UPMSP Vocational Mobile Repairing cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In mobile circuits, current draw at a fixed voltage reveals resistance faults (excess current = short).

Series resistance adds (R = R1 + R2); parallel resistance divides (1/R = 1/R1 + 1/R2). Two equal resistors in parallel give half the value.

A silicon PN junction drops about 0.6–0.7 V forward; a Schottky diode about 0.2–0.3 V. Diode-mode multimeter readings near 0 V indicate a short, OL indicates open.

Capacitors block DC and pass AC; they store charge Q = C × V and smooth supply rails. A shorted capacitor reads near 0 Ω in both directions; an open one reads OL.

Inductors oppose changes in current; they are used in buck/boost converters and RF matching. Continuity through a coil is normal; infinite resistance means a broken winding.

AC in a phone is not mains AC: switching regulators create high-frequency square waves. Use an oscilloscope or frequency counter, not a plain multimeter, to verify clock and PWM signals.

Digital logic levels: logic high is near the rail voltage (commonly 1.8 V or 3.3 V), logic low near 0 V. A node stuck between these levels indicates a leakage or pull-up fault.

Ground reference matters: always connect the meter's black probe to a known good ground (shield can or ground pad) before measuring a rail. Floating ground gives false readings.

Main blocks: SoC (CPU+GPU+modem), RAM, eMMC/UFS storage, PMIC, RF transceiver + PA + front-end module, display/touch connector, camera connectors, audio codec, sensors, and charging IC.

The PMIC generates multiple rails (VBUCK, VLDO) from the battery; a missing rail points to the PMIC, its inductor, or its feedback network rather than to the load.

RF section: antenna → front-end module (switch/filter) → power amplifier → transceiver → baseband. Signal faults can be isolated by testing at each stage.

eMMC/UFS stores bootloader, OS, and user data. Failure modes include corrupted partitions (software) and dead NAND (hardware); distinguish by ISP/EDL access.

Connectors are frequent failure points: display, battery, charging port, and antenna spring contacts. Reflow or replace before condemning the IC driving them.

Water-damage indicators and corrosion on shields/connectors often explain intermittent faults better than component-level testing.

Board-to-board connectors carry power and high-speed data; bent pins cause no-boot, no-charge, or no-display symptoms depending on the pin group affected.

Standard toolkit: precision screwdrivers (Phillips, Y-type, Pentalobe), plastic pry tools, suction cup, tweezers, heat mat/hot air, anti-static wrist strap, and a magnetic mat for screw mapping.

Heat the frame to roughly 60–80 °C to soften adhesive before prying; excessive heat damages OLED panels and battery cells.

Disconnect the battery connector first and reconnect it last; this prevents short circuits while other connectors are handled.

Screw mapping is mandatory: screws differ in length, and a long screw in a short hole can puncture the board or battery.

Adhesive strips under the battery are pulled slowly and parallel to the chassis; prying with metal tools risks puncturing the cell.

After assembly, verify display, touch, cameras, speaker/mic, charging, and network registration before sealing the device.

Keep screws and small parts organized by step; lost or swapped screws are a leading cause of post-repair faults.

Start with the symptom history: drop, water, heat, or software update. Match the symptom to the most likely block before opening the device.

Divide-and-conquer: measure the supply rail at the load; if present, the fault is downstream (component), if absent, upstream (PMIC/inductor/feedback).

Current-draw signature on a DC supply: near-zero = open/no boot; high steady = short; pulsing = boot loop. Compare with a known-good board.

Thermal camera or isopropyl alcohol evaporation locates shorted components; the warm or fast-drying part is suspect.

Diode-mode testing on connectors and rails finds shorts to ground quickly; compare readings with a known-good board.

Substitution testing (known-good battery, screen, flex) isolates the fault to device or accessory in minutes.

Record findings per repair; repeat faults on the same model often share a common failure point.

Lead-free solder (SAC305) melts around 217–220 °C; set iron tips to roughly 320–360 °C for general work and hot air to 350–400 °C with airflow matched to part size.

Use flux for every rework step; it removes oxides and improves wetting. Clean residue with isopropyl alcohol after the joint cools.

A good joint is shiny, concave, and wets both pad and lead; a dull, ball-shaped joint indicates cold solder or insufficient flux.

