Continuity Testing and Signal Tracing

A technician measuring parts on a circuit card with both multimeter probes on the board (US Navy photo, public domain).

Continuity testing is checking with a multimeter, with the power off, that two points that should be joined are joined and that points that should be separate are not. Signal tracing is following a signal through a powered circuit, stage by stage, with a logic probe or an oscilloscope, until the point where it goes missing or goes wrong. Together they find open tracks, cracked joints, broken wires in cables and connectors, and the stage of a circuit that has failed. They locate the fault; the repair itself is a separate job, covered by the technique pages linked below.

Continuity testing does not find a shorted supply rail or a failed chip on its own. A dead rail is covered in Finding Short Circuits, and a chip that drives the wrong levels in Logic Chip Testing.

When to use it

• One output, port or function is dead while the rest of the machine works. The signal leaves a chip that is otherwise working and does not reach the connector. The Sinclair QL RGB Port Repair is a worked example: the television picture proves the ZX8301 is driving its outputs, so the fault lies in the copper between the branch point and the monitor socket.
• A board has had battery or capacitor leakage on it. Check every track through the stained area, including tracks that look intact.
• A cable, lead or connector is suspect. Each pin is checked end to end, and against every other pin.
• A fault comes and goes when the board is flexed, warmed or cooled. Chemtronics lists testing circuit traces for continuity, testing boards for stress fractures and isolating thermally intermittent components among the uses of its Freez-It cold spray.

Once the open is found, repair it with PCB Trace Repair, Jumper Wire Repair, Via and Through-Hole Repair or PCB Pad Repair; repair a cable with Ribbon and Flex Cable Repair or Connector Pin Replacement and Crimping.

Safety

• Make continuity and resistance measurements only with the circuit de-energised. Unplug the machine, and discharge any capacitor that could still hold a charge; Fluke warns that a charged capacitor gives false readings.
• In a machine with a CRT, follow the CRT Discharge Procedure before putting probes anywhere near the tube or its boards.
• A machine with an internal mains power supply has mains voltage inside it while it runs. Keep signal tracing to the low-voltage logic board, and see Recommended Tools on isolation transformers.

Tools and materials

• A digital multimeter with a continuity (beeper) range. The beep is set off by a resistance threshold that differs between meters. Fluke gives a typical figure of 40 Ω or less on a 400.0 Ω range; its Fluke 10 meter beeps below 25 Ω and detects opens and shorts that last longer than 250 µs; SparkFun describes the threshold as a few ohms. Read the display as well as listening for the beep.
• The machine's circuit diagram and component layout, so that every point on the net can be found. Most platform pages link to the machine's hosted service manual.
• Fine, sharp meter probes, and a magnifier for inspecting the stretch of board between two test points.
• A logic probe for digital signals. Hewlett-Packard's 545A, for example, switches between TTL thresholds (logic one at 2.0 V, logic zero at 0.8 V) and CMOS thresholds set at 70 % and 30 % of the supply it is connected to.
• An oscilloscope with a ×10 probe, for clocks, video, audio and anything where the level or shape matters. Sinclair lists a multimeter with a continuity range and an oscilloscope with a ×10 probe as the minimum test equipment for the QL. See Recommended Tools on oscilloscope bandwidth.
• Optionally a logic pulser, which drives a node briefly to the opposite state so that its effect can be watched downstream.
• Optionally a freeze spray made for electronics, such as Chemtronics Freez-It (a non-flammable HFC spray that the maker rates to cool to −51 °C), for intermittent faults.

A meter with its dial set to the combined diode and continuity position, marked with the diode symbol and sound waves. The display shows "1", this meter's open-circuit reading (SparkFun, CC BY-SA 4.0).

Procedure

Prepare

1. Find the net on the circuit diagram: the pin that drives it, every pin it reaches, any connector pins, and any series parts such as resistors or buffers. Mark them on a copy of the component layout.
2. Switch off, unplug the power supply and remove any cables from the ports.
3. Set the meter to continuity. Touch the probes together: the meter should beep, which shows that the leads and the range work.

