Reference
Bench Mode
Glanceable lookup for the workbench — the numbers, codes and failure habits you need mid-repair. Power-off rules and program-specific values still apply; see each note.
First 5 minutes steps
Before any powered measurement. Eyes and process first.
- Strap on, tested, and logged before opening the tote. Handle the board by its edges.
- Read the work order / failure ticket fully. Note the expected current draw and any tighter-than-default rail specs.
- Confirm the board revision / dash number and pull the BOM for that exact rev. A 'same' board at a different rev may legitimately differ.
- Inspect under magnification with good lighting: burns, discolored mask, bulged or vented caps, cracked or cold joints, solder bridges, white flux haze, corrosion.
- Smell for burnt components and look for the crater a shorted tantalum leaves.
- Check connectors early: bent / recessed pins, fretting corrosion, cracked solder tails from cable strain. They are the #1 field-failure site.
- With power off, ohms / diode-test each rail to ground to find a dead short before you ever apply power.
- Write numbers down as you go. 'Rail was 4.82 V' is data; 'looked okay' is not.
Diode-test signatures table
Illustrative readings only: actual test current, compliance voltage, device type and temperature matter. Isolate all sources and verify stored energy absent. Parallel paths can mask any individual part.
| Junction | Forward | Reverse | Verdict |
|---|---|---|---|
| Silicon (rectifier / signal) | 0.5–0.7 V | OL | Healthy |
| Schottky | 0.15–0.45 V | OL | Healthy — low forward is normal, not a defect |
| Germanium (old equipment) | 0.2–0.3 V | OL | Healthy |
| LED | 1.6–3.3 V (lights faintly) | OL | Healthy — blue/white may exceed some meters' test voltage |
| Zener (forward) | 0.5–0.7 V like silicon | OL | Forward check only; reverse leakage/breakdown requires the specified current-limited test and ratings |
| Any junction | low (≈0 V) | low (≈0 V) | Low-resistance path; distinguish the component from parallel paths through authorized isolation |
| Unspecified device | OL | OL | Inconclusive: may be normal for a bidirectional TVS or exceed meter range; identify and test per datasheet |
Logic-level thresholds table
A voltage is read as 1 or 0 against these thresholds. The band between LOW and HIGH is undefined — a signal loitering there (float, contention, weak driver) is itself a fault signature. Remember: a DMM averages, so a toggling line reads mid-rail too. Scope before declaring weirdness.
| Family | Supply | Input LOW below | Input HIGH above |
|---|---|---|---|
| TTL | 5 V | 0.8 V | 2.0 V |
| LVTTL | 3.3 V | 0.8 V | 2.0 V |
| 5 V CMOS (HC etc.) | 5 V | ~1.5 V (0.3×Vdd) | ~3.5 V (0.7×Vdd) |
| 3.3 V LVCMOS | 3.3 V | 0.8 V | 2.0 V |
| Lower core rails | 2.5 / 1.8 / 1.2 V | proportional to Vdd | proportional to Vdd |
Resistor color code (4-band & 5-band) table
4-band = digit, digit, multiplier, tolerance. 5-band (precision) = digit, digit, digit, multiplier, tolerance. Example: yellow-violet-red-gold = 47 × 100 = 4.7 kΩ ±5%.
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1 k | — |
| Yellow | 4 | ×10 k | — |
| Green | 5 | ×100 k | ±0.5% |
| Blue | 6 | ×1 M | ±0.25% |
| Violet | 7 | ×10 M | ±0.1% |
| Gray | 8 | ×100 M (rare) | ±0.05% (rare) |
| White | 9 | ×1 G (rare) | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
SMD resistor codes table
Chip resistors carry a printed code. Tiny 0201/0402 parts are unmarked — use the BOM.
| Notation | Example | Reads as | Value |
|---|---|---|---|
| 3-digit | 472 | 47 × 10² | 4.7 kΩ |
| 3-digit | 100 | 10 × 10⁰ | 10 Ω |
| 4-digit (1%) | 4702 | 470 × 10² | 47 kΩ |
| R-notation | R10 | decimal point at R | 0.10 Ω |
| R-notation | 4R7 | decimal point at R | 4.7 Ω |
| Zero-ohm jumper | 0 or 000 | — | 0 Ω (link) |
Capacitor 3-digit code table
First two digits = value, third = number of zeros, result in picofarads (pF). A trailing letter is the tolerance. Decoupling caps on logic rails are typically 100 nF (code 104).
| Code | Calculation | Picofarads | Common name |
|---|---|---|---|
| 104 | 10 × 10⁴ pF | 100,000 pF | 100 nF (0.1 µF) |
| 223 | 22 × 10³ pF | 22,000 pF | 22 nF |
| 103 | 10 × 10³ pF | 10,000 pF | 10 nF |
| 474 | 47 × 10⁴ pF | 470,000 pF | 470 nF |
| 101 | 10 × 10¹ pF | 100 pF | 100 pF |
| 475 | 47 × 10⁵ pF | 4,700,000 pF | 4.7 µF |
Polarity & pin-1 marks table
Recite this cold. Backwards installation is both something you must never do in rework and something to check for on a failed unit — assembly errors escape into the field.
