UPMSP Radio & Colour TV Technique flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

For a linear conductor at constant temperature, current is directly proportional to voltage and inversely proportional to resistance: I = V/R, with V in volts, I in amperes and R in ohms.

RMS value of sine wave

The RMS (root-mean-square) value of a sinusoidal voltage or current equals the peak value divided by the square root of two, about 0.707 times peak; it produces the same heating as an equal DC value.

Series and parallel resistance

Series resistances add directly: Rt = R1 + R2 + ... Parallel resistances combine reciprocally: 1/Rt = 1/R1 + 1/R2 + ...; two in parallel equal their product divided by their sum.

Multimeter current measurement

To measure current, break the circuit and connect the meter in series with the load, selecting the correct current range; never connect a current range directly across a voltage source because it causes a short circuit.

CRO volts per division

A cathode-ray oscilloscope displays voltage against time; peak-to-peak voltage equals vertical divisions occupied by the waveform multiplied by the volts/division setting, after accounting for probe attenuation.

Soldering good joint

A good solder joint is shiny, smooth and concave, formed by heating the joint and applying 60/40 tin-lead rosin-core solder; a dull, ball-shaped or grainy joint is a cold joint and must be reworked.

Resistor colour code

The resistor colour code assigns black=0, brown=1, red=2, orange=3, yellow=4, green=5, blue=6, violet=7, grey=8 and white=9; the third band is the multiplier and gold or silver gives tolerance.

Zener diode regulation

A zener diode is operated in reverse breakdown, where its voltage stays nearly constant; a series resistor drops the excess supply voltage and limits current, giving a simple shunt voltage regulator.

Transistor current gain beta

For a bipolar junction transistor in the active region, beta (hFE) equals collector current divided by base current; a small base current controls a much larger collector current, allowing current amplification.

Bridge rectifier output

A full-wave bridge rectifier uses four diodes, conducts on both half-cycles, and gives a DC output near 0.636 times peak voltage; ripple frequency is twice the AC supply frequency.

Capacitor input filter ripple

In a capacitor-input filter the peak-to-peak ripple is approximately load current divided by (ripple frequency multiplied by capacitance); larger capacitance or higher ripple frequency reduces ripple but increases diode surge current.

Three-terminal regulator 7805

The 7805 is a fixed positive linear regulator giving 5 volts output, with input, ground and output pins; it needs about 2 volts dropout and a heatsink when load current and voltage drop cause high dissipation.

Dynamic microphone

A dynamic microphone uses a moving coil attached to a diaphragm inside a magnetic field; it is robust, needs no power supply, and has low output impedance, making it suitable for stage and PA use.

Class B amplifier efficiency

A class B push-pull amplifier conducts each transistor for half a cycle, giving maximum theoretical efficiency of 78.5 percent; it suffers crossover distortion unless biased slightly into class AB.

Loudspeaker impedance

A loudspeaker's nominal impedance, typically 4, 8 or 16 ohms, is the load presented to the amplifier; connecting a lower impedance than rated can overheat the output stage.

Superheterodyne receiver

A superheterodyne receiver mixes incoming radio frequency with a local oscillator to produce a fixed intermediate frequency, which is amplified and detected; this gives good selectivity and stable gain.

AM image frequency

The image frequency is an unwanted signal separated from the wanted signal by twice the intermediate frequency; it can pass through the mixer, so RF selectivity and a high IF reduce image interference.

FM de-emphasis

FM broadcasting pre-emphasises high audio frequencies at the transmitter; the receiver applies complementary de-emphasis, commonly 50 or 75 microseconds, to restore flat response and reduce high-frequency noise.

Half-wave dipole length

A half-wave dipole has an electrical length of half a wavelength and a feed-point impedance near 73 ohms; a practical length in metres is approximately 143 divided by frequency in megahertz.

VSWR formula

Voltage standing wave ratio equals (1 + reflection coefficient) divided by (1 - reflection coefficient); a value of 1:1 means a perfect match, while higher values indicate mismatch and reflected power.

Skywave propagation

Skywave propagation uses refraction by the ionosphere to carry high-frequency signals beyond the horizon; it supports long-distance communication, depends on frequency and solar activity, and has fading and skip zones.

PAL line frequency

A 625-line, 50-field PAL television system has 25 frames per second and a horizontal line frequency of 15,625 hertz, with each line lasting 64 microseconds including blanking.

Composite video signal

A composite video signal combines luminance, blanking and synchronising pulses in one waveform, typically 1 volt peak-to-peak into 75 ohms, with sync at the most negative level.

Interlaced scanning

Interlaced scanning divides each frame into two fields, first scanning odd lines then even lines; this doubles the field rate, reducing perceived flicker without increasing bandwidth.

TV sound intercarrier

In a television receiver the sound intermediate frequency is obtained as the difference between vision and sound carriers, 5.5 MHz in system B/G; this intercarrier is amplified and demodulated by an FM detector.

Flyback transformer function

The flyback transformer generates the high anode voltage, typically 15 to 20 kilovolts, and other supplies for a CRT, using the horizontal output stage's retrace pulse; it also supplies focus and screen voltages.

PAL colour subcarrier

The PAL colour subcarrier frequency is 4.43361875 megahertz in a 625-line system; its phase is reversed on alternate lines to cancel hue errors, while the burst provides a reference.

NTSC colour subcarrier

The NTSC colour subcarrier is 3.579545 megahertz in a 525-line, 60-field system; it carries I and Q colour-difference signals by quadrature amplitude modulation, with a 6-megahertz channel.

DTH LNB

A direct-to-home low-noise block downconverter receives Ku-band satellite signals, amplifies them, and converts them to a lower intermediate frequency, typically using a 9.75 or 10.6 gigahertz local oscillator.

LCD panel backlight

A liquid-crystal display does not emit light; it modulates a backlight, usually LED edge-lit or direct-lit, through polarisers and a thin-film transistor array that controls each pixel.

Isolation transformer use

An isolation transformer couples AC power magnetically with no direct connection between primary and secondary, so servicing a live-chassis television is safer; it does not reduce voltage.

CRT anode discharge

Before servicing a CRT, discharge the anode under the rubber cap to the chassis through an insulated resistor, because the tube capacitance can store a dangerous high-voltage charge after power is removed.