and the driver circuit shown it should give
±4V for 0.5MHz input variation.
EXAMPLE of Circuit Description:
The driver stage Tr1 takes a small sample signal from the i.f. strip but this should be large enough to drive Tr1 into saturation. That is to say Tr1 is a limiter stage so that the signal amplitude applied to the discriminator coil L2 stays constant over the normal range of signal levels. Trl is biased at approximately 7mA which, according to the original report ("Simple a.f.c. system for 625 -line TV receivers" by P. Bissmire, PHILIPS Technical Communications, March, 1970), gives the best limiting performance. C1, R
14
and R3 damp the stage to prevent oscillation. C2 decouples the power
feed and should be close to the circuit. The coil former and can are the
normal ones used for TV sets and so should be easily obtainable: the
former diameter is 5mm. and length 40mm. and winding details are given
in Fig. 5.
Therefore a stable AFC circuit is developed:
A
superheterodyne receiver having an automatic intermediate
frequency control circuit with means to prevent the faulty
regulation thereof. The receiver has means for receiving a radio
frequency signal and mixing the same with the output of a
superheterodyne oscillator. This produces an intermediate frequency
signal which is coupled to a frequency or phase discriminator to
produce an error signal for controlling the frequency of the
superheterodyne oscillator. A regulation circuit is provided having
an electronic switch to interrupt the feedback circuit when only
unwanted frequencies tend to produce faulty regulation of the
superheterodyne oscillator.
- The EHT Output is realized with a selenium rectifier.
The EHT selenium rectifier which is a Specially designed selenium rectifiers were once widely used as EHT
rectifiers in television sets and photocopiers. A layer of selenium
was applied to a sheet of soft iron foil, and thousands of tiny discs
(typically 2mm diameter) were punched out of this and assembled as
"stacks" inside ceramic tubes. Rectifiers capable of supplying tens of
thousands of volts could be made this way. Their internal resistance was
extremely high, but most EHT applications only required a few hundred
microamps at most, so this was not normally an issue. With the
development of inexpensive high voltage silicon rectifiers, this
technology has fallen into disuse.A selenium rectifier is a type
of metal rectifier, invented in 1933. They were used to replace vacuum
tube rectifiers in power supplies for electronic equipment, and in high
current battery charger applications.
The photoelectric and rectifying properties of selenium were observed by C. E. Fitts around 1886 but practical rectifier devices were not manufactured routinely until the 1930s. Compared with the earlier copper oxide rectifier, the selenium cell could withstand higher voltage but at a lower current capacity per unit area.
Left side all signal parts, and right side power supply and EHT.
Even with some PCB parts the chassis has some handwired sections.
Tuner unit is completely keyboard "preomat" developed.
The tuning circuits has a large knob potentiometers tuning system which use voltage controlled capacitances such as varactor diodes as the frequency determining elements.
How AFC Circuit Works in B/W Analog Television Receiver:
Push-Button
tuning on u.h.f. while being very convenient often leaves a margin of
mistuning, especially after some wear and tear has occurred on the
mechanism. Even dial tuning can lead to errors due to the difficulty
many people experience in judging the correct point. Oscillator drift
due to temperature changes can also cause mistuning. Automatic frequency
control (a.f.c.) will correct all these faults. The vision carrier when
the set is correctly tuned on u.h.f. is at 39.5MHz as it passes down
the i.f. strip. Thus if at the end of the i.f. strip a discriminator
tuned circuit is incorporated centred on 39.5MHz the discriminator
output will be zero at 39.5MHz and will move positively' one side of
39.5MHz and negatively the other as the tuning drifts. This response is
shown in Fig. 1.If the tuning is not correct then the discriminator output is not zero and if this output is applied to change the reverse bias on a tuning diode mounted in the oscillator section of the u.h.f. tuner it will correct most of the error. Tuning, varicap or varactor diodes-to give them a few of their names-are junction diodes normally operated with reverse bias but not sufficient to bias them into the breakdown region in which zener diodes operate. The greater the reverse bias the lower their capacitance: a typical curve, for the PHILIPS BB105 or STC BA141 tuning diode, is shown in Fig. 2. All diodes e
xhibit
this basic type of characteristic but special diodes have to be used
for u.h.f. because they must not introduce any excessive loss into the
tuned circuits they control. In other words, just as a coil has to have a
good Q so does a varicap diode. Normally, we don't worry about the Q of
a capacitor as it is usually very good. However, a tuning diode is not a
true capacitor. It has, for example, leakage current so the Q of the
diode is a factor which has to be considered. The diode manufacturer
however will have considered these points and if you buy a diode
specified for u.h.f. use you will have no trouble. These points have
been mentioned to clear up any misunderstandings and to show why any old
diode won't do.Basic AFC System
To return to our TV set, if the oscillator frequency is too high then the vision carrier frequency will also be too high and in the simple arrangement shown in Fig. 3 the discriminator will give a negative signal to decrease the bias on the tuning diode thus increasing its
capacitance
and in turn reducing the oscillator frequency and correcting the error.
