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BJT Amplifier Troubleshooting
11 questions By Tony R. Kuphaldt
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Question 1 of 11
As an instructor of electronics, I am called upon frequently to help students troubleshoot their malfunctioning lab circuits. When I approach a student’s self-built circuit to troubleshoot it, though, I often begin the process with a very different mindset than if I were troubleshooting a malfunctioning circuit on a real job site.
Aside from different safety considerations and a very different work environment, what else do you think I might consider differently when approaching a student-built circuit? Specifically, how might the range of probable faults differ between a professionally-installed electronic system that malfunctions and a student’s lab project that malfunctions? What generalizations might you make about this difference in troubleshooting perspective, regarding the construction and operational history of the circuit in question?
Reveal answerIf the circuit in question is untried, literally anything could be wrong with it.
Notes:Although sound troubleshooting technique will ultimately yield a solution, asking “pre-diagnostic” questions such as this will greatly enhance your efficiency as a troubleshooter. Discuss this with your students, enlightening them if possible with anecdotes from your own troubleshooting experiences.
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Question 2 of 11
Examine the following “component” stereo system closely:
The CD player generates the audio signal to be amplified, while the equalizer/preamp modifies the tone of the signal to suit the listener’s preferences and the power amplifier provides adequate power to drive the speakers.
Suppose this system has a problem: no sound at all coming out of either speaker. All components in the system are turned on, as indicated by power lights on the front panels. All control knobs seem to be set to their proper positions, as well. The CD player indicates the disk is being played, and that it is presently playing a song. Despite all these good indicators, though, no sound is heard from the speakers.
Being prepared at all times to troubleshoot electronic systems, you have a digital multimeter close by which you may use to check for the presence of audio signals (set the meter to measure AC millivolts). All audio signal cables (including the speaker cables) may be unplugged to provide access for your meter’s test probes.
At what point in the system would you begin testing for the presence of an audio signal? Explain why you chose that point, and describe your subsequent actions based on the results of that test.
Reveal answerMy personal preference would be to unplug the output cables from the equalizer/preamp unit and test for signal output there. I’ll leave the other steps up to you, to elaborate on in class discussion with your peers!
Notes:Discuss with your students how this is an ideal application for the “divide-and-conquer” strategy of troubleshooting, where you divide the signal path into halves, checking for the presence of a signal at each half-way point, narrowing in on the location of the faulted component in a rapid manner.
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Question 3 of 11
Here are a few good steps to take prior to applying any specific troubleshooting strategies to a malfunctioning amplifier circuit:
- Measure the output signal with an oscilloscope.
- Determine if the amplifier is receiving a good input signal.
- Check to see that the amplifier is receiving good-quality power.
Explain why taking these simple steps may save a lot of time in the troubleshooting process. For example, why bother checking the amplifier’s output signal if you already know it isn’t outputting what it’s supposed to? What, exactly, constitutes “good-quality” power for an amplifier circuit?
Reveal answerIt is usually a good idea to verify the exact nature of the malfunction before proceeding with troubleshooting strategies, even if someone has already informed you of the problem. Seeing the malfunction with your own eyes may illuminate the problem better than if you simply acted on someone else’s description, or worse yet your own assumptions.
The rationale for checking the input signal should be easy to understand. I’ll let you answer this one!
“Good-quality” power consists of DC within the proper voltage range of the amplifier circuit, with negligible ripple voltage.
Follow-up question #1: suppose you discover that the “faulty” amplifier is in fact not receiving any input signal at all? Does this test exonerate the amplifier itself? How would might you simulate a proper input signal for the amplifier, for the purposes of testing it?
Follow-up question #2: explain how to measure power supply ripple voltage, using only a digital multimeter. How would you measure ripple using an oscilloscope?
Notes:In my own experience I have found these steps to be valuable time-savers prior to beginning any formal troubleshooting process. In general terms, check for output, check for input, and check for power.
New technicians are often surprised at how often complex problems may be caused by something as simple as “dirty” power. Since it only takes a few moments to check, and can lead to a wide range of problems, it is not wasted effort.
