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.

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.