TESTING COMPONENTS "IN-CIRCUIT"

You can test components while they are IN CIRCUIT, but the surrounding components
will have an effect on the results.
You can get all sorts of "In-Circuit" testers. They are expensive and offer little more
accuracy than a multimeter.
In-Circuit testing with a multimeter can give you the same results as a tester.
All you have to do is turn the project ON and use a multimeter (set to voltage) to
determine the voltage at various points. It is best to have a circuit of the equipment so
you can what to expect at each point.
Only major departures from the expected can be located in this way.
Obviously the first thing to look for is burnt-out components. Then feel components
such as transistors for overheating.
The look for electrolytics that may be dry. Sometimes these have changed colour or
are slightly swollen.
If they are near hot components, they will be dry.
For the cost of a few dollars I change ALL THE ELECTROLYTICS in some pieces of
equipment, as a dry electrolytic is very difficult to detect.
Testing a transistor "in-circuit" is firstly done with the supply ON. That's because it is
quicker.
Measure the voltage between ground and collector.
In most cases you should get a voltage of about half-rail. If it is zero, or close to rail
voltage, you may have a problem.
Turn off the supply and use the multimeter on low-ohms to measure all six resistances
between the leads.
A low resistance in both directions on two leads will indicate a fault.
Resistors almost NEVER go "HIGH." For instance, a 22k will never go to 50k. However
a low-value resistor will "burn-out" and you will read the value of the surrounding
components.
Don't forget, some low-value resistors are designed to burn-out (called fusible
resistors) and anytime you find a damaged low-value resistor, you will need to look for
the associated semiconductor.
You can replace the resistor quickly and turn the circuit ON to see it burn out again.
Alternatively you can trace though the circuit and find the shorted semiconductor.
It's always nice to "see the fault" then "fix the fault."
Sometimes a transistor will only break-down when a voltage is present, or it may be
influenced by other components.
When the piece of equipment is turned OFF, you can test for resistance values. The
main thing you are looking for is "dry joints" and continuity. Dry joints occur around
the termination of transformers and any components that get hot. Rather than wasting
time checking for dry joints, it is better to simply go over the connections with a hot
iron and fresh solder.
You may need to check the continuity of a track (trace) and it may go from one side of
the PC board to the other.
Use a multimeter set to low-ohms and make sure the needle reads "zero-ohms."
It is very dangerous to do any testing on a project using a multimeter set to "amps" or
"milliamps."
You cannot test "current flowing through a component" by placing the probes across a
component. You will simply over-load the rest of the circuit and create a problem.
To find out if current is flowing though a circuit or a low-value resistor, turn the project
ON and measure the voltage either across the component or the voltage on one end
then the other.
A voltage-drop indicates current is flowing.
That's about it for testing "in-circuit." Use the rest of this eBook to help you with
diagnosis.
Don't think an IN-CIRCUIT COMPONENT TESTER is going to find a fault any faster than
a multimeter. They all use a multimeter principle.