Note that in this diagram the reverse bias on the diode is applied to
its cathode. It is therefore positive with respect to ground so that a
negative signal from the discriminator will reduce the positive voltage
on the diode thus reducing its bias and increasing its capacitance. In
this arrangement the diode is biased somewhere near the mid point of its
characteristic by the positive d.c. bias fed into one side of the
discriminator. The discriminator thus adds to or subtracts from this
d.c. bias.AFC Loop Gain:
The amount by which the error is reduced depends on the gain of the circuit. An estimate of the gain required must first be made by guessing how much error is likely to be given by your push -buttons or hand tuning: 1MHz would be an outside figure as a tuning error of that magnitude would produce a very bad picture of low definition in one direction and badly broken up in the other. This error should be reduced to
about
100kHz to be really unnoticeable, indicating a required gain of ten. In
fitting a.f.c. to an existing set some measure- ments should be done as
an experiment before finally deciding on the circuit gain. The first
thing to do is to add the suggested discriminator to the i.f. strip. As
the circuit (Fig. 4) shows a Foster -Seeley type discriminator is used
and with the coils specified and the driver circuit shown it should give
±4V for 0.5MHz input variation.EXAMPLE of Circuit Description:
The driver stage Tr1 takes a small sample signal from the i.f. strip but this should be large enough to drive Tr1 into saturation. That is to say Tr1 is a limiter stage so that the signal amplitude applied to the discriminator coil L2 stays constant over the normal range of signal levels. Trl is biased at approximately 7mA which, according to the original report ("Simple a.f.c. system for 625 -line TV receivers" by P. Bissmire, PHILIPS Technical Communications, March, 1970), gives the best limiting performance. C1, R
14
and R3 damp the stage to prevent oscillation. C2 decouples the power
feed and should be close to the circuit. The coil former and can are the
normal ones used for TV sets and so should be easily obtainable: the
former diameter is 5mm. and length 40mm. and winding details are given
in Fig. 5.The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the developed apparates both tubes or transistors.
Therefore a stable AFC circuit is developed:
A
superheterodyne receiver having an automatic intermediate
frequency control circuit with means to prevent the faulty
regulation thereof. The receiver has means for receiving a radio
frequency signal and mixing the same with the output of a
superheterodyne oscillator. This produces an intermediate frequency
signal which is coupled to a frequency or phase discriminator to
produce an error signal for controlling the frequency of the
superheterodyne oscillator. A regulation circuit is provided having
an electronic switch to interrupt the feedback circuit when only
unwanted frequencies tend to produce faulty regulation of the
superheterodyne oscillator.
- The EHT Output is realized with a selenium rectifier.
The EHT selenium rectifier which is a Specially designed selenium rectifiers were once widely used as EHT
rectifiers in television sets and photocopiers. A layer of selenium
was applied to a sheet of soft iron foil, and thousands of tiny discs
(typically 2mm diameter) were punched out of this and assembled as
"stacks" inside ceramic tubes. Rectifiers capable of supplying tens of
thousands of volts could be made this way. Their internal resistance was
extremely high, but most EHT applications only required a few hundred
microamps at most, so this was not normally an issue. With the
development of inexpensive high voltage silicon rectifiers, this
technology has fallen into disuse.A selenium rectifier is a type
of metal rectifier, invented in 1933. They were used to replace vacuum
tube rectifiers in power supplies for electronic equipment, and in high
current battery charger applications.
The photoelectric and rectifying properties of selenium were observed by C. E. Fitts around 1886 but practical rectifier devices were not manufactured routinely until the 1930s. Compared with the earlier copper oxide rectifier, the selenium cell could withstand higher voltage but at a lower current capacity per unit area.
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