TechTalk

Simple Troubleshooting

Welcome to Tech Talk, Pico Macom's series of technical notes, featuring useful information and equipment tips. This installment of Tech Talk discusses use of voltage/ohm/current meters to troubleshoot voltage losses on a coaxial cable.

"Swap-and-Drop"

All technicians would like to be able to finish their job and move on to the next as quickly as possible. Unfortunately there are times when problems arise that involve troubleshooting the system to find a fault. Without the proper tools and test instruments, technicians are left to blindly “swap-and-drop” or replace equipment in hopes that this will resolve the problem so they can move on.

There are two big drawbacks with this approach: First, much of the returned equipment will not actually be damaged. Second, the technician hasn’t identified the true cause and, when presented with a similar situation, will simply swap-and-drop by replacing equipment.

As an alternative to the swap-and-drop approach, we will provide some simple troubleshooting techniques.

Test Equipment
This Tech Talk focuses on volt/ohm/amp meters. There are two basic types commonly in use.

An analog meter has a pin that deflects across the meter face; the range is on the face of the meter. The maximum meter deflection limit is printed on the front by the selector knob. For example: If you know the voltage to be measured is around 120 volts AC, rotate the knob to the 250 ACV position. Then, on the meter face scale that shows 250 max on the right, read the measured voltage at the point to where the pin deflects. The procedure is the same for resistance and current.

The digital meter is available with a scaled selection and an auto-ranging selection. The scaled selection works as mentioned above in the analog version, with the exception that the display is a digital LCD. This eliminates the user interpretation of meter deflection and provides an absolute value. The auto-ranging version provides the additional feature of a single selection for each type of measurement. This greatly simplifies the operation and the cost difference is very slight. Insert the probes in the proper jacks, move the selection switch to the position that corresponds with the type of voltage, and the meter will show the value on the display. Product: Pico Macom MUL-280

Although we don’t often think of them this way, the probes and adapters we use to connect to the circuit are extremely important pieces of test equipment. Always use care when handling probes and adapters. A damaged probe can provide false readings, leading to conclusions that will increase troubleshooting time.

Measurements

Measuring AC Voltage

  1. Insert the red plug into the red jack marked ACV. It may simply have a sine wave symbol (~) along with a V for voltage.
  2. Insert the black plug into the black jack. It will usually be designated as COM for common.
  3. Switch the meter knob to the position and range of the AC voltage to be measured.
  4. Place the black probe end so it makes continuous contact with the grounded/shielded/chassis portion of the device under test.
  5. Place the red probe end so it makes continuous contact with the circuit/center conductor/connector pin portion of the device under test.
  6. The meter face will display the AC voltage value.

Measuring DC Voltage

  1. Insert the red plug into the red jack marked DCV. It may simply have a direct current symbol ( DC ) along with a V for voltage.
  2. Insert the black plug into the black jack. It will usually be designated as COM for common.
  3. Switch the meter knob to the position and range of the DC voltage to be measured.
  4. Place the black probe end so it makes continuous contact with the grounded/shielded/chassis portion of the device under test.
  5. Place the red probe end so it makes continuous contact with the circuit/center conductor/connector pin portion of the device under test.
  6. The meter face will display the DC voltage value.

Measuring AC Current
Important: The circuit must be interrupted to properly measure current.

  1. Insert the red plug into the red jack marked ACA. It may simply have a sine wave symbol (~) along with an A for amperage.
  2. Insert the black plug into the black jack. It will usually be designated as COM for common.
  3. Switch the meter knob to the position and range of the AC current to be measured.
  4. Place the black probe end so it makes continuous contact with the load side of the device under test.
  5. Place the red probe end so it makes continuous contact with the voltage source side of the device under test.
  6. The meter face will display the AC ampere value.

Diagram 1

Measuring DC Current
Important: The circuit must be interrupted to properly measure current.

  1. Insert the red plug in the red jack marked DCA. It may simply have a direct current symbol ( DC ) along with an A for amperage.
  2. Insert the black plug into the black jack. It will usually be designated as COM for common.
  3. Switch the meter knob to the position and range of the DC current to be measured.
  4. Place the black probe end so it makes continuous contact with the load side of the device under test.
  5. Place the red probe end so it makes continuous contact with the voltage source side of the device under test.
  6. The meter face will display the DC ampere value.

Diagram 2

Open/Short/Resistance
Important: Analog meters require calibration prior to resistance measurements.

  1. To measure resistance in a circuit, insert the red plug into the red jack marked OHM. It may simply have an Omega symbol Omega for Ohm.
  2. Insert the black plug into the black jack. It will usually be designated as “COM” for common.
  3. Switch the meter knob to the position and range of the resistance to be measured.
  4. Before making the measurement, short the probes together so they make continuous contact.
  5. Calibrate the meter by adjusting the calibration wheel for deflection to the highest resistance in the range.
  6. Place the black probe end so it makes continuous contact with the grounded/shielded/chassis portion of the device under test.
  7. Place the red probe end so it makes continuous contact with the circuit/center conductor/connector pin portion of the device under test.
  8. The meter face will display the resistive value.
    • If there is no meter pin deflection, the circuit is open.
    • If the meter pin deflects to the highest point in the range, change the range and recalibrate before testing again.
    • If the meter pin continually deflects to maximum, this is an indication of a shorted circuit.
    • If the meter pin falls into the middle of the range, this is an indication of resistance on the line.

Diagram 3