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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.
A 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 voltages can 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.
- Check the supply to the circuit: The first steps in checking the circuit are to ensure that it has power being supplied to it. This is easily done using a multimeter set to a voltage range. Measure the voltage using a multimter at the points where the supply enters the circuit board. If the multimeter indicates that there is no supply voltage then there can be a number of possibilities to investigate:
- Battery could be flat if the equipment is battery powered
- If the transistor equipment is battery powered, the battery could have been left in for many months and corroded the battery compartment. Check for this and if there are signs of leakage then clean and remove all signs of corrosion being careful not to touch any of the residue.
- On-off switch faulty. This can be checked by disconnecting any power source and checking for continuity across the switch
- Corroded connector. One common problem is that connectors become corroded with time and connections can become very poor, especially of the equipment has not been used for some time. To overcome this it can help to unplug and then re-mate the connector.
- Check for any broken wiring that would prevent the power reaching the circuit board.
- Check the outputs from the board: In the same way that broken connections may exist for the power line, the same may be true of the outputs from the board. Again it is worth checking any connectors that may have corroded or oxidised with time, and check for any broken connections.
- Check the inputs to the circuit: Likewise, if the signal inputs are not reaching the board then it will not be able to perform. Again any switches, and connectors along with any broken wires should be checked. Often a multimeter can be used to check the continuity of the wires, but first ensure there is no power applied to the circuit.
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.
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:
- Assume the circuit is operating in a linear mode, i.e. it is not a switching circuit.
- Assume the circuit is operating in a common emitter mode as shown in the diagram.
- Assume the circuit has a resistive collector load.
If the assumptions above are true, then the following voltages can be expected. If not then allowances need to be made for the changes.
- The collector voltage should sit at approximately half the rail voltage. More specifically it should sit at half the rail voltage less the emitter voltage. In this way the greatest voltage swing can be obtained. If the transistor has an inductive load, as in the case of the intermediate frequency amplifier in a radio which may have an IF transformer in the collector circuit, then the collector should sit at virtually the same voltage as the rail voltage.
- The emitter voltage should sit at around a volt or two. In most class A common emitter circuits an emitter resistor is included to give some DC feedback. The voltage across this resistor is typically a volt or so.
- The base voltage should sit at the PN junction turn on voltage above the emitter. For a silicon transistor, which is the most common type, this is around 0.6 volts.
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.
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