Odisha CHSE +2 Vocational Mobile Technology cheat sheet

A smartphone block chain is: battery → power management IC (PMIC) → SoC (CPU+GPU+modem) → display, camera, audio and radio sections; each section has its own regulator rail.

Battery nominal voltage is 3.7 V (Li-ion); full charge is 4.2 V and cutoff is about 3.0–3.3 V, so a reading below 3.3 V under load indicates a discharged or failing cell.

The SoC (System on Chip) integrates CPU, GPU, modem, image signal processor and memory controller; the eMMC/UFS chip holds firmware and user data and is separate from RAM.

Antenna sections are separate: cellular main/diversity, Wi-Fi/BT, GPS and NFC each use their own feed and matching network; a shorted feed line drops only that radio.

Charging path order: USB port → charging IC → battery FET → PMIC; a phone that runs on charger but not on battery points to the battery or battery FET.

Display connectors carry both power rails and MIPI data lines; a bent pin or lifted pad on the connector causes no display or lines on screen.

Water-damage indicator (LDI) turns from white to red/pink on contact with moisture; corrosion on shield cans and under ICs is the usual cause of post-liquid faults.

Speaker, earpiece, microphone and vibrator are driven by the audio codec IC; a dead earpiece with working speaker usually means the earpiece coil or its flex, not the codec.

Cellular generations: 2G (GSM/CDMA, voice+SMS), 3G (UMTS/WCDMA, HSPA), 4G (LTE, all-IP), 5G (NR, sub-6 GHz and mmWave); each uses different bands and RF front ends.

GSM uses SIM and TDMA/FDMA; LTE uses USIM and OFDMA downlink / SC-FDMA uplink; 5G NR uses OFDMA both ways with flexible numerology.

Frequency bands are operator-assigned: in India, LTE commonly uses 850/1800/2100/2300/2500 MHz and 5G NR uses 3300–3600 MHz (n78); a phone must support the band to register.

Signal strength is reported as dBm (negative): about −50 dBm is excellent, −100 dBm is weak, and below −110 dBm causes dropped calls or no service.

SINR and RSRP/RSRQ describe LTE quality; high RSRP with low SINR means interference, not distance from the tower.

SIM/USIM holds IMSI, Ki and operator profile; no service with another working SIM points to SIM reader, SIM power rail, or blocked IMEI.

IMEI is a 15-digit device identity; a blacklisted or null IMEI causes network rejection even with a valid SIM.

Wi-Fi (2.4/5 GHz), Bluetooth (2.4 GHz ISM) and GPS (1.5 GHz L1) are receive/transmit paths independent of the cellular modem; test each separately.

Android architecture layers: Linux kernel → HAL → native libraries and Android Runtime (ART) → Java/Kotlin framework → apps; iOS uses XNU kernel with a closed app sandbox.

Factory reset erases user data and restores the OS image but does not reflash firmware; flashing rewrites partitions such as boot, system and vendor.

Fastboot/download mode and recovery mode are separate boot paths: fastboot flashes partitions over USB, recovery applies OTA packages and wipes cache/data.

Boot loop causes split into software (corrupt update, bad partition, wrong firmware) and hardware (eMMC failure, power rail sag); safe mode or fastboot access distinguishes them.

Android Debug Bridge (ADB) commands: adb devices, adb reboot, adb shell, adb logcat; fastboot devices and fastboot flash for firmware.

A bootloader lock blocks flashing unsigned images; unlocking wipes data and may void warranty, so record the customer's consent before unlocking.

App crashes with logs showing FATAL EXCEPTION point to app or framework faults; kernel panic or repeated eMMC read errors point to storage hardware.

OTA updates require sufficient battery charge (typically above 30–50%) and free storage; interrupted OTA is a common cause of soft-bricked devices.

ESD control: wrist strap with 1 MΩ resistor to ground, grounded mat, and ionized air for plastics; CMOS ICs can fail from discharges below human perception (~100 V).

Soldering station temperature ranges: leaded solder 300–350 °C, lead-free 350–400 °C; hot-air rework 350–400 °C with airflow 3–5 and preheat to reduce thermal shock.

Multimeter modes: continuity/diode for track and short checks, DC volts for rails, resistance for coils; measure voltage with power applied, resistance with power removed.

DC power supply with current limit (e.g., 0.5–1 A) locates shorts: a shorted rail draws current immediately at low voltage, a healthy board draws near zero.

Insulated tweezers, plastic pry tools, and anti-static brushes prevent mechanical damage; metal spudgers near batteries risk puncturing cells and thermal runaway.

