UPMSP Radio & Colour TV Technique cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R, with V in volts, I in amperes, R in ohms and P in watts.

Series resistances add (Rt = R1 + R2 + ...); parallel resistances combine as 1/Rt = 1/R1 + 1/R2 + ...; two parallel resistors equal product/sum.

Unloaded voltage divider: Vout = Vin × R2/(R1 + R2); loading by the following stage lowers the output because R2 is shunted.

For a sine wave, Vrms = Vpeak/sqrt(2) = 0.707 Vpeak; Vavg (full-wave) = 2Vpeak/pi = 0.637 Vpeak; half-wave Vavg = Vpeak/pi = 0.318 Vpeak.

Reactance: XL = 2pifL and XC = 1/(2pifC); impedance Z = sqrt(R² + (XL - XC)²); at series resonance XL = XC, Z = R and current is maximum.

Resonant frequency fr = 1/(2pisqrt(LC)); a series circuit is capacitive below resonance and inductive above resonance.

Kirchhoff's laws: KCL - algebraic sum of currents at a node is zero; KVL - algebraic sum of voltages around any closed loop is zero.

RC charging: one time constant tau = RC gives 63.2% of final voltage; five tau is taken as practically full charge; inductor tau = L/R.

Transformer relations: Vs/Vp = Ns/Np = Ip/Is; an ideal transformer conserves power, so a step-up in voltage steps current down.

A voltmeter connects in parallel with the circuit and should have very high resistance; an ammeter connects in series and should have very low resistance.

An analog multimeter's sensitivity is quoted in ohms per volt; a 20,000 ohm/V meter on its 10 V range has 200 kohm input resistance and loads a circuit more than a digital meter.

Never measure resistance or continuity on a live circuit; the meter's own cell supplies the test current and external voltage can damage the instrument.

On a CRO, voltage = vertical divisions × volts/div; time = horizontal divisions × time/div; a 10× probe multiplies the displayed voltage by 10.

A megger (insulation tester) applies a high test voltage, commonly 500 V DC, to measure insulation resistance in megohms; the circuit must be dead and capacitors discharged.

Soldering: use 60/40 or 63/37 tin-lead rosin-core solder at about 300-350 degrees C, heat the joint and feed solder into it, not onto the iron tip.

A cold solder joint looks dull, grainy or ball-shaped and is electrically intermittent; reheat and reflow it rather than adding more solder.

An isolation transformer isolates the equipment from earth, protecting the technician from shock when servicing a live-chassis TV; it does not reduce the output voltage.

ESD control: wear a 1 Mohm wrist strap connected to a grounded mat, and store MOSFETs and ICs in conductive foam or antistatic bags.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold = +/-5%, silver = +/-10%, no band = +/-20%.

Capacitor code 104 = 10 × 10^4 pF = 100 nF; 103 = 10 nF; 223 = 22 nF; a letter gives tolerance (J = +/-5%, K = +/-10%, M = +/-20%).

Silicon diode conduction begins at about 0.7 V forward; germanium at about 0.3 V; reverse breakdown is not normal conduction except in zener or avalanche devices.

A zener diode regulates in reverse breakdown: Vout = Vz, and series resistor Rs = (Vin - Vz)/(Iz + IL) must allow minimum zener current at no load.

BJT relations: IE = IC + IB; beta = IC/IB; VBE = 0.7 V for silicon; saturation VCE(sat) = 0.2 V; cutoff when VBE < 0.6 V.

A JFET or MOSFET is voltage-controlled and has very high gate input impedance; a MOSFET gate is easily damaged by static, so handle with grounded tools.

An SCR latches on when the gate receives a pulse and remains on until anode current falls below the holding current; a triac is equivalent to two SCRs back-to-back for AC control.

An LED's forward voltage is about 1.8-2.1 V for red and 3.0-3.3 V for blue/white; a series resistor limits current: R = (Vs - Vf)/If.

