Technically speaking: Troubleshooting techniques 101
Over the years the requirements of a land mobile radio technician have changed considerably. In spite of all the changes, one thing still separates the “men from the boys”–the troubleshooting ability of the technician, not age or experience. Of course, age and experience certainly tend to enhance the ability of the seasoned technician, but one must have the potential to begin.
You have probably seen “technicians” come and go who could never seem to properly diagnose problems effectively. The “hunt and miss” techniques they used just didn’t cut it in the busy land mobile radio shop where turning out work on a timely basis is survival.
There is no single “right” way to troubleshoot a given piece of communication equipment. Two technicians, given the same arsenal of test equipment, may proceed differently in troubleshooting the same problem. The best method would be the one that leads to the fastest repair of the malfunctioning equipment. In this series we will look at some of the proven methods of troubleshooting that have evolved over the years, and some of the unorthodox methods as well.
The workbench To troubleshoot efficiently, the workbench must be properly organized and equipped. The workbench should have all the test equipment that is commonly used in troubleshooting land mobile equipment. The equipment should be within easy reach from the technician’s normal working position. The center piece of the equipment arsenal is the service monitor. The service monitor normally contains virtually all of the basic test equipment requirements in an integrated package for convenience and accessibility. Other test equipment should include a high-quality bench power supply with voltage and current metering, including adjustable foldback-current-limiting to protect both the supply and the equipment under test.
The workbench should provide sufficient space to place the service manual without taking up all the needed work space. The workbench should be kept uncluttered and should be cleaned between each service job. A cluttered workbench leads to confusion, resulting in inefficiency and, thus, longer service time. A complete set of interconnecting test cables should be nearby and kept in good repair. A bad or intermittent test cable can send the best technician on an occasional wild goose chase!
Of course, good lighting is a must. In addition, a lighted magnifier should be attached to the workbench. The age of microminiaturization necessitates this. Sometimes, a fluorescent light fixture can generate lots of noise, which may interfere with receiver sensitivity tests. Still, fluorescent lighting is desired–just make sure the lighting isn’t causing noise to appear in the receiver.
Talk to the customer The way a customer describes a problem may be confusing, and the technician should make an effort to confirm weird complaints. Remember, the customer is not familiar with technical terms and may describe a problem in an unusual manner. Quiz the customer until you gain an understanding of the problem. It may be a case of the customer not operating the equipment properly.
The equipment should be removed to the workbench only when it is determined that the problem does not lie in the installation or improper operation. Once the equipment is on the workbench, all complaints should be verified through external test and measurement procedures.
Confirming the symptoms When faced with a malfunctioning piece of equipment, often the most difficult part of troubleshooting is deciding where to begin. Let the symptoms be your guide.
Suppose you are handed a transceiver, and you are told that the receiver is inoperative. That complaint is not definitive and, therefore, will require some “external” tests to provide a more definitive answer as to what type of problem exists within the receiver.
The logical step is to hook up the transceiver on the test bench and test the controls, such as squelch and volume. Also, when using the ammeter on the bench power supply, check to make sure the transceiver is drawing the proper amount of current. A large current drain in the receive standby mode would indicate a probable short circuit in the transceiver.
Run a sensitivity check on the receiver. Use both the SINAD and the 20dB-quieting method, and compare results with receiver specifications. If the sensitivity is poor with both methods, then one or more stages of the receiver apparently are malfunctioning.
If you cannot force a high-level signal through the receiver, it is likely that one of the local oscillators (first or second) is not working (assuming the audio section is working). Usually, unless a local oscillator is dead, you will be able to force a high-level (1,000 microvolts or greater) signal through the receiver. It may not sound good, but you should be able to hear it in the speaker.
Operation of the local oscillator(s) can be determined through the use of a spectrum analyzer with a “sniffer” probe held near the oscillator or mixer. In synthesized receivers, the local oscillator will be a voltage-controlled oscillator (VCO). Some sets are single-conversion–there will be no second oscillator. Crystal oscillator failure is most often caused by a defective crystal. In synthesized sets, if the synthesizer is “out of lock,” the entire transceiver usually is disabled by the lock detector. An out-of-lock condition can be caused by a variety of problems within the synthesizer–missing reference oscillator, missing VCO signal, alignment problems, programmable divider problems and a host of other ills.
To find the defective component(s), troubleshooting must follow some systematic procedure. Generally speaking, the trouble must be localized to a section. Once the defective section is determined, the trouble is next localized to a stage. Then within the stage, the trouble must be localized to the defective component(s). Again, the method used to implement this process may vary from technician to technician; however, there are some general rules that can apply.
Next time we will examine several techniques for signal tracing and signal injection that are used to troubleshoot typical land mobile radio communications gear. We will look at some short cuts that can lead you to a dead or seriously impaired stage quickly.
So, until next time–stay tuned!