Check continuity along a net

Finding an open track. With one probe left on the connector pin (D), the other probe is moved back along the net. D to C reads connected, D to B reads open, so the break is between C and B. Drawn for this wiki.

1. Put one probe on the pin at one end of the net, for example the connector pin, and the other on the pin at the far end, for example the chip's pin. Probe a pin, pad or via where the copper is bare; solder mask over a track insulates it.
2. Read the resistance. A track or a wire should read as a very low resistance, close to the lead resistance.
3. If the reading is open, leave the first probe where it is and move the second probe back along the net, one point at a time: vias, component legs and pads that the diagram shows on the same net. SparkFun's advice is to follow the route of the copper until the break is found.
4. Stop at the first point that reads open. The break lies between that point and the last point that read connected.
5. Inspect that section of the board under magnification, on both sides, for a cracked or eaten track, a corroded via or a cracked joint.
6. Check the net for shorts as well: measure from it to the nets on either side of it and to ground. Continuity also shows two points joined that should not be, such as neighbouring surface-mount pins.
7. Where a chip is in a socket, measure from the chip's own pin as well as from the socket's solder tag. Retro Repairs and Refurbs had a Commodore 64 that showed readable text only while the PLA was pressed down; under its non-factory socket two vias had been damaged.

In circuit, a reading can pass through other parts on the net. Fluke's advice is to make sure the part under test is isolated from the rest of the circuit, so lift one end of a part, or take the chip out of its socket, before trusting a reading that does not make sense. On a supply rail, a short beep that stops is the rail's capacitors charging, not a short.

Buzz out a cable or connector

1. Find the pinout in the machine's manual. A socket drawn as seen from the back of the machine and a plug seen from its pin side are mirror images of each other; the Sinclair QL RGB Port Repair gives an example where the manufacturer's own drawings disagree.
2. Check each pin to its partner at the other end of the cable. Each should read as a direct connection.
3. Check each pin against every other pin, and against the shell. None should read as connected unless the pinout joins them.
4. Leave the probes on a pair of pins and move the cable, along its length and where it enters each plug, while watching the display. A meter that catches short opens, such as the Fluke 10 with its 250 µs detection, shows a wire that breaks only when the cable moves. The QL guide uses the same test with the machine running, moving the lead at the socket while watching the picture.

Find an intermittent open

1. Leave the meter on a pair of points that read connected.
2. Gently prod or twist the board around the suspect area with an insulating stick, watching the reading. Goldwasser uses the same test on a running power supply to see whether a symptom can be made to change.
3. Spray the suspect area with electronics freeze spray in short bursts. A crack that opens when cold shows as an open reading.
4. Note where the reading drops out, and inspect that spot under magnification.

Trace a signal through a powered circuit

An Elenco LP-560 logic probe: HI and LO lamps near the tip, a TTL/CMOS switch, a PULSE/MEM switch, and red and black leads with clips for the circuit's supply (photo: Plusea, CC BY 2.0).

1. Check the supply first, at the supply pins of the chips in the path. 74-series TTL is specified for 4.75 to 5.25 V.
2. Connect the logic probe's power leads to the circuit's supply and ground, and set it to TTL or CMOS to suit the chips. In CMOS mode the thresholds follow the supply the probe is clipped to.
3. Start at the source of the signal, for example the output pin of the chip that generates it, and confirm the signal is there.
4. Move to the next point along the path: the input of the next stage, then its output, and so on towards the connector.
5. Read the probe against the table at the end of this section.
6. For infrequent pulses, clip the probe to the node, reset its pulse memory and wait; the memory lamp lights when the node changes state.
7. Where the signal is analogue (video, audio) or its shape and timing matter (clocks, sync), use the oscilloscope with the ×10 probe and its ground clip connected near the point being measured. HP notes that its logic probe catches 10 ns pulses with the short ground lead connected near the source, and only about 20 ns pulses without it.
8. Compare each reading with the service manual's expected value or waveform, or with the same point on a known-good board. Sinclair's QL service manual works this way. The Atari 800XL Video Output Repair and Amiga 500 Video Output Repair guides give expected readings stage by stage, from Sams' service data and Commodore's service manual.
9. The fault lies between the last point where the signal is correct and the first point where it is not. If the signal is present at a chip's output pin but missing at the next chip's input, the copper between them is open; switch off and check that stretch for continuity as above. HP's article notes that voltage readings can locate an open circuit on a node, but tell little else once a node is found faulty.
10. If a node is stuck high or low at every point, the driver may be dead, or something on the node may be holding it there. HP's current tracer article sets out how to tell the two apart by following the current; see Finding Short Circuits and Logic Chip Testing.