Semiconductor heat sinks reduce junction temperature; thermal runaway occurs when rising temperature increases current, which further increases temperature in poorly biased BJTs.

Half-wave rectifier: one diode, output Vdc = 0.318 Vm, ripple frequency = supply frequency, ripple factor 1.21, maximum efficiency about 40.6%.

Full-wave centre-tap rectifier: two diodes, Vdc = 0.636 Vm, ripple frequency = 2f, ripple factor 0.482, PIV = 2Vm.

Bridge rectifier: four diodes, Vdc = 0.636 Vm, ripple frequency = 2f, PIV = Vm; the bridge needs no centre-tapped transformer.

Capacitor-input filter: Vripple(pp) = Idc/(fr × C); increasing C or fr reduces ripple but raises the diode surge current and peak repetitive current.

A bleeder resistor across the filter capacitor discharges it after switch-off and improves voltage regulation slightly, at the cost of wasted current.

Zener shunt regulator design: Rs = (Vin(min) - Vz)/(Iz(min) + IL(max)); the zener must dissipate at least Vz × Iz(max).

78xx series regulators give fixed positive outputs (7805 = 5 V, 7812 = 12 V) with about 2 V dropout; 79xx gives negative outputs; both need input/output decoupling capacitors.

LM317 is an adjustable positive regulator with Vout = 1.25 V × (1 + R2/R1); it needs a minimum load current and a heatsink at high dissipation.

SMPS converts AC to high-frequency switching, giving efficiency of 70-90% and small transformers; it requires PWM control, feedback isolation and an input EMI filter.

Dynamic microphone: moving coil in a magnetic field, no power supply, robust, low impedance (about 200-600 ohm); condenser microphone needs phantom power, usually 48 V.

Balanced lines use two signal conductors plus a shield and a differential input to reject common-mode hum; XLR connectors are standard for balanced microphone lines.

Loudspeaker nominal impedance is typically 4, 8 or 16 ohm; amplifier load must not be lower than the rated minimum, or output devices overheat.

Amplifier classes: A conducts 360 degrees (efficiency up to about 25-30%), B conducts 180 degrees (78.5% maximum), AB reduces crossover distortion, D switches at high frequency (over 90%).

Voltage gain in decibels: dB = 20 log10(Vout/Vin); power gain: dB = 10 log10(Pout/Pin); -3 dB is the half-power point.

Negative feedback reduces gain but improves bandwidth, lowers distortion and stabilises the operating point; closed-loop gain is approximately 1/beta for high open-loop gain.

Audio range is 20 Hz-20 kHz; total harmonic distortion (THD) and signal-to-noise ratio are key amplifier specifications.

A crossover network splits audio into frequency bands for woofer, midrange and tweeter; a capacitor in series with a tweeter is a simple first-order high-pass filter.

Constant-voltage 100 V PA distribution uses a step-up transformer at the amplifier and step-down transformers at speakers, allowing many speakers on one line.

Superheterodyne blocks: RF amplifier -> mixer -> local oscillator -> IF amplifier -> detector -> AF amplifier; AGC controls RF/IF gain.

AM broadcast IF is commonly 455 kHz (465 kHz in some designs); FM broadcast IF is 10.7 MHz; the IF amplifier provides most of the selectivity and gain.

Image frequency: fimage = fs + 2fIF for high-side injection; the RF tuned circuit must reject it before the mixer.

AM envelope detector: a diode rectifies the modulated RF and an RC network removes the RF, leaving the audio envelope; the time constant must be short compared with the highest audio period.

FM demodulators: Foster-Seeley discriminator and ratio detector convert frequency deviation to amplitude; a limiter removes amplitude noise before detection.

FM receiver de-emphasis is 75 microseconds in the Americas and 50 microseconds in Europe/India; it complements the transmitter's pre-emphasis and reduces high-frequency noise.

Receiver specifications: sensitivity is the minimum input for a stated output and S/N; selectivity is the ability to reject adjacent channels; fidelity is the accuracy of reproduction.

