Precautions for Measuring DC Voltage with a Multimeter
Under powered‑on conditions, use the DC‑voltage range of a multimeter to measure DC supply voltage and operating voltages of peripheral components. Measure the DC voltage to ground at each pin of the IC and compare the readings with normal reference values, so as to narrow down the fault scope and locate defective components.
Precautions for DC Voltage Measurement
① The multimeter shall have sufficiently high internal resistance, at least 10 times greater than the resistance of the measured circuit, to avoid considerable measurement errors.
② Generally turn all potentiometers to their mid‑position. For television sets, apply a standard color‑bar signal generator as the signal source.
③ Take anti‑slip measures for test‑leads or probes. Any momentary short‑circuit may easily damage the IC. A practical anti‑slip method: fit a section of bicycle valve core over the tip of the test‑lead, extending about 0.5 mm beyond the metal tip. This ensures reliable contact with measuring points and effectively prevents slipping. Even if it touches adjacent points, short‑circuit can be avoided.
④ When the measured voltage of a certain pin deviates from the normal value, analyze whether this pin voltage is critical for normal IC operation and check corresponding voltage variations of other pins before judging whether the IC is faulty.
⑤ IC pin voltages are affected by peripheral components. Leakage, short‑circuit, open‑circuit or parameter drift of peripheral devices, as well as different wiper positions of connected variable potentiometers, will all cause changes in pin voltages.
⑥ If voltages at all IC pins are normal, the IC is generally considered intact. If several pins show abnormal voltages, start troubleshooting from the pin with the largest deviation. Check peripheral components first; if no peripheral fault is found, the IC is most likely damaged.
⑦ For dynamic receiving devices such as televisions, IC pin voltages differ between signal‑present and signal‑absent conditions. If unexpected large voltage variations occur on pins that should remain stable, or no variation appears on pins supposed to change with signal amplitude or adjustable‑component positions, the IC can be confirmed defective.
⑧ For devices with multiple operating modes such as video recorders, IC pin voltages vary under different operating modes.
