Before you can diagnose and repair a phone, there are some skills that you need to
learn. These skills are:
- Testing using a multimeter
- Jumper setting
Let us briefly discuss each skill in turn.
Soldering is a process in which two or more metal items are joined together by
melting and flowing a filler metal into the joint. The filler metal has a relatively lower
Steps In Soldering
1. Prepare the following materials:
- Soldering Iron,
- Solder paste,
- Long Nose Pliers,
- PCB holder,
- Electronic Components (Capacitor,Resistors, Diode etc.)
2. Plug and pre-heat the soldering iron.
3. Heat both items at the same time by applying the soldering iron to the
copper pad and the component lead.
4. Continue heating and apply a few millimeters of solder. Remove the iron and
allow the solder joint to cool naturally.
5. It only takes a second or two to make the perfect joint, which should appear
Desoldering is the removal of solder and components from a printed circuit
board for troubleshooting, repair, replacement, and salvage.Steps in desoldering
1. Use a solder wick (finely braided copper) to wick away excess solder from a desoldered connection.
2. Apply the solder wick and use the soldering iron to the de-soldered
connection. The solder wick will draw the excess solder off the PCB pad.
Testing a phone using a multimeter
We hope you still remember that a multimeter is a device that is used to measure
the voltage, current and resistance of various components of a mobile phone.
We shall now discuss how to measure resistance, voltage and current using a
a) Measuring Resistance
To measure resistance follow these steps:
1. Plug your red and black probes into the
appropriate sockets on your multimeter.
2. Choose the appropriate resistance
measurement setting on your millimeter’s
3. Hold the probes against the resistor.
4. Check the resistor value on the display.
b) Measuring voltage
Testing for proper supply voltage is usually the first step when troubleshooting a
circuit. To measure voltage you should follow these steps:
1. Select V~ (ac) or V (dc), as desired.
2. Plug the black test probe into the COM input jack. Plug the red test probe
into the V input jack.
3. If the DMM has a manual range only, select the highest range so as not to
overload the input.
VΩ m A jack
Take Note: Always turn off the power supply to your circuit before
Mobile Phone Repair and Maintenance
4. Touch the circuit with the tips of the probes
5. Read the number in the display window and take note of the unit of
c) Measuring Current
1. Turn off power to the circuit.
2. Cut or unsolder the circuit, creating a place where the meter probes can be
3. Select A~ (ac) or A (dc) as desired.
4. Plug the black test probe into the COM input jack. Plug the red test probe
into the amp or milliamp input jack, depending on the expected value of the
5. Connect the probe tips to the circuit across the break so that all current will
flow through the DMM (a series connection).
6. Turn the circuit power back on.
Jumpering means to temporarily complete a circuit or to bypass a break in a circuit
by making a connection from one point to another.
A good conductor wire is used to make a jumper which by-passes the components
and passes on a signal or supply line for further uses.
When wire is used as a jumper, it must have some special specifications as required.
These jumper wires can mainly be of two types i.e. insulated and non-insulated. In
the mobile phone insulated wires are used for jumpers. The length of a jumper
depends on the two points connected in between.
Why do Jumpering
While repairing mobile phones, we find that certain faulty components are very
difficult to get from the market. To repair such mobile phones the only immediate
option is the use of jumpers. By use of jumpers we will bypass the faulty components
How to Jumper
1. Disassemble mobile phone and place it on a PCB holder.
2. Using a multimeter, check track and find the fault or the missing track that
3. Apply liquid soldering flux to the points where you need to solder jumper
4. Cut jumper wire to desired length and remove its lamination using blade
5. Hold one end of the jumper wire and solder it to one point of the faulty
circuit track. Use a good quality tweezers to hold the wire and good quality
of soldering iron and solder wire to solder.
6. Now hold the other end of the jumper wire and solder to the other point of
7. Using a multimeter check the jumper.
The Figure 31 Below shows jumper settings in of the jumpers may look like on
your motherboard. In this example, the jumper is the white block covering two of the
three gold pins. Also, next to the pins is a silkscreen description of what the pins do,
in this case when pins 1-2 are jumped the computer is operating normal, when 2-3
are jumped it is set into configuration mode, and when open the computer will be in
You now know the skills that you need in order to diagnose and repair a mobile
phone. Let us now learn how to diagnose and repair mobile phones.
Mobile Phone Diagnosis