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Transistor circuit test and fault finding using a multimeter
- some of the key points to note and hints and tips for testing and fault finding a transistor circuit including those in a radio with a multimeter.
This test meter tutorial is split into several pages: Analogue multimeter
 DMM digital multimeter
 Basic instructions on how to use a multimeter
 Multimeter tutorial and video
 Making a multimeter voltage measurement
 How to measure current with a multimeter
 Making a multimeter resistance measurement
 Simple diode and transistor test
 Testing and fault finding a transistor circuit
One of the main uses for multimeters whether they are analogue multimeters or digital multimeters, DMMs is to test and fault find circuits like those in a transistor radio. Multimeters are ideal items of test equipment for finding many faults in a transistor circuit. However to use a multimeter to test a circuit and find faults it is necessary to have a little knowledge about the circuit, and also to adopt a logical approach in tracking down any faults that may exist.
To find out exactly how to use a multimeter refer to the other pages under the "Related Articles" section on the left hand side of this page under the main menu.
Word of warning!! Some transistor equipment may be mains powered. Only qualified persons should attempt to repair mains powered equipment or equipment that contains high or hazardous voltages. High voltage scan kill so be warned!
Look for obvious faults
The first step when looking at tracing any faults and testing a transistor circuit of any sort is to look for the obvious or major faults. Fortunately most faults with electronics equipment such as transistor radios are relatively major and therefore easy to find. Accordingly the first step in any fault finding is to look for the major problems.
By using a multimeter for the fault finding, it is possible to find many of the obvious faults that can occur. If the problem cannot be found, and it appears that the correct power is reaching the transistor circuit, and the inputs are all connected and present as well as the output lines being intact, then further fault finding on the transistor circuit board itself may be needed. Again a multimeter can assist in this.
Expected voltages in a transistor circuit
If all the inputs to the board appear to be correct, further tests can be undertaken using a multimeter for the fault finding and to track down the problem. Again a systematic approach should be adopted.
When testing a particular transistor circuit, a multimeter can be used to determine if the voltages around the circuit are correct. To test and fault find a particular transistor circuit, it is necessary to have an idea what the steady voltages should be. The circuit below is a typical basic transistor circuit. Many circuits are similar to it, and it provides good starting point to explain some of the points to note.
Expected voltage readings when testing a transistor circuit with a multimeter
The circuit shows several of the points where the voltage can be measured in a circuit. Most of them are measured with respect to ground. This is the easiest way to make a voltage measurement because the "common" or negative probe can be clipped to a suitable ground point (many black probes used for the negative line have a crocodile or alligator clip for this purpose). Then all the measurements can be made relative to ground.
there are normally a number of points around a transistor circuit that are easy to measure, and the expected voltages can be anticipated for the most part if a few assumptions are made:
If the assumptions above are true, then the following voltages can be expected. If not then allowances need to be made for the changes.
Indications of the types of voltage expected can be seen on the circuit diagram.
In addition to this there are many other types of circuit which may need fault finding. Switching circuits are quite common these days where transistors are used to drive other elements such as relays or other devices. These do not operate in a linear mode. Instead all voltages are either on or off. The collector voltage will either be approximately zero when the transistor is on, and approximately the rail voltage when off. The emitter will usually be connected to ground, and the base voltage will be high, i.e. approximately 0.6 volts for a silicon transistor when the transistor is on (i.e. collector near zero), and low, (zero volts) when the transistor is off and the collector is high.
A multimeter is an ideal piece of test equipment to help with fault finding an electronics transistor circuit. Often circuits like transistor radios fail after they have been used for many years, and it is useful to be able to mend them Also when constructing equipment, circuits do not always work first time and it is necessary to fault-find these circuits. While it will not be possible to solve all problems using a multimeter, it is one of the most useful basic tools for any fault finding job.