AFC in FM receivers corrects local-oscillator drift by feeding a control voltage from the discriminator to a varactor or reactance stage.

AM MW band is 531-1602 kHz with 9 kHz spacing; FM broadcast band is 87.5-108 MHz.

Half-wave dipole: length = 143/f(MHz) metres (468/f(ft)); feed impedance about 73 ohm; radiation pattern is a figure-eight broadside to the wire.

Quarter-wave vertical monopole: length = 71.5/f(MHz) metres; feed impedance about 36 ohm; it needs a ground plane or radials.

Characteristic impedance: Zo = sqrt(L/C) for a lossless line; coaxial cable is commonly 50 ohm (RF) or 75 ohm (TV), twin line 300 ohm.

Reflection coefficient Gamma = (ZL - Zo)/(ZL + Zo); VSWR = (1 + |Gamma|)/(1 - |Gamma|); VSWR 1:1 is a perfect match, and higher values mean reflected power.

A balun matches a balanced antenna to unbalanced coax, and an impedance-matching network maximises power transfer; mismatch raises VSWR and feed-line loss.

Propagation modes: ground wave below about 2 MHz, skywave by ionospheric refraction at HF, space wave line-of-sight at VHF/UHF; the ionosphere's F layer supports long-distance HF.

Skip distance is the ground distance from the transmitter to the first skywave return; the MUF is the highest frequency that the ionosphere returns for a given path.

Yagi-Uda antenna: driven element, reflector behind and directors in front; it gives unidirectional gain, and a folded dipole has about 300 ohm impedance.

Antenna gain in dBi compares to an isotropic radiator; ERP = transmitter power × antenna gain, with feed-line loss subtracted.

PAL 625/50: 625 lines per frame, 50 fields/s, 25 frames/s, line frequency 15,625 Hz, line period 64 microseconds; NTSC 525/60: 525 lines, 59.94 fields/s, 15,734 Hz.

Interlaced scanning: two fields per frame (odd and even lines), so the field rate is twice the frame rate; this reduces flicker for a given bandwidth.

Composite video: 1 V peak-to-peak into 75 ohm, with about 0.3 V for sync, 0.7 V for video, and blanking at the black level.

Horizontal blanking includes front porch, sync pulse (about 4.7 microseconds) and back porch; the back porch carries the colour burst in PAL/NTSC.

Vertical sync is broad pulses with equalising pulses before and after; it occurs during the vertical blanking interval of about 25 lines.

Video bandwidth for standard-definition TV is about 5-6 MHz; vestigial sideband transmission reduces occupied bandwidth.

Aspect ratio is 4:3 for standard definition and 16:9 for widescreen/HD; resolution is the number of active pixels per line by active lines.

Sync separator strips sync from composite video using amplitude slicing and RC filtering; it separates horizontal and vertical sync for the deflection stages.

Camera/display scanning converts light intensity into a voltage raster; in a CRT, the electron beam is deflected horizontally and vertically to form the raster.

Monochrome TV signal path: antenna -> tuner (RF amp, mixer, local oscillator) -> IF amplifier -> video detector -> video amplifier -> CRT; AGC controls tuner and IF gain.

In system B/G (India), vision IF is 38.9 MHz and sound IF is 33.4 MHz; the intercarrier sound is at 5.5 MHz.

The video detector recovers composite video; the video amplifier drives the CRT cathode and includes contrast and brightness controls.

Sound channel: intercarrier take-off -> 5.5 MHz band-pass -> FM detector -> audio amplifier -> loudspeaker.

Sync separator separates H and V sync; the vertical oscillator runs at 50 Hz and the horizontal oscillator at 15,625 Hz, locked by sync pulses.

Horizontal output stage drives the line deflection coils and the flyback/EHT transformer; the damper diode provides the retrace current path.

EHT for a monochrome CRT is typically 15-20 kV; focus and screen (G2) controls set beam focus and background brightness.

Keyed AGC is gated by the horizontal flyback pulse so that AGC responds to sync-tip level and is immune to noise between lines.

