NORDTON (ZANUSSI) MOD. H24 CHASSIS BS212.4 INTERNAL VIEW.

The ZANUSSI CHASSIS BS212.4 is the last chassis for B/W television sets using tubes mixed with semiconductors.

tubes used:

- PL84
- PY88
- PL504
- PCL805
- PCF200


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 exhibit 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, R14 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.

Thermionic tubes, also known as vacuum tubes or electron tubes, are electronic devices that were widely used in the early 20th century as amplifiers, oscillators, and rectifiers in electronic circuits. These devices are made of a sealed glass envelope that contains a vacuum, and they rely on the flow of electrons from a heated cathode to a positively charged anode to produce a current. The cathode is typically made of a material that has a low work function, such as tungsten or thoriated tungsten, and is heated by a filament to emit electrons through a process called thermionic emission. The anode, on the other hand, is usually made of a metal that can withstand high temperatures and is shaped to form a series of plates or grids that control the flow of electrons.

Vacuum tubes have always been used to control the flow of electricity through electronic components.

These devices became a key component of electronic circuits for the first half of the twentieth century. They were crucial to the development of radio, television, radar, sound recording and reproduction, long-distance telephone networks, and analog and early digital computers. Although some applications had used earlier technologies such as the spark gap transmitter for radio or mechanical computers for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, and created the discipline of electronics.

Of course, there must be a hard vacuum (about one millionth of normal air pressure at sea level) inside the bulb (modern light bulbs often contain gas), otherwise gas molecules would block the movement of electrons. To preserve this hard vacuum, after a thermionic valve has had all the air pumped out of it, a "getter" made of reactive metal such as barium is fired by means of an electrical discharge and/or RF induction heating through the glass wall, which creates a silvery coating on the inside of the glass that will absorb any remaining gas molecules, including ones that can be released from the valve materials.

Classification
One classification of thermionic vacuum tubes is by the number of active electrodes. A device with two active elements is a diode, usually used for rectification. Devices with three elements are triodes used for amplification and switching. Additional electrodes create tetrodes, pentodes, and so forth, which have multiple additional functions made possible by the additional controllable electrodes.
Other classifications are:

•  by frequency range (audio, radio, VHF, UHF, microwave)
•  by power rating (small-signal, audio power, high-power radio transmitting)
•  by cathode/filament type (indirectly heated, directly heated) and warm-up time (including "bright-emitter" or "dull-emitter")
•  by characteristic curves design (e.g., sharp- versus remote-cutoff in some pentodes)
•  by application (receiving tubes, transmitting tubes, amplifying or switching, rectification, mixing)
•  specialized parameters (long life, very low micro phonic sensitivity and low-noise audio amplification, rugged or military versions)
•  specialized functions (light or radiation detectors, video imaging tubes)

 tubes used to display information (Nixie tubes, "magic eye" tubes, vacuum fluorescent displays, CRTs)
Tubes have different functions, such as cathode ray tubes which create a beam of electrons for display purposes (such as the television picture tube)

Within the space of a mere fourteen years, television has developed from a scientific toy to a public service which daily provides entertainment and instruction to hundreds of thousands of people on both sides of the Atlantic. The skepticism which originally prevailed concerning the possibility of exploiting the new art has given place to complete confidence in both technical and commercial circles, and television now supports a thriving industry which is assured, by plans already well advanced, of rapid growth and development not only in Great Britain and in the U.S.A. but also in European countries which, s o far, have not instituted a television service.

Many problems had to be solved before television could been brought to its state of perfection at which, from the technical point of view, mass popularity can be assured. But mass popularity depends not only upon technical perfection but also upon the availability of receivers of good performance at low cost. The production of such equipment was, of course, the task of the set maker. In the design of high-quality television receivers, the tubes employed played an important part.

+ Tubes originally produced for normal broadcast reception, however efficient they may be for that purpose, are not necessarily the most suitable for television reception. Indeed, such have been  developments that it is safe to say that unless tubes which have been specially developed for television service are employed, receivers of the highest performance combined with low cost cannot be realized.

HERE below the Datasheets Manuals of the  Thermionic tubes used in the TV SET in the article:NORDTON (ZANUSSI)  MOD. H24 

 

 

Note 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.