Li-ion battery safety: never charge a swollen or venting cell; store damaged cells in a fire-resistant container and discharge before disposal per local e-waste rules.

Isopropyl alcohol (99%) and a soft brush clean flux and corrosion; avoid water-based cleaners on powered boards and never power a wet board.

Personal protection: safety glasses for desoldering and cutting, fume extraction for flux smoke, and eye wash access when handling battery electrolyte.

Fault-finding order: confirm complaint → reproduce → visual and liquid-damage inspection → power rail measurement → section isolation → component test → repair → full function test.

No power path: check battery voltage, then charging port, then PMIC input and output rails; a missing rail names the faulty regulator or its load.

Dead short rule: if a rail reads near 0 Ω to ground, inject a current-limited voltage and find the heating component with a thermal camera or isopropyl evaporation.

No charging: test charger, cable, port pins, charging IC input, battery FET and battery; a port with bent VBUS or ground pins is the most common cause.

No display but phone boots (vibration, sound, PC detection): isolate display flex, connector, backlight rail and display IC in that order.

No network: swap SIM, check SIM reader pins and SIM power, inspect antenna feed and matching parts, then verify IMEI and band support.

Intermittent faults often come from cold solder joints, cracked solder balls under the SoC or PMIC, or a loose battery connector; flexing the board while monitoring the rail exposes them.

Record measured values (voltage, resistance, current draw) before and after repair; a comparison against a known-good board shortens diagnosis.

Flux removes oxides and improves wetting; clean residue with isopropyl alcohol after soldering because some fluxes are corrosive and conductive.

Good solder joint: shiny, concave fillet, smooth wetting on pad and lead; a dull, ball-shaped or cracked joint is a cold joint and must be reworked.

Solder wick (desoldering braid) removes excess solder from pads; a desoldering pump removes molten solder from through-holes; add fresh solder and flux before wicking old joints.

Hot-air rework: preheat board to 100–150 °C, apply airflow around the IC (not directly on it), lift with vacuum pen only after solder turns liquid (about 217 °C for SAC305).

BGA and QFN parts have pads under the body and cannot be hand-soldered with an iron; they need stencil-applied paste, hot air or infrared, and reballing.

Component replacement order: remove with heat and flux, clean and level pads, apply paste or solder, align by silkscreen or dot marking, reflow, then inspect under magnification.

ESD and thermal limits: never exceed the component datasheet reflow profile; plastic connectors and displays deform above about 250 °C, so shield them with kapton tape.

Continuity and diode-mode checks after soldering detect bridges between adjacent pins; a bridge on a power rail shows as a short and must be removed before power-up.

Service intake record: customer name, contact, IMEI, model, reported fault, accessories received, cosmetic condition and customer signature; this protects both parties.

Backup before repair: warn that data may be lost on factory reset or board-level work and obtain written consent before erasing data.

Diagnosis-first workflow: give an estimate and expected time before starting work; get approval for any cost above the quoted figure.

Warranty rules: physical damage, liquid damage and unauthorized repair usually void warranty; check seals and indicators before promising free service.

Repair quality checks before handover: call test, speaker/earpiece/mic, camera, touch, charging, Wi-Fi/Bluetooth, sensors and network registration.

Cleaning and preventive maintenance: remove lint from charging port and speaker mesh, check battery health, and advise on charger and cable quality.

Customer communication: explain the fault in plain language, state what was replaced, and hand over old parts when the customer asks.

Data privacy: never copy, view or retain customer photos or messages; erase test data from devices used for comparison.

Break-even point in units = fixed cost ÷ (selling price per unit − variable cost per unit); a repair shop must know this number to price services.

Pricing a repair: parts cost + labour time × hourly rate + overhead share + margin; quote before work and record the final invoice.

GST registration is required in India once turnover crosses the threshold (₹40 lakh for goods, ₹20 lakh for services in most states); keep purchase and sale invoices.

Inventory control: record IMEI or serial for every part and device, use first-in-first-out, and reconcile stock monthly to detect loss.

Employment basics: written offer letter, working hours, leave policy and wage payment as per applicable labour law; apprenticeships follow the Apprentices Act.

Workplace safety duties: provide PPE, train staff on battery and soldering hazards, keep a first-aid kit and fire extinguisher, and log incidents.

Customer trust builds repeat business: transparent estimates, itemized bills, warranty on repairs and a complaint register with response times.

Digital presence: accurate business listing, service photos and reviews; respond to negative reviews with a fix, not an argument.