MONOCHROME TV RECEIVER CIRCUIT
Out of the numerous receiver circuits that are in use, one developed by Bharat Electronics Ltd. (BEL) has been chosen for detailed discussion. It is a multichannel fully solid-state receiver conforming to CCIR 625-B system.
It employs three ICs. The design of the receiver is so simple that it can be assembled* with a minimum of tooling and test facilities. Such an exercise can be very instructive for fully grasping alignment and servicing techniques of a television receiver. Fig. on the foldout
page shows the circuit diagram of this receiver. A brief description of the circuit follows:
The receiver employs a turret type tuner and provides all channels between 3 to 10 in the VHF range. A high pass filter with a cut-off frequency of 40 MHz is used at the input to reduce interference due to IF signals. The tuner operates from a + 12 V supply and has an effective AGC range of 50 db.
The video IF sub-system consists of IC, BEL CA**3068 and other associated components housed in a modular box to avoid any possible RF interference.
All interconnections to this module are *The technical manual of the receiver can be obtained on request from M/s Bharat Electronics Ltd. Jalahalli, P.O., Bangalore 560013 India.
The manual gives all necessary circuit details, coil data, list of components, assembly and alignment details. either through feed through capacitors or insulated lead throughs.
The IF sub-assembly provides (i) a gain around 75 db to the incoming signal from the tuner,
required selectivity and bandwidth, (iii) attenuation to adjacent channel interfering frequencies, (iv) attenuation of 26 db to sound IF for intercarrier sound, (v) sound IF and video outputs and (vi) AGC voltage to the IF section and tuner.
The important functions performed by this sub-system are:
to amplify inter carrier IF signal available from the picture IF amplifier,
to recover sound signal,
to amplify the sound signal and deliver at least 2 watts of audio power to the loudspeaker.
The circuit of BEL CA*3065 IC consists of
a regulated power supply,
a sound IF amplifier-limiter,
an FM limiter,
an electronic attenuator and a buffer amplifier, and
an audio driver.
The sound section operates from a + 12 V supply. It employs transistor BC 148B as a boot-strapped driver and matched transistor pair, 2N5296 and 2N6110, at the output stage. The bandwidth of the audio amplifier is from 40 Hz to 15 KHz and can deliver 2 watts of useful audio power.
This section of the receiver uses transistors BF 195 C as driver (buffer), BD115 as video amplifier and BC147B in the blanking circuit. The video amplifier delivers 90 V p-p signal to the cathode of picture tube 500-C1P4.
The current limiting is provided by diode 0A79 and associated circuitry. Transistor Q 503 switches amplifier transistor Q502 only during the time when video signal is present and turns it off during horizontal and vertical sync periods.
The horizontal blanking pulses of 60 V p-p and vertical blanking pulses of 40 V p-p are applied to the base of transistor Q503. High voltage output of 1.1 KV from the horizontal output circuit is rectified and fed to focusing grids of the picture tube through a potential divider. Brightness control operates from a + 200 V supply. The contrast control functions by varying input signal amplified to the video amplifier transistor Q 502.
This section employs transistor Q 401 (BC 148 A), IC, CA 920 and associated passive
components. Composite video signal from the IF section is applied to pin 8 of IC 401 (CA 920). The functions of the horizontal subsystem are:
To generate a line frequency signal, the frequency of which can be current controlled,
To separate sync information from the composite signal,
To compare the phase of sync pulses with that of the oscillator output and generate a control voltage for automatic tuning of the oscillator.
Phase comparison between the oscillator waveform and middle of the line flyback Pulse, to generate a control voltage for correction of the switching delay time in the horizontal driving and output stages, and
Shaping and amplification of the oscillator output to obtain pulses capable of driving the horizontal deflection driver circuit.
Output from the horizontal oscillator is applied to the base of horizontal driver transistor Q802 through a coupling capacitor (C 803). The transistor switches from cut-off to saturation when a pulse is applied to its base and provides the necessary drive for Q803 (BU 205).
The output transistor (Q803) drives the line output transformer to provide deflection current to the yoke coils. In addition, the output circuit (i) generates flyback pulses for blanking, AFC and AGC keying, (ii) provides auxiliary power supplies and generates high voltage (+ 18 KV) for anode of the picture tube and (iii) produces 1.1 KV dc for the focusing grids. The heater supply of 6.3 V for the picture tube is taken across winding 10-11 of the line output transformer.
As shown in Fig. 29.1, the circuit operates from a 40 V dc supply obtained through the line output transformer. This sub-system is a self-oscillatory synchronized oscillator with a matched pair of output transistors.
Clipped vertical sync pulses are fed at the base of Q 70 (BC 148 C) to provide it with a stable drive. Resistor R 724 senses yoke current and feeds a voltage proportional to this current back to the base of Q 702 (BC148 B) for adjustment of the picture height. Coupling network between the collector of Q 701 and the base of Q 702 incorporates the necessary ‘S’ correction and provides linearity of the deflection current. Hold control forms part of the input circuit of transistor Q 701.
The power supply circuit is a conventional transformerless half-wave rectifier and filter circuit.
It provides 200 V for feeding various sections of the receiver. Front panel control. Four controls i.e. on/off volume control, channel selector and fine tuning, contrast control and brightness control are brought out at the front panel of the receiver.
In addition, vertical hold and horizontal hold controls are available on the side panel for occasional adjustments.