How to Check Electronic Components on a PCB Using a Multimeter

I need to do more electronics circuit repairs and not just theory so I have been trying to repair faulty electronics that I need to repair. My ubiquiti Lite AP 5AC 120 or LBE-5AC-120 radio is malfunctioning and I have been trying to repair it. I wrote a note on repairing Ubiquiti antenna and radio and in Ubiquiti Power Adaptor LED Blinking Repair. I gathered some helpful notes on how to check electronics components like resistor, capacitor, inductor, diode and transistors, what are different ways of finding faulty component on PCB circuit boards. This note is about these things.

Testing components while still on the PCB can be tricky due to parallel circuits affecting readings. Here’s a step-by-step guide to checking capacitors, resistors, inductors, diodes, transistors, and voltage regulators with a multimeter.

1. General Testing Rules

✅ Power Off & Discharge Capacitors (use a resistor or screwdriver to short leads).
✅ Desoldering Helps (parallel components can give false readings).
✅ Diode Mode & Resistance Mode are most useful.

2. Testing Each Component

🛠️ A. Capacitors (Electrolytic & Ceramic)

Method 1: Capacitance Mode (Best for Exact Value)

• Steps:

  1. Set multimeter to Capacitance (F) mode.
  2. Place probes on capacitor terminals (no need for ground).
  3. If value matches rating (±20%) → Good.
  4.  If OL (Open) or 0 → Dead.

Method 2: Resistance Mode (Quick Check for Short/Leakage)

• Steps:

  1. Set to Ohms (Ω) mode (start at 20kΩ+).
  2. Place probes across capacitor.
  3. Good: Starts low (charging) → climbs to OL.
  4. Bad: Stays at 0Ω (short) or no change (open).

⚠️ Note:

• On PCB: Other components may interfere.

• For SMD caps: Test in-circuit—if reading is near 0Ω, it’s likely shorted.

🛠️ B. Resistors

Method: Resistance Mode (Ω)

• Steps:

  1. Set multimeter to Ohms (auto-range or higher than expected value).
  2. Place probes across resistor.
  3. Good: Value matches marking (±5-10%).
  4. Bad: OL (open) or 0Ω (shorted).

⚠️ Note:

• On PCB: Parallel paths can alter readings—desolder one leg for accuracy.

🛠️ C. Inductors (Coils)

Method 1: Continuity Mode (Check for Open)

• Steps:

  1. Set to Continuity (Beeper) mode.
  2. Place probes across inductor.
  3. Good: Beeps (low Ω, usually <10Ω).
  4. Bad: No beep (open coil).

Method 2: Inductance Mode (If Available)

• Steps:

  1. Set to Inductance (H) mode.
  2. Measure across inductor.
  3. Good: Close to rated value.
  4. Bad: OL or 0H.

⚠️ Note:

• On PCB: Hard to test accurately—desolder if possible.

🛠️ D. Diodes

Method: Diode Test Mode (Best)

• Steps:

  1. Set to Diode Mode (⎓►| ).
  2. Forward Bias: Red probe on Anode (+), Black on Cathode (-) → 0.3V-0.7V (Si) or 0.2V (Schottky).
  3. Reverse Bias: Swap probes → OL (no conduction).
  4. Bad: 0V (shorted) or OL both ways (open).

⚠️ Note:

• On PCB: Other diodes in parallel may affect readings.

🛠️ E. Transistors (BJT & MOSFET)

Method 1: Diode Mode (BJT)

• Steps (NPN):

  1. Red probe on Base (B), Black on Emitter (E) → ~0.6V.
  2. Red on B, Black on Collector (C) → ~0.6V.
  3. Reverse probes → OL.
  4. Bad: 0V or OL in all directions (shorted/open).

Method 2: MOSFET Check

• Steps:

  1. Set to Diode Mode.
  2. Gate (G) to Source (S): Should read OL (unless charged).
  3. Drain (D) to Source (S): Should act like a diode (~0.5V one way, OL other).

⚠️ Note:

• On PCB: Hard to test accurately—desolder if possible.

🛠️ F. Voltage Regulators (e.g., 7805, LM317)

Method: Power-On Test (Best)

• Steps:

  1. Power the PCB (check input voltage).
  2. Measure Input (Vin), Ground (GND), Output (Vout).
  3. Good: Output matches spec (e.g., 5V for 7805).
  4. Bad: 0V, too low, or fluctuating.

Method 2: Resistance Check (Off)

• Steps:

  1. Set to Diode Mode.
  2. Check between Vin & GND (should not be shorted).
  3. Check Vout & GND (should not be shorted).

3. Quick Summary Table

4. Pro Tips

🔹 For accurate readings, desolder at least one leg of the component.
🔹 Use a component tester (like TC1) for quick checks.
🔹 If a component tests bad, check nearby traces for shorts.