* 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.
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.
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 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.
1. 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.
2. 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.
3. 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 sued 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.
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