UPMSP Radio & Colour TV Technique key terms

The relationship stating that current through a conductor equals the voltage across it divided by its resistance (I = V/R), provided temperature stays constant. It underpins every voltage, current and resistance calculation in receiver servicing.

The total opposition a circuit presents to alternating current, combining resistance with reactance and measured in ohms. Unlike plain resistance it also causes a phase shift between voltage and current.

The frequency-dependent opposition offered by a capacitor or inductor to alternating current, measured in ohms. Capacitive reactance falls as frequency rises, while inductive reactance rises with frequency.

The principle that the algebraic sum of all voltage drops and sources around any closed loop of a circuit equals zero, used to analyse multi-mesh networks such as power-supply and amplifier stages.

A general-purpose test instrument that measures voltage, current and resistance, and often continuity and diode drop. Digital types show numeric readings; analogue types use a moving pointer.

An instrument that plots a signal's voltage against time on a screen, letting a technician observe waveform shape, amplitude, frequency and phase in receiver and video circuits.

A tin-lead or lead-free alloy melted to join component leads to pads, together with flux that removes oxide and helps the alloy wet the surfaces. Correct heat and clean surfaces give a shiny, reliable joint.

A dull, grainy or balled connection caused by too little heat, dirt or movement while the solder solidified. It conducts intermittently and is a common cause of crackling or dead sections.

A component of two conductors separated by a dielectric that stores charge and blocks direct current while passing alternating current. Capacitance is measured in farads, usually microfarads or picofarads in receivers.

A silicon diode designed to operate in reverse breakdown at a specified voltage, holding that voltage nearly constant. It is widely used for simple shunt regulation and over-voltage clamping in power supplies.

Setting fixed DC voltages and currents at a transistor's terminals so it works at the chosen point on its characteristic curves. Correct bias keeps the amplifier stage linear and prevents clipping or thermal runaway.

A four-diode arrangement that converts both half-cycles of AC into pulsating DC, giving full-wave output without a centre-tapped transformer. It is the commonest rectifier block in TV and radio power supplies.

The ratio of the AC ripple voltage present in a rectifier's output to its average DC voltage. A low value means smoother DC; filtering and regulation reduce it.

An integrated series regulator with input, output and common pins that holds its output at a fixed voltage such as 5 V, 9 V or 12 V, with internal current limiting and thermal shutdown.

The final stage of an audio chain that raises a small signal to a level able to drive a loudspeaker, usually working in class AB with negative feedback to lower distortion.

The nominal AC resistance of a loudspeaker voice coil, typically 4, 8 or 16 ohms, used to match it to an amplifier's rated output and avoid overload or mismatch.

A microphone in which a small coil attached to a diaphragm moves in a magnetic field, generating a signal by electromagnetic induction. It is rugged and needs no supply.

A receiver architecture that mixes incoming signals with a local oscillator to produce a fixed intermediate frequency, where most amplification and selectivity take place, giving stable tuning and uniform performance across the band.

The fixed frequency, 455 kHz for AM and 10.7 MHz for FM, to which every received signal is converted before detection. Tuned IF transformers provide the receiver's main selectivity and gain.

A feedback circuit that senses signal strength and adjusts RF or IF stage gain so that strong and weak stations produce similar audio output, preventing overload and fading.

A straight conductor, usually a half wavelength long, fed at its centre, which radiates or receives best broadside to its axis. It is the reference against which other antennas' gain is quoted.

The ratio of voltage to current for a wave travelling along a uniform transmission line, determined by its inductance and capacitance per unit length and independent of line length, typically 50 or 75 ohms.

The ratio of maximum to minimum voltage along a transmission line, showing how well the line and antenna impedances match. A value near 1:1 means little reflected power.

The left-to-right movement of the electron beam across the screen, driven by a sawtooth current in the deflection coils, which builds the picture line by line at a high rate.

The single signal containing picture information, blanking and synchronising pulses with their correct timing. It lets one cable carry both brightness detail and the timing needed to reconstruct the raster.

A pulse added to the video signal that cuts the beam off while it flies back to start the next line or field, so retrace lines are not visible on the screen.

A stage that slices sync pulses from the composite video signal and separates them into horizontal and vertical trains, which then trigger the deflection oscillators to keep the picture locked.

The very high direct voltage, typically above 20 kV in colour CRTs, that accelerates the electron beam. It is produced by the flyback transformer and is dangerous to service.

The part of the colour video signal that carries brightness information, formed from weighted red, green and blue camera outputs. It is compatible with monochrome receivers.

A short sample of the colour subcarrier, about ten cycles, transmitted during back porch time. It references the subcarrier phase so the receiver can correctly demodulate colour.

Phase Alternating Line, a colour system that reverses the phase of the red-difference subcarrier on alternate lines, so hue errors cancel and appear as small saturation errors instead.

A perforated metal sheet inside a colour CRT, placed behind the screen, that ensures each electron beam strikes only its own colour phosphor dots, preserving colour purity.

A light source, usually edge-lit LEDs or a CCFL tube, behind an LCD panel. The liquid crystals only control light transmission, so without it the picture would be invisible.

The low-noise block downconverter mounted at a satellite dish's focus. It amplifies weak Ku-band signals and converts them to a lower IF band for the cable and set-top box.

A receiver that demodulates and decodes digital satellite or cable transmissions, then outputs audio and video to the TV. It also handles channel lists, conditional access and menus.

Connecting exposed metalwork and the chassis to the earth so fault current trips protection and the user is not shocked. It is a mandatory safety requirement in mains-powered equipment.

Static electricity discharge that can destroy MOSFETs, ICs and laser diodes during handling. Prevention uses wrist straps, grounded mats and antistatic packing.