CREZAR TV12 YEAR 1977.

The CREZAR  TV12 is a  little portable B/W television has 12 Inches screen format.

It has a very uncommon tuning feature:

- Some preselection programs are adressed to only UHF channels

- Others are adressed to VHF channels respectively first band and third band of VHF.

- Tuning search for each preselection is acted in potentiometric way.

One " " new " " tuning system currently being incorporated in some television receivers uses a varactor tuner which overcomes some of the disadvantages of mechanical turret tuner by accomplishing tuning electronically. As the name indicates, the heart of such a tuner is a varactor diode which is used as a capacitive tuning element in the RF and local oscillator sections. In this system, channel selection is made by applying a given reverse bias voltage to the varactor to change its electrical capacitance. The channel selection biasing can be performed by mechanically or electrically switching approximately 5 or many more preset potentiometers. The problem with such arrangement is that it quite seriously limits the number of channels available to the consumer. Additionally, it suffers from the drawback that all potentiometers require adjusting for the desired channels. The VHF channels are usually factory adjusted while the six UHF channels require on-location adjustment. Moreover, using this arrangement, the only indication--during adjustment--of which channel is selected is by station identification.

This set was featuring  good to average picture quality even with a non excellent black level stabilization 
feedback, and was very reliable and almost faultless.

Author Comment about a A Need for Black level clamping on these tv..................

 As its name indicates the purpose of a black -level clamp is to hold the black level of a video signal at a constant level irrespective of changes in the content of the video signal. 

The problem arises when the video signal is a.c. coupled. A simple method of maintaining the black level of the signal with a.c. coupling is to use a d.c. restorer which clamps the sync pulse tips at a constant level, see Fig. I. A d.c. restorer thus holds the black level constant irrespective of changes in picture content but the black level will still vary with overall video signal amplitude. The answer to this is to clamp the black level itself so that it is held at a constant level irrespective of picture content and signal amplitude. 

Clamping is essential with colour receivers of course but this feature has been almost wholly absent from monochrome sets-it is simpler to use a.c. coupling and rely on the picture generally being at a mean level. In my view however the extra expense involved in adding a black -level clamp is well worthwhile: dark pictures look dark instead of like a bright and often streaky fog and the dark detail in bright pictures does not become lost in shadow. Furthermore the general contrast level for the same video drive is improved since once adjusted the brightness control is always at its optimum setting. These features also make the clamp very useful for DX -TV purposes since the brightness level can be set to the optimum level re- quired for photography, independent of the state of the received signal.

Circuit Principle:

The basic action of this clamp circuit is shown in Fig. 2: the heart of the circuit is transistor Q1. This transistor is non conducting during the line period and as the c.r.t. cathode is more negative than the black level DI is reversed biased. The c.r.t. is drawing current however so that Cl charges-the potential at its right- hand side as a result becoming steadily more positive there would be no current flow through CI. To ensure that CI can still charge and the black -level clamp operate under these conditions RI is required. The time -constant of RI, Cl must be shorter than the shortest time -constant in the preceding video stages. These are normally quite high anyway (around 100- 500mS). Now the clamp appears at first sight to have a time -constant of 500mS (C7, R17 in Fig. 3) but the aiming potential of C7 is the h.t. line at + 200V com- pared with a mean signal level of around + 30V to +60V. Thus the effective time -constant is more like 100mS. There is an advantage to be gained in having a short time -constant: if it is appreciably less than 20mS the clamp will effectively remove any mains hum on the video signal. Such hum does tend to be more noticeable when the picture is uniformly dark. A short time - constant also produces horizontal streaking on the picture if there is a ghost present however as the black level at the end of the line sync pulses has superimposed on it the displaced video signal of the ghost. Thus if the clamp has too short a time -constant it is able to follow these unwanted variations in black level. Note that as long as the time -constant Cl, RI (Fig. 2) is reasonably large compared with the line period CI will charge only a fraction of a volt during the line period. There will be no noticeable change in brightness from left to right across the picture therefore due to Cl charging. So much for the basic thinking behind the clamp circuit.

It has a Transistorized horizontal deflection circuits  made up of a horizontal switching or output transistor, a diode, one or more capacitors and a deflection winding. The output transistor, operating as a switch, is driven by a horizontal rate square wave signal and conducts during a portion of the horizontal trace interval. A diode, connected in parallel with the transistor, conducts during the remainder of the trace interval. A retrace capacitor and the deflection yoke winding are coupled in parallel across the transistor-diode combination. Energy is transferred into and out of the deflection winding via the diode and output transistor during the trace interval and via the retrace capacitor during the retrace interval.
In some television receivers, the collector of the horizontal output transistor is coupled to the B+ power supply through the primary windings of the high voltage transformer.


CREZAR was a little Italian manufacturer of cheap television sets in the 70's.
Crezar was a combination of " Crespi e Zaretti " - Milano -
It's dead at the end of the 70's !!
These were build in the way as here this tellye in collection shown is cheap & Simple.