No raster with sound OK suggests EHT, horizontal output or CRT heater; no sound with raster suggests the sound IF/detector/audio stages.

Additive colour: red, green and blue primaries combine to white; complementary colours are cyan (green + blue), magenta (red + blue) and yellow (red + green).

Luminance equation: Y = 0.299R + 0.587G + 0.114B; colour-difference signals R-Y and B-Y carry hue and saturation.

PAL: 625/50, 8 MHz channel, colour subcarrier 4.43361875 MHz, phase alternation on alternate lines, 25 Hz offset to reduce visible dot crawl.

NTSC: 525/60, 6 MHz channel, subcarrier 3.579545 MHz, I and Q quadrature modulation, and a 60 Hz field rate (59.94 Hz).

SECAM: 625/50, sequential FM chroma with subcarriers 4.40625 MHz (Dr) and 4.25 MHz (Db), line-sequential transmission.

PAL decoder uses a 64-microsecond glass delay line to average adjacent lines, cancelling phase (hue) errors; the burst gates the reference oscillator.

Colour killer mutes chroma when the burst is absent or weak; it avoids random colour snow on monochrome broadcasts.

RGB matrix derives R, G and B from Y, R-Y and B-Y; G-Y = -0.51(R-Y) - 0.19(B-Y).

Convergence aligns the three electron beams at the shadow mask; purity adjusts beam landing on the correct phosphor triad.

CRT: electron gun, deflection yoke, shadow mask/aperture grille, phosphor triad; EHT 20-30 kV; a CRT stores dangerous charge after power-off.

LCD: backlight -> polariser -> TFT array -> liquid-crystal layer -> colour filter -> front polariser; it is non-emissive and needs a backlight.

LED-backlit LCD uses edge-lit or direct-lit LEDs with local dimming; OLED pixels emit their own light, giving true blacks and wide viewing angles.

Plasma displays use UV-emitting gas cells to excite phosphors; they are largely obsolete because of power consumption and burn-in.

DTH: Ku-band downlink 10.7-12.75 GHz, dish focuses signal into LNB, LNB converts to 950-2150 MHz IF, coax 75 ohm feeds the set-top box.

LNB local oscillator is typically 9.75 GHz (low band) or 10.6 GHz (high band); 22 kHz tone and DiSEqC select band/switch.

Set-top box: tuner -> demodulator (DVB-S/S2) -> MPEG-2/MPEG-4 decoder -> video/audio output (HDMI, composite); smart card or CAS decrypts pay channels.

Digital compression uses MPEG-2 or MPEG-4 AVC; lower bit rate reduces quality; DVB-S2 uses LDPC and higher-order modulation for greater capacity.

HDMI carries digital video and audio with HDCP copy protection; composite video carries SD analogue video on a single coax.

Fault-finding sequence: confirm the symptom, inspect visually, check supplies, use half-split, signal trace, then substitute a known-good module.

Dead set: check mains plug, fuse, SMPS/chopper, rectifier and startup circuit; a blown fuse may indicate a shorted chopper transistor or output diode.

No raster: check EHT, horizontal output transistor, flyback, CRT heater and G2 supply; excessive EHT can cause X-ray protection shutdown.

No sound: inject a signal at the volume control; if the speaker responds, the fault is before the volume control, in IF/detector stages.

Safety: use an isolation transformer for live-chassis servicing, keep one hand behind the back, and discharge CRT anode and filter capacitors before touching.

Never replace a fuse with a higher rating; use the same type and rating, and investigate the cause of failure first.

ESD damage is latent or immediate; use grounded wrist straps, antistatic mats and conductive bags for ICs and MOSFETs.

Environmental: e-waste contains lead, mercury and cadmium; return CRTs, batteries and circuit boards to authorised recyclers, and follow RoHS lead-free soldering.

After repair, perform safety checks: insulation resistance, earth continuity and visual inspection for pinched wires and correct fuse rating.