Analog Board Troubleshooting

Study an annotated diagram

Analog circuits don't have the clean 1/0 verdicts of digital — signals are degraded, not just dead. The compensation: analog circuits are chains of recognizable stages, each with predictable DC conditions you can check with a DMM before any signal even matters. Learn the stages, learn their DC fingerprints.

1. Power supplies — a useful starting point

Verify power early because many functions depend on it. Two families:

Linear (the simple chain)

Input → transformer (if AC) → rectifier → bulk filter cap → linear regulator → output caps → load

DC fingerprints, in order:

1. Input present? (Fuse, connector, switch)
2. After rectifier: DC ≈ peak of AC input (×1.41 minus diode drops). Ripple depends on load, capacitance and topology; compare with specified conditions before diagnosing a capacitor.
3. Regulator input ≥ output + dropout (~2V classic, ~0.1–0.5V LDO). A regulator with insufficient input headroom isn't broken — it's starved.
4. Output at nominal ±tolerance, nearly ripple-free.

Classic faults: open fuse (find why), shorted rectifier diode (often takes the fuse), dried bulk cap (hum, sag under load), shorted regulator (output = input voltage — dangerous downstream!), regulator in thermal shutdown (cycles on/off as it heats — use authorized non-contact observation).

Switching (SMPS) — efficient, busier

Input → input filter → switching element (MOSFET) at tens–hundreds of kHz → inductor/transformer → rectifier (diode or synchronous FET) → output caps → feedback divider → controller IC

Bench approach:

1. Output dead: check input voltage AND controller enable pin (sequencing! — see 07 — Digital Board Troubleshooting §3). Then is the switch node switching (scope: a hard square waveform)? No switching with good input+enable = controller or its supply. Switching but no output = inductor/rectifier/shorted output.
2. Output wrong value: feedback path — the divider sets the output; a drifted/cracked divider resistor literally re-programs the supply.
3. Output present but ripply/noisy: output caps (ESR) — a possible wear-out mechanism, not the only explanation.
4. "Hiccup" (output pulses up and collapses repeatedly): check overcurrent, undervoltage, sequencing, feedback and protection conditions before assigning a cause (04 — DMM Mastery §4).
5. Shorted main MOSFET: usually obvious — blown fuse, burnt part, near-0Ω across it. Verify the driver, rectifier and downstream load before authorized replacement; do not shotgun companion parts.

⚠️ Mains-fed converters are outside independent practice in this course. Work requires task-specific training, safe isolation and stored-energy verification, rated equipment and an approved probing method. A differential probe alone does not make the task safe.

2. Op-amp stages — the analog workhorse

DC approximation for a stable op-amp inside its input, output, load and bandwidth limits with negative feedback: the two inputs sit at (very nearly) the same voltage (the "virtual short"), and the output sits wherever the feedback math puts it.

The three-measurement op-amp check:

1. Supply pins (V+ and V−; could be +15/−15, +5/0, etc.) — right there at the pins, not just at the rail TP.
2. Both inputs: record which is non-inverting and inverting. Unequal voltages can reflect saturation, a common-mode violation, insufficient loop gain or a feedback problem; they do not by themselves prove a failed amplifier.
3. Output: calculate the expected level and check the datasheet's output swing and load limits. Check external loading and stability as well as feedback continuity before deciding the device is faulty.

Then signal: inject/follow AC through the stage — gain ≈ resistor ratio (inverting: −Rf/Rin; non-inverting: 1+Rf/Rg). Distorted peaks = stage running out of supply headroom or bias shifted.

Comparators look like op-amps but run open-loop (inputs NOT equal — that's normal for them) and often have open-drain outputs needing a pull-up.

3. The rest of the analog zoo — quick fingerprints

• Voltage references (e.g., 2.500V, 4.096V parts): one DMM reading. A drifted reference skews every measurement the board makes — on precision/instrumentation CCAs check it early; ATE often flags it as many simultaneous "out of tolerance" failures.
• Filters (RC/LC): passives — verify with the in-circuit resistance rules and capacitor checks (03 — Component Identification & How Each Part Fails, 04 — DMM Mastery). Symptom of a dead filter cap: noise/garbage passing where it shouldn't.
• Analog switches/multiplexers: verify control logic levels, then continuity through the selected channel (powered, signal-level test). A blown channel (overvoltage) passes nothing or leaks.
• ADCs/DACs: boundary parts — verify reference, rails, clock, then known-input vs reported-code (functional test territory).
• Sensor front-ends (instrumentation amps, bridges): check excitation voltage, then the amp with the op-amp ritual.
• Trimmers/pots: mechanical, vibration-sensitive — wipers go intermittent/open with age. Don't re-adjust sealed/calibrated trimmers casually: in aerospace, calibration adjustments are controlled operations — a paint-sealed trimmer is sealed for a reason; breaking the seal without authorization is a process violation.

4. Analog failure signatures

Multichannel comparison is the analog superpower: instrumentation and aerospace boards often have 4/8/16 identical channels. The good channels are your schematic-free reference — measure the same node on each; the outlier helps bracket a discrepancy; compare operating conditions before naming a failed part.

5. Grounding subtleties worth respecting

Analog ground topology varies by design; do not assume separated grounds or a single-point tie. Two repair implications:

• A measurement referenced to the wrong ground can be millivolts-to-volts off — use the specified reference and verify its relationship to protective earth before attaching a scope ground.
• A cracked joint in the ground tie (or a corroded chassis standoff) creates offsets, hum, and crosstalk that look like component failures. When a board has multiple inexplicable analog symptoms at once, measure ohms across the ground system before chasing each symptom individually.

6. Self-check

1. Linear regulator: 28V at input, 4.96V at output (5V nominal). Verdict? Reveal answer
2. Op-amp inputs read +2.1V and +2.1V; output is mid-range. Op-amp health? Reveal answer
3. Op-amp output slammed at +14.7V (15V rail); non-inverting input is 3.0V and inverting input is 1.2V. Reveal answer
4. SMPS output pulses up to 1V repeatedly, never reaches 5V. Reveal answer
5. 8-channel board, channel 5 reads offset by 0.4V vs siblings at the second op-amp stage. Next move? Reveal answer

Next: 09 — Teradyne Machines & Automated Test (ATE)