Hot-air rework: preheat the board, apply airflow around (not directly on) the part, and lift with tweezers only after the solder is fully liquid.

BGA reballing requires a stencil, solder balls of the correct diameter, flux, and a reflow profile; misalignment causes bridges under the package.

Never exceed the component's datasheet reflow temperature; connectors and plastic parts deform easily. Use kapton tape as a heat shield.

After rework, inspect under magnification for bridges, tombstones, and lifted pads, then verify with a multimeter before powering.

Flashing erases and rewrites firmware partitions. Always confirm the exact model and firmware build; a mismatched image can brick the device.

Bootloader/fastboot, EDL (Emergency Download), and ISP are recovery paths. EDL requires a signed programmer file and the correct Firehose loader for the chipset.

A full flash typically includes bootloader, modem, system, vendor, and userdata partitions; a partial flash can leave version mismatches that cause boot loops.

Back up user data before flashing; most flash tools wipe storage by default.

After flashing, clear cache/data and verify IMEI, baseband, and network registration; a missing baseband indicates a firmware or partition fault.

FRP and account locks are security features; bypassing them is not a legitimate repair procedure. Restore the device to the owner.

Keep a log of tool version, firmware file, and result for every flash; this shortens diagnosis of repeat failures.

Cellular generations: 2G (GSM), 3G (UMTS/WCDMA), 4G (LTE), 5G (NR). Each uses different bands and modulation; a phone may support some but not all.

Signal strength is measured in dBm; values closer to 0 are stronger. Roughly −50 to −70 dBm is excellent, −90 to −100 dBm is weak, below −110 dBm is marginal.

SINR/RSRQ indicate signal quality; high RSRP with poor SINR points to interference, not distance from the tower.

Antenna faults show as low or no signal on all bands; a single-band failure points to the front-end module or filter for that band.

APN settings control data connectivity; wrong APN gives voice but no data.

Wi-Fi and Bluetooth use 2.4/5 GHz radios separate from cellular; test them independently to isolate the fault.

Flight mode toggling and network reset are quick software checks before condemning the RF hardware.

Li-ion cell nominal voltage is about 3.7 V; full charge is 4.2 V (4.35–4.4 V on some high-voltage cells); cutoff is around 3.0 V.

Charging profile is CC/CV: constant current until the cell reaches ~4.2 V, then constant voltage with tapering current.

Charging IC controls current and voltage; a phone that charges slowly or not at all may have a faulty IC, port, or cable rather than a dead battery.

Measure battery voltage under load; a cell that reads 3.7 V open but collapses under load is failing.

Battery health below about 80% capacity causes shutdowns under peak load; recommend replacement.

Never charge a swollen battery or puncture a cell; isolate and dispose of it per local e-waste rules.

USB-C PD and fast-charge protocols negotiate voltage/current; a non-compliant charger may charge slowly or not at all.

ESD precautions: grounded wrist strap, ESD mat, and ionized air where needed. Handle boards by edges; never touch connector pins or IC leads.

Store sensitive parts in anti-static bags or conductive foam; never in plain plastic bags.

Lithium batteries are a fire risk: keep a Class D or lithium-rated extinguisher nearby; never use water on a burning cell.

Soldering fume extraction is required; rosin flux fumes cause respiratory irritation. Work in a ventilated area.

Safety glasses when clipping leads or using hot air; hot tools rest in stands, never on the bench.

E-waste: separate batteries, PCBs, and displays; dispose through licensed recyclers. Do not burn or landfill.

Keep the bench dry and uncluttered; spills near live equipment are a shock hazard.

Price repairs by parts cost + labor + risk margin; publish a diagnostic fee that is credited toward the repair.

Written job sheets record device condition, reported fault, agreed price, and customer signature; they protect both parties.

Warranty terms should state duration, coverage (parts vs. labor), and exclusions (water damage, physical damage).

Turnaround estimates should be realistic; under-promising and over-delivering builds repeat business.

Track inventory and parts usage; stock-outs and dead stock both cost money.

Customer communication: explain the fault in plain language, show the old part, and confirm before extra work.

Comply with local tax, licensing, and data-protection rules; never access customer data beyond what the repair requires.