Tools and Test Equipment

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A repair shop’s tools fall into three tiers. Students should own Tier 1 by Week 2 of the course. A new shop should open with Tier 1 and Tier 2. Tier 3 is added as board-level work grows.

Tool tiers

Tier 1 — Hand tools (module-level repair)

Tier 2 — Soldering and diagnostic station (component-level repair)

Tier 3 — Advanced

The digital multimeter (DMM) in depth

The multimeter is the most important instrument in the shop. Buy a true-RMS, auto-ranging meter with fast continuity beep and diode mode. Probes should have fine, sharp tips; add needle-tip probes for phone boards.

Functions you will use

Golden rules of measurement

1. Voltage is measured with the circuit powered; resistance, continuity and diode mode with the circuit unpowered (battery and adapter disconnected). Measuring resistance on a powered circuit gives wrong readings and can damage the meter.
2. Black probe to ground for voltage measurements. Use a screw hole, shield, or large ground pad.
3. Never leave the red lead in the A (current) socket when measuring voltage — that puts a near short across whatever you probe and can blow the meter fuse or damage the board.
4. On tiny boards, keep probes perpendicular and steady to avoid bridging two adjacent pins.

Diode-mode measurement (“diode readings”)

This technique is used throughout Chapters 11 and 16. With the board unpowered:

• Put the red probe on ground and the black probe on the test point (this is the convention used by most phone and laptop board technicians; it measures the reverse direction through the protection diodes built into the chips on that line).
• Read the value on the display (typically shown in volts or as a three-digit number such as 0.452 or 452).
• Compare with a known-good board of the same model, or with a published reference value.

How to interpret:

Values are typical, not universal: most data and power lines on phones and laptops read somewhere between about 0.3 and 0.8 in this mode, but the only correct reference is a known-good unit of the same model and board revision.

Measuring resistance to ground

On large rails (e.g. a laptop CPU core rail, PPBUS, VBAT), resistance to ground may legitimately be low because of the load. CPU core rails can measure only a few ohms or less on a healthy board. A reading of 0–1 Ω on a rail that should be hundreds of ohms or more indicates a short. Again: compare with a known-good reference.

The bench DC power supply

A linear or switching lab supply with adjustable voltage and adjustable current limit. In repair it is used three ways:

1. As a battery substitute for phones and laptops — powering the board through the battery connector (or a dedicated cable) while watching the current display. The current pattern during power-on is a powerful diagnostic signal (Chapter 16).
2. As an adapter substitute for laptops — supplying 19–20 V through the DC jack with a current limit to see whether the board draws current, and how much.
3. For short injection — feeding a low voltage into a shorted rail to heat the faulty component.

Short injection procedure

1. Confirm the rail is shorted (diode mode / resistance near 0).
2. Identify the rail’s normal operating voltage from the schematic. Never inject a voltage higher than the rail’s normal voltage, and stay at or below the lowest voltage rating of components on that rail. For low-voltage rails start at about 0.5–1 V.
3. Set the current limit low (start around 0.5–1 A) and connect: red lead to the rail (a capacitor on that rail), black lead to ground.
4. Raise the current gradually. The supply will be in current-limit (constant-current) mode because the short pulls the voltage down.
5. Find the component that heats: thermal camera, a thin film of IPA (evaporates first over the hot part), rosin smoke (freeze-spray then watch where frost melts first), or a fingertip (carefully).
6. Remove the hot component and re-measure. If the short is gone, replace it with a correct part.

Caution: If the short is inside a large chip (CPU, PMIC, SoC), injection will heat that chip. Do not inject high current for long into expensive ICs. Diagnose and decide.

Thermal camera

A thermal camera turns heat into a picture, showing shorted components within seconds of injection. Models designed for board repair have close-focus lenses. A smartphone-attachment thermal camera with a macro lens is an affordable starting point.

Microscope

Stereo zoom microscopes (7–45×) give depth perception, which you need for soldering. Mount it on a stable boom or pillar stand with an LED ring light. Optional: a trinocular port with a camera for recording work and teaching — very useful in a classroom.

Oscilloscope basics

Use an oscilloscope to see signals that change too fast for a multimeter:

• PWM switching at a buck converter’s phase node (a square wave confirms the controller is switching).
• Crystals/clocks (sine or square wave at the expected frequency).
• Data lines (activity bursts on SPI, I²C).

Set the probe to ×10, connect probe ground to board ground with the shortest possible lead, and use auto-set as a starting point.

Software toolkit

Keep these on a USB drive (preferably using a multi-boot tool such as Ventoy, which lets you copy several ISO files onto one drive and choose at boot):

Watch and read

Review questions

1. Why must resistance and diode-mode measurements be taken with the device unpowered?
2. In the diode-mode technique described, which probe goes to ground?
3. A data line reads OL in diode mode while the known-good reads 0.520. What does this indicate?
4. Describe the safety limits when injecting voltage into a shorted rail.
5. Name three methods to locate a heating component during short injection.

Lab exercises

Lab 3.1 — Multimeter drills. Measure: a 9 V battery, a USB port’s 5 V, ten resistors (compare to code), continuity of a ribbon cable, and the forward voltage of a silicon diode, a Schottky diode and an LED.

Lab 3.2 — Known-good database. Using two identical scrap boards (one working), record diode-mode readings on 20 points (charge port pins, battery connector pins, main rails). Start your own reference notebook — a technician’s known-good database is one of their most valuable assets.

Lab 3.3 — Short injection. On a scrap board, the instructor creates a short by soldering a deliberately shorted capacitor onto a low-voltage rail. Locate it using injection and a thermal camera or IPA.