| Part | Mark | Means |
|---|---|---|
| Aluminum electrolytic | Stripe on can | NEGATIVE lead |
| Tantalum | Stripe / bar | POSITIVE lead ⚠ opposite of electrolytic |
| Diode | Band | Cathode |
| LED | Flat side / shorter lead | Cathode |
| DIP / SOIC / QFP | Dot or notch | Pin 1 |
| BGA | Corner mark | Ball A1 |
Reference-designator letters table
The letter prefix on a silkscreen designator tells you the component class at a glance.
| Letter | Component |
|---|---|
| R | Resistor |
| RV / VR | Variable resistor / potentiometer (VR also used for voltage regulator on some schematics — confirm from the part) |
| C | Capacitor |
| L | Inductor / coil |
| FB | Ferrite bead |
| D / CR | Diode (CR common on aerospace & mil schematics) |
| Q | Transistor (BJT / MOSFET) |
| U | Integrated circuit |
| Y / X | Crystal / oscillator |
| F | Fuse |
| K | Relay |
| T | Transformer |
| J | Jack / connector (fixed half) |
| P | Plug / connector (mating half) |
| S / SW | Switch |
| BT | Battery |
| TP | Test point |
Failure modes by component table
Knowing the failure habits of each species is half the repair job. 'How it fails' is the habit; 'confirm' is the bench check.
| Component | How it fails | How to confirm |
|---|---|---|
| Aluminum electrolytic | Age and heat can reduce capacitance and raise ESR; short circuits are also possible. | Inspect, then verify capacitance, ESR or leakage using the specified isolated test; ripple alone does not identify a capacitor. |
| Tantalum | Fails SHORT, sometimes ignites. A burnt crater on the board is often a tantalum. | Ohms / diode-test to ground reads near 0 Ω; visible char. Confirm by lifting a leg. |
| MLCC ceramic | Cracks from board flex or thermal shock → intermittent or short. Crack can be invisible. | Classic 'rail reads ~0.5 Ω to ground'. Find by mV-gradient / thermal hunt, then lift to confirm. |
| MOSFET | Fails drain-source SHORT typically. Gate oxide is ESD-fragile. | Identify device topology and protection structures from the datasheet. A body diode may be expected; gate charging and parallel paths can mislead. |
| TVS diode | Can fail short, open or leaky; a transient is a possible cause, not automatically proven. | Identify unidirectional/bidirectional type. Isolate to distinguish parallel shorts. Diode mode does not verify clamping performance. |
| Via | Cracked plated barrel from thermal cycling → intermittent: works cold, fails hot (or inverse), flex-sensitive. | Localize continuity changes using an approved, de-energized environmental/mechanical test within assembly limits. |
| Connector | Bent or recessed pins, fretting corrosion, damaged joints or cable strain. | Visual under magnification; continuity pin-to-solder-tail; mV-drop under load on suspect contacts. |
| Crystal / oscillator | Mechanically fragile — a dropped board with a dead clock is a prime suspect. | Verify setup, probe loading, supply, enable and specified clock test point; a flatline alone does not prove the crystal failed. |
| Resistor | Mostly fails OPEN or drifts HIGH, usually from an overload elsewhere — often the victim of a short. | Verify contact, energy state, part identity and meter setup; confirm with an authorized isolated measurement. |
| Solder joint | Cracks, insufficient/excess solder, poor wetting or misplaced parts; appearance depends on alloy and process. | Inspect against the controlled acceptance criteria and electrically localize. Reflow recovery alone does not establish the mechanism. |
Power-rail check order & tolerance cards
Use the applicable test limits and operating conditions. There is no universal ±5% acceptance rule. Check sequence, input, enable, load and the specified measurement setup.
ESD & FOD bench rules steps
Static below ~3 kV (you can't feel it) destroys or latently wounds modern CMOS. A wounded part passes test and fails in flight — which is why these rules are absolute, not ceremonial.
- Wrist strap properly worn, connected, and tested (daily). The ~1 MΩ series resistor is for your safety — never substitute a plain wire.
- Work only at a protected station: dissipative mat, common-point ground, grounded iron tip, ESD smock.
- Keep insulators out of the ESD area — tape dispensers, foam cups, bubble wrap, synthetic fleece. They can't be grounded and field-zap parts at a distance.
- Boards travel in shielded bags or conductive totes — never bare-handed across the shop, never on random surfaces.
- Handle boards by the edges, away from connector pins and components. Remember dry air (winter) is ESD season.
- FOD: account for every tool and consumable; clean as you go; capture clipped leads, never flick them.
- A clipped lead, solder ball, or screw left inside an LRU becomes a flying short circuit — inspect the work area before closing any assembly.
- Report any unaccounted-for item immediately. Tool control (shadow boards, check-in/out) exists for this reason.
Generic expected values (textbook reference) table
GENERIC / TEXTBOOK starting points only — typical healthy values for orientation. PROGRAM-SPECIFIC values (rail nominals, tolerances, expected current draw, signature limits) MUST come from your own authorized documentation — the test spec, traveler, and BOM for this board revision — not from this table. When in doubt, stop and ask; an escape costs far more than a question.
| Quantity | Generic / textbook value |
|---|---|
| Silicon junction forward drop