How to repair PCBs (circuit boards) and replace electronic components

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Tools needed for replacing components on a circtuit board

You need at least:
-A soldering iron with a precision tip
-A desoldering pump
-Solder wire with flux
-A digital multimeter
-A set of regular precision tools: screwdrivers, pliers, etc.

The digital multimeter is used to test components (not explained here), and used on its 20K resistance reading position, always marked with a diode symbol.

Testing the semiconductors on a circuit board.

Prior to testing electronic components on a circuit board, it is necessary to disconnect all wires and cables that connect to it, and any source of power including batteries.

Semiconductor junctions are to return a low reading when the digital multimeter leads are used in one way, and an open circuit reading when reversing the red and black leads.
Open circuit (high reading, out of range) or zero reading (short-circuit) are signs of a component failure.
Testing different types of semiconductors like transistors, thyristors, regulators and ICs, etc. is not explained in this pictorial.

It is a good practice to mark each tested component that passes, not get confused and forget which device on the PCB was tested and which one was not.

Once you've determined the failing component, you need to buy a replacement as follows:
-Go online and type the number of the component so the store returns equivalents to you
-You might have doubt about which one is the number to use, just try all the alphanumerical ones shown on the body of the component
-Corroded devices are to be carefully cleaned in order to get the number
-The replacement electronic component has to match in size, leg assignment, and electrical characteristics

Do not use solvents when cleaning very old metal body electronic components. You can brush them very gently by hand and use alcohol, WD40, electronic cleaner, a wet towel or any combination to eliminate rust and dirt and have access to the component number.

Removing an electronic component from a circuit board

Make sure not to breathe the solder vapor or smoke that will result from melting it, work in a well ventilated area, pull away your face from the smoke and/or wear a good protective mask.
Proceed as follows:

Heat your soldering iron and put little solder on its tip, if it's the first use
Apply the soldering iron tip directly to the solder around the leg of the component to remove, add little solder from your solder wire to initiate the melting process faster
Once melted, apply immediately the desoldering pump which you should have pre-loaded, and press the button
Repeat the process on each leg and until holes have been created aroud the legs of the electronic component, so the latter can be taken out very easily without forcing with any tool.

Transistor Q17 has been removed from this circuit board after having pumped out all solder around its legs:

Buy your soldering kit or station here:

The use of the desoldering pump in particular, requires some patience and you'll need to train on its use.
The circuit board has to be firmly sitting on some hard surface so you can press with the pump.
You need to activate its button at the same time when its tip touches the melted solder.

Refitting an electronic component to the circuit board (PCB)

Using long nose pliers, bend the legs of the component like this BT152-600R thyristor to let them match the holes in the board and in a manner that the component sits vertically once introduced in its place.

Weld the legs to the board before cutting them.
Make sure to apply the soldering iron tip to the component leg, which heat should melt the solder wire, not to melt the solder wire on a cold leg.
Once the solder spreads, move the soldering iron tip in half-circle to spread well the solder and make sure it looks like a cone.
Still, you have to be a quick as possible, excess heat applied to the component leg can damage the semiconductor in a non-reversible manner.

Once done with the electronic component soldering, you can use cutting pliers to cut the excess length of legs.
Make sure your solder cones do not touch each other, or touch anything nearby.

Additional tips for electronic components replacement on boards

Some circuits are resin encapsulated which makes them literally impossible to fix.
This resin aged enough and got deteriorated; it was possible to mechanically remove it without damaging the components beneath, but then, a thorough cleaning of the board had to be applied prior to testing and replacing semiconductors.
Solvents cannot be used since most components have plastic bodies, also solvent damages the insulating varnish that covers PCBs; use exclusively alcohol to clean your boards.

When electronic components are fitted to a heat sink like these transistors, it is necessary to use heat sink compound upon refitting them, to ensure optimum heat transfer between the metal body of the component and the heat dissipator. Tightening the screw to the max is also a must.

This transistor has a heat dissipator inserted in its legs to let its body dissipate heat from beneath.