PCBgogo

The schematic (circuit diagram) acts as a map to the PCB itself. It represents components as symbols, just as a map represents real features as symbols. The designer will often prepare a troubleshooting schematic as part of the product’s documentation, and this schematic might have annotations showing expected voltages or signals at various points, and what the limits on those should be.

Fault-finding often starts with a visual inspection (if there is anything obvious, like a burned component, that is a highly suspicious place to start looking), followed by a run through to check all of the voltages indicated on the schematic are within their published norms. Of course this requires that the board in question is powered up. Service technicians will often have a test harness or rig that they can use to power up and work on a board safely.

If a test voltage or signal is not as expected, the technician must then reason about what he sees compared to what he should see, and what sort of failure would account for it. Often the position of an incorrect voltage will immediately pinpoint a likely problem area, if the designer has provided enough test points. For example, if the voltages going into a transistor stage are nominal, but the voltage on the collector or emitter is wrong, it would point to that transistor being faulty. Given that hypothesis, the technician would verify it by testing additional voltages to confirm the diagnosis.

With experience, a technician can find faults very quickly, and often particular boards and products are prone to common failures that they’ll recognise. Unusual and difficult sleuthing to track down a fault is quite rare, but often more interesting because of it.

The schematic (circuit diagram) acts as a map to the PCB itself. It represents components as symbols, just as a map represents real features as symbols. The designer will often prepare a troubleshooting schematic as part of the product’s documentation, and this schematic might have annotations showing expected voltages or signals at various points, and what the limits on those should be.

Fault-finding often starts with a visual inspection (if there is anything obvious, like a burned component, that is a highly suspicious place to start looking), followed by a run through to check all of the voltages indicated on the schematic are within their published norms. Of course this requires that the board in question is powered up. Service technicians will often have a test harness or rig that they can use to power up and work on a board safely.

If a test voltage or signal is not as expected, the technician must then reason about what he sees compared to what he should see, and what sort of failure would account for it. Often the position of an incorrect voltage will immediately pinpoint a likely problem area, if the designer has provided enough test points. For example, if the voltages going into a transistor stage are nominal, but the voltage on the collector or emitter is wrong, it would point to that transistor being faulty. Given that hypothesis, the technician would verify it by testing additional voltages to confirm the diagnosis.

With experience, a technician can find faults very quickly, and often particular boards and products are prone to common failures that they’ll recognise. Unusual and difficult sleuthing to track down a fault is quite rare, but often more interesting because of it.