Troubleshooting notes from days past, TTL, Linear

General Troubleshooting

• Follow safety precautions.
• Always think “what if”.
  • Analytical procedures
  • Precautions when probing equipment
  • Insulate all but last 1/8” of probe tip
• Learn from mistakes
• Don’t start with electronic test equipment, start with analytical thinking.
• If you get stuck, sleep on it.
• Many problems have simple solutions.
• Whenever possible, try to substitute a working unit.
• Don’t blindly trust test instruments.
• Coincidences do happen, but are relatively rare.
• Confirm the problem before attempting repair.
• Get used to working without a schematic.
• Make notes and diagrams.
• Use trays/pill bottles/ice cube trays to hold small parts and screws.
• Work in an open, well-lit area.
• Understand ESD risks.

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Integrated Circuits

• Defining the problem
  • Define problem and record the failure mode
  • Check with the system operator for information
  • Make a record of the failure mode to refer to when future failures of a similar nature occur in the same type of system.The record should become a permanent part of the records for that particular system.
• Isolate the faulty unit
  • Check manual for test points for waveforms and voltage levels
  • Signal injection works well with divide and conquer
• Locating the faulty circuit
  • Signal injection and tracing
• Identify the faulty component
  • Make voltage, current, and resistance checks
• Repair
  • Replace the faulty device or component
• Test the system
  • Make sure it works before returning it.

PRECAUTIONS:

Do not short pins together using large test probe leads

Do not use excessive heat when desoldering components

NEVER insert or remove an IC while power is supplied, excessive surge currents can occur and destroy the IC.

Troubleshooting IC units

• Check the power supply input at the IC pins.If it is within its rated value, and the ripple level is low, proceed to the next step.
• Check for the proper input signal at the IC pin indicated as input on the IC diagram.
• Check the output pin of the IC for the correct output signal.
• Visually check and measure with an ohmmeter for any open or short circuits in the copper track to the IC.
• If the IC must be removed from the circuit by desoldering, take the extra time necessary to remove the solder completely from each pin with a good desoldering tool, so that the IC can be lifted out without placing strain on the copper tracks of the printed circuit board.

Typical IC Failures

• Catastrophic
  • Occurs when the device is overloaded
  • Can result from
    • A shorted output
    • A condition that causes excessive current flow over a long period of time
    • Or when excessive voltages or voltages of the wrong polarity are applied to the device
  • Transient power supply spikes on the voltage supply line can cause a device to the destroyed.
  • Identify and eliminate the cause of the failure before replacing the failed device
  • Most difficult problem to solve is power line spike.
    • If replacement soon fails, and all other possible causes have been corrected, look to the power supply
• Gradual
  • Long periods of usage may cause degradation of the IC’s operating parameters
    • Particularly true if operating in high temperature or high humidity
  • Will show as gradual deterioration in system performance
  • Replace the device and try to alter its environment
• Poor performance
  • May be result of external components or improper connections
  • If device is being used properly, check that the external components are of the correct value.They could have changed value because of excessive heat used during soldering, leakage paths, or unwanted feedback paths.Solder bridges or leakage paths provide a path for current and signals to go to the wrong location.

Linear Integrated Circuits for Technicians - Frank R. Dungan

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Digital Troubleshooting

• Look for signal activity
  • Usually no need to be concerned about propagation delay unless involved in R & D
  • Check to see where Pin 1 is
  • All signal readings should be take at the IC pin on top of the board.One leg can be bent under the chip.Sockets can develop bad connections.
  • Try to divide the circuit in half, check it, and repeat if necessary.
• Missing signal
  • Should it be there?
  • Is it being disabled by another pin (reset/set, enable/disable, chip select, or an interrupt)?
• Pin stuck high or low
  • Inputs good, outputs bad
    • Internal short in the IC connected to the bad pin
    • Internal open in the source IC
    • Short on PC board (external)
    • Proper input conditions may not be present.Check with oscilloscope or a comparator.
    • Turn off power, measure pin with an ohmmeter from the pin to supply voltage and then to ground.
      • If lowest resistance is less than an ohm, look for a short on the circuit board trace.
      • If the lowest resistance is a few ohms, either the source or one of the load ICs is shorted.
      • If the ohmmeter shows no short circuit, it is most likely that the source IC is open internally.
• TTL pin stuck at about 1.5V
  • Possible when a TTL output open circuits
  • Input circuits following pull to about 1.5V (in the middle of “no man’s land” between the acceptable logic limits.
• CMOS open circuit
  • Extremely sensitive.The circuit may or may not change states with the slightest input circuit disturbance, such as the application of a finger or a probe.
• Signal level is bad
  • Short between input signals will tie two other circuit outputs together.If they agree, output will be acceptable.If they don’t agree, they will conflict and cause a bad logic level.Look for the chip or circuit board short that is tying two inputs together.You should be able to identify another output signal that has a similar problem and then be able to identify the chip that has an input from each of the two signals.Divide the circuit up and check parts of it at a time until all faults are located.
• No missing signal found
  • An AND, NAND, X-OR, or X-NOR gate with an internal open input can mask a problem.The output will ignore one of the inputs.REASON:one input will pull high.There will be no response to the outside signal, as if the input is always high.OR and NOR gates can not cause this problem – they will “lock-up” and won’t pass signals if one of the inputs goes high.
  • A counter could have shorted or open circuited and still produce an output, though it is not valid.Use a comparator – all the outputs and inputs of a known good and a suspect IC are compared in full operation at full speed.
  • Two inputs of a single chip may short together, causing a poor level part of the time as the two inputs conflict.Use an oscilloscope to help verify this as a possible problem.
  • May need to use a logic pulser to single step logic circuits.

Practical Electronics Troubleshooting

James Perozzo

Internal Digital IC faults

1. Malfunction in internal circuitry
   1. Caused by an internal component failing or operating outside its specifications.The IC outputs do not respond properly to the inputs.
2. Input internally shorted to ground or supply
   1. IC will be stuck in Low or High state.This will affect the output of the device that is generating the signal.
3. Output internally shorted to ground or supply
   1. Output pin will be stuck Low or High.Has not effect on the inputs.
4. Open circuited input or output
   1. Very fine conducting wire connecting the pin to the internal circuit will break producing an open circuit.
   2. TTL will respond as if it were a Logic 1
   3. CMOS will respond erratically and may become damaged from overheating
5. Short between two pins
   1. Logic signals at those two pins will be identical

External Faults

1. Open signal lines
   1. Broken wire
   2. Poor solder connection, loose wire-wrap connection
   3. Crack or cut trace on a printed circuit board (may be hairline)
   4. Bent or broken pin on an IC
   5. Faulty IC socket such that the pin(s) does not make good contact
   6. Detect with visual inspection and an ohmmeter
2. Shorted signal lines
   1. Sloppy wiring – stripping too much wire insulation off wires in close proximity
   2. Solder bridges – short 2 or more pins together
   3. Incomplete etching
   4. Detect with visual inspection and an ohmmeter
3. Faulty power supply
   1. Fault in internal circuitry or the circuits it is powering are drawing more current that the supply is designed for.A faulty component could draw more current.
   2. Check voltage levels to see that they are in their specified ranges.Check the regulation and AC ripple using and oscilloscope.
   3. Common sign of a faulty power supply:one or more chips operating erratically.Check power and ground levels at each of these chips.
   4. When a digital IC has its output connected to too many inputs, its output current will be exceeded and the output voltage will fall into the indeterminate zone.

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