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| |[pic] |[|Transistor circuit test and fault finding using a multimeter |[|[pic] |

| | |p|- some of the key points to note and hints and tips for testing and fault finding a transistor circuit including those in a |p| |

| |[pic] |i|radio with a multimeter. |i| |

| |Home |c|[pic][pic][pic][pic][pic][pic]One of the main uses for multimeters whether they are analogue multimeters or digital multimeters,|c| |

| |Audio and Video |]|DMMs is to test and fault find circuits like those in a transistor radio. Multimeters are ideal items of test equipment for |]| |

| |Basic concepts | |finding many faults in a transistor circuit. However to use a multimeter to test a circuit and find faults it is necessary to |[| |

| |Circuits - analogue | |have a little knowledge about the circuit, and also to adopt a logical approach in tracking down any faults that may exist. |p| |

| |Electronic components | |To find out exactly how to use a multimeter refer to the other pages under the "Related Articles" section on the left hand side |i| |

| |Radio | |of this page under the main menu. |c| |

| |Ham radio | | |]| |

| |Constructional | |Word of warning!!   Some transistor equipment may be mains powered. Only qualified persons should attempt to repair mains |[| |

| |techniques | |powered equipment or equipment that contains high or hazardous voltages. High voltage scan kill so be warned! |p| |

| |Testing and test equipment | | |i| |

| |Links | |[pic] |c| |

| |[pic] | |Look for obvious faults |]| |

| |Bookshop | |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.|P| |

| |[pic] | |This is easily done using a multimeter set to a voltage range. Measure the voltage using a multimter at the points where the |r| |

| | | |supply enters the circuit board. If the multimeter indicates that there is no supply voltage then there can be a number of |a| |

| |  Related articles | |possibilities to investigate: |c| |

| |Analogue multimeter | |Battery could be flat if the equipment is battery powered |t| |

| | | |If the transistor equipment is battery powered, the battery could have been left in for many months and corroded the battery |i| |

| |DMM digital multimeter | |compartment. Check for this and if there are signs of leakage then clean and remove all signs of corrosion being careful not to |c| |

| | | |touch any of the residue. |a| |

| |Basic instructions on how to use | |On-off switch faulty. This can be checked by disconnecting any power source and checking for continuity across the switch |l| |

| |a multimeter | |Corroded connector. One common problem is that connectors become corroded with time and connections can become very poor, |E| |

| | | |especially of the equipment has not been used for some time. To overcome this it can help to unplug and then re-mate the |l| |

| |Multimeter tutorial and video | |connector. |e| |

| | | |Check for any broken wiring that would prevent the power reaching the circuit board. |c| |

| |Making a multimeter voltage | | |t| |

| |measurement | |Check the outputs from the board:   In the same way that broken connections may exist for the power line, the same may be true |r| |

| | | |of the outputs from the board. Again it is worth checking any connectors that may have corroded or oxidised with time, and check|o| |

| |How to measure current with a | |for any broken connections. |n| |

| |multimeter | | |i| |

| | | |Check the inputs to the circuit:   Likewise, if the signal inputs are not reaching the board then it will not be able to |c| |

| |Making a multimeter resistance | |perform. Again any switches, and connectors along with any broken wires should be checked. Often a multimeter can be sued to |s| |

| |measurement | |check the continuity of the wires, but first ensure there is no power applied to the circuit. |H| |

| | | |By using a multimeter for the fault finding, it is possible to find many of the obvious faults that can occur. If the problem |a| |

| |Simple diode and transistor test | |cannot be found, and it appears that the correct power is reaching the transistor circuit, and the inputs are all connected and |n| |

| | | |present as well as the output lines being intact, then further fault finding on the transistor circuit board itself may be |d| |

| |Testing and fault finding a | |needed. Again a multimeter can assist in this. |b| |

| |transistor circuit | |[pic] |o| |

| | | |Expected voltages in a transistor circuit |o| |

| | | |If all the inputs to the board appear to be correct, further tests can be undertaken using a multimeter for the fault finding |k| |

| | | |and to track down the problem. Again a systematic approach should be adopted. | | |

| |Latest added and updated pages | |When testing a particular transistor circuit, a multimeter can be used to determine if the voltages around the circuit are |I| |

| |Ham radio contesting hints and | |correct. To test and fault find a particular transistor circuit, it is necessary to have an idea what the steady voltages should|a| |

| |tips | |be. The circuit below is a typical basic transistor circuit. Many circuits are similar to it, and it provides good starting |n| |

| |Ham radio contests and calendar | |point to explain some of the points to note. |S| |

| |Darlington pair transistors | |[pic] |i| |

| |Understanding transistor | |Expected voltage readings when testing a transistor circuit with a multimeter |n| |

| |specifications | |The circuit shows several of the points where the voltage can be measured in a circuit. Most of them are measured with respect |c| |

| |Choosing replacement transistors | |to ground. This is the easiest way to make a voltage measurement because the "common" or negative probe can be clipped to a |l| |

| |How does a diode work | |suitable ground point (many black probes used for the negative line have a crocodile or alligator clip for this purpose). Then |a| |

| |How does a transistor work | |all the measurements can be made relative to ground. |i| |

| | | |there are normally a number of points around a transistor circuit that are easy to measure, and the expected voltages can be |r| |

| | | |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. |J| |

| | | | |o| |

| | | |Assume the circuit is operating in a common emitter mode as shown in the diagram. |h| |

| | | | |n| |

| | | |Assume the circuit has a resistive collector load. |D| |

| | | | |u| |

| | | |If the assumptions above are true, then the following voltages can be expected. If not then allowances need to be made for the |n| |

| | | |changes. |t| |

| | | |The collector voltage should sit at approximately half the rail voltage. More specifically it should sit at half the rail |o| |

| | | |voltage less the emitter voltage. In this way the greatest voltage swing can be obtained. If the transistor has an inductive |n| |

| | | |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. |[| |

| | | | |p| |

| | | |The emitter voltage should sit at around a volt or two. In most class A common emitter circuits an emitter resistor is included |i| |

| | | |to give some DC feedback. The voltage across this resistor is typically a volt or so. |c| |

| | | | |]| |

| | | |The base voltage should sit at the PN junction turn on voltage above the emitter. For a silicon transistor, which is the most |a| |

| | | |common type, this is around 0.6 volts. |m| |

| | | | |a| |

| | | |Indications of the types of voltage expected can be seen on the circuit diagram. |z| |

| | | |In addition to this there are many other types of circuit which may need fault finding. Switching circuits are quite common |o| |

| | | |these days where transistors are used to drive other elements such as relays or other devices. These do not operate in a linear |n| |

| | | |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|c| |

| | | |volts) when the transistor is off and the collector is high. |o| |

| | | |[pic] |.| |

| | | |Summary |u| |

| | | |A multimeter is an ideal piece of test equipment to help with fault finding an electronics transistor circuit. Often circuits |k| |

| | | |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 |p| |

| | | |not be possible to solve all problems using a multimeter, it is one of the most useful basic tools for any fault finding job. |i| |

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