What the HP 545A logic probe's lamp shows (Hewlett-Packard Journal, December 1976)

Lamp	Meaning at the probe tip
Off	Logic zero
Bright	Logic one
Dim	A voltage between the two logic levels, an open circuit, a floating input, or a three-state output that is switched off
Flashing at about 10 Hz	The node is toggling between zero and one
One flash of about 50 ms	A single pulse

Many probes, such as the Elenco LP-560 pictured, have separate high and low lamps in place of one lamp; check the probe's own instructions for what each lamp means.

Inject a signal

A ×10 oscilloscope probe with its spring hook removed to show the tip, and the ground lead with its clip (photo: Smial, Free Art License).

Where nothing is moving on a node, a logic pulser can supply the activity. HP's 546A drives a node to the opposite state for a short pulse each time its button is pressed, high nodes low and low nodes high, without unsoldering anything, and can also send single pulses or pulse streams of 1, 10 or 100 Hz.

1. Clip the pulser's power leads to the circuit's supply.
2. Put the pulser tip on the node and the logic probe on the point downstream that should respond.
3. Pulse the node and watch the probe. A response shows the path between the two points works.

Checking the repair

1. With the power off, measure the repaired net end to end again. It should read as a direct connection, close to the lead resistance.
2. Measure from the repaired net to its neighbours and to ground, to make sure the repair has not bridged anything.
3. Power up and check the signal at the far end of the path with the logic probe or oscilloscope.
4. Flex the board or cable gently while watching the signal or the machine's output.

Common mistakes

• Measuring resistance or continuity on a powered board. The readings are wrong, and the meter is put across live circuits.
• Trusting the beep alone. The beeper only says the resistance is under the meter's threshold, which can be tens of ohms.
• Reading through a parallel path. A reading taken in circuit can go through other parts on the net.
• Taking the short beep of charging capacitors for a short on a supply rail.
• Using a logic probe without its power leads on the circuit's own supply, or on the wrong logic family. The CMOS thresholds are a fraction of the probe's supply voltage.
• Counting connector pins from the wrong side.
• Condemning a chip because its output is missing at the connector, without checking the copper between the chip and the connector first.

Worked examples on this wiki

• Sinclair QL RGB Port Repair: a continuity table from each pin of the monitor socket J7 back to the ZX8301, and a test that uses the television picture to prove the chip is working.
• Atari 800XL Video Output Repair: Sams' checks for each stage from the GTIA through the buffer U20 and the video amplifier Q3 to the monitor socket.
• Amiga 500 Video Output Repair: Commodore's checks from Denise through the buffers U40 and U41 and the video hybrid to the RGB and composite outputs.
• Atari 800XL SIO Port Repair: Sams' checks for each line of the serial port, from POKEY and the PIA to the socket.
• Super Nintendo Entertainment System Power Circuit Repair: following the supply from the adapter socket through the filter, fuse, diode and switch to the regulator, one stage at a time.

Related pages

• Finding Short Circuits
• Logic Chip Testing
• Testing Diodes, Transistors and Regulators
• Fuse Identification and Replacement
• PCB Trace Repair
• Jumper Wire Repair
• Via and Through-Hole Repair
• Ribbon and Flex Cable Repair
• Edge Connector and Cartridge Slot Cleaning
• Battery Explosion, Capacitor or Corrosion Damage
• Recommended Tools