Introduction: Testing Bad Transistors on PCBs

Testing transistors on a printed circuit board (PCB) can be crucial for troubleshooting a wide range of electronic devices. This article offers a detailed guide on how to identify and test bad transistors, focusing on practical methods and real-world scenarios.

Understanding Transistor Failures

A transistor is considered bad if its performance does not meet the required specifications or it causes unnecessary expense for the same performance. In this article, we will explore a specific case where a bad NPN transistor in an AM radio receiver caused unexpected issues and how to diagnose and resolve such issues.

Distinguishing Between Good and Bad Transistors

The process of testing a transistor on a PCB involves visual inspection, measuring supply voltages, and using specialized equipment. If you have access to a schematic diagram or are familiar with the circuit, you can power up the device and measure voltages across different stages. However, if you're working without a schematic, you may need to remove the transistor from the PCB to test it more accurately.

Physical Testing of Transistors

One effective method to test a transistor is to remove it from the PCB and connect it to an ohmmeter. This allows you to determine whether the transistor is PNP or NPN and whether it is functioning correctly.

Schematic and Component Verification

Once you've physically removed the transistor, you can use an ohmmeter to measure its resistance between the various leads. If the resistance values do not match the expected range, the transistor may be failing. For instance, the HFE (Current Gain) of an NPN transistor like the C9011 should be around 52 or another typical value for that type. If the HFE is significantly different, the transistor is likely faulty.

Case Study: A Bad Transistor in an AM Radio Receiver

Consider the scenario where a bad transistor in an AM radio receiver caused the device to fail. The C9011 NPN transistor was used as a mixer-oscillator in a battery-powered AM radio. Despite normal supply voltages and circuit testing, the radio failed to receive signals.

Upon signal injection testing, the radio stations were received again, indicating that the mixer-oscillator stage was indeed the culprit. When the C9011 was removed and tested with a digital multimeter, the HFE was found to be within the normal range, suggesting that the transistor was not the immediate cause. However, after replacing the transistor, the radio functioned normally, highlighting the importance of thorough testing.

Common Issues and Fixes

Typically, transistors can fail in three ways: they can open, short out, or partially fail. An open transistor means that the circuit between the Emitter and Collector is broken, while a shorted transistor means that the Emitter-Base or Base-Collector junctions are connected, causing a loss of control over current flow.

Testing Techniques

To test a transistor in circuit, it's essential to perform the test with a stable signal. If the Emitter to Base and Base to Emitter resistances are significantly different, the transistor is likely functioning properly. If the resistance is close or zero, the transistor may be shorted or open.

Signal Injection for Troubleshooting

Signal injection is an effective method to pinpoint problematic stages in a circuit. By injecting a known signal into a suspected stage, you can verify if the device functions correctly. If the device responds, it confirms the stage is the source of the issue.

Conclusion

Testing transistors on a PCB can be a complex but essential task. By understanding the common failure modes and employing effective testing techniques, you can diagnose and resolve issues in electronic devices with confidence. Whether you're a hobbyist, an engineer, or a repair professional, this guide offers valuable insights into transistor testing and troubleshooting.