GSM Circuit Tracing SS2 Computer Hardware & GSM Repair Lesson Note

Class: SS2

Subject: Computer Hardware and GSM Repair

The Invisible Road

Imagine you are trying to turn on a lightbulb in your room, but it won’t shine. You check the bulb, and it’s fine. You check the switch, and it’s fine. The problem must be the wire hidden inside the wall.

Inside a mobile phone, there are thousands of tiny “invisible roads” called Tracks. They carry electricity from the battery to the screen, the speaker, and the charging port. Circuit Tracing is the act of using a Multimeter to follow these roads to see where they are “broken.”

The Secret Weapon: Continuity Mode

To trace a circuit, we almost always use the Continuity Mode on our digital multimeter.

• The Icon: Look for a symbol that looks like a small “Wi-Fi” icon or a musical note (sound waves).
• How it works: When you touch the two metal probes (Red and Black) together, the meter makes a BEEP sound.
• The “Human” Side: The beep means “The road is clear!” If you touch two ends of a track and it doesn’t beep, it means the road is “broken” (Open Circuit).

How to Trace a Path (Step-by-Step)

Let’s say a customer brings a phone where the Loudspeaker isn’t working, but the speaker itself is new. Here is how we trace it:

1. Find the Points: Identify the two gold pads on the motherboard where the speaker touches.
2. Black Probe on Ground: Put the Black probe on any large metal shield on the board (this is “Ground”).
3. Red Probe on the Path: Touch the gold pads with the Red probe.
4. Listen for the “Value”: * If the meter shows a number (like 450 or 600), the line is connected to the “Boss” (the Power IC).
   1. If it shows “1” or “OL” (Open Line), the track is broken somewhere.

Finding a “Short Circuit”

A “Short” is when electricity takes a shortcut it wasn’t supposed to take. This usually happens when a Capacitor gets damaged and starts touching the “Ground.”

• The Symptom: The phone gets very hot, or the battery drains in 10 minutes.
• The Test: 1. Set the meter to Continuity (Beep) mode. 2. Put the Black probe on a metal shield (Ground). 3. Touch the Red probe to both sides of a brown capacitor.
• The Result: A healthy capacitor should only beep on one side. If it beeps on both sides, you have found a “Short!”

Using “Jumper Wires” to Fix Breaks

Once you use your multimeter to find a break in the track, how do you fix a road you can’t see? We use a Jumper.

• What is it?: A very thin copper wire coated in a tiny layer of plastic (insulation).
• The Fix: You solder one end of the wire to where the signal starts and the other end to where it needs to go. You are essentially building a “bridge” over the broken part of the road.
• Pro Tip: Always use a tiny bit of “Solder Flux” to make the wire stick properly to the small pads.

Summary: Troubleshooting Guide

Student Practical Activity

“Take a dead motherboard and pick two points that look connected. Use your meter to ‘hunt’ for the beep. If you find a path that should beep but doesn’t, try to see if you can spot a tiny scratch on the board that might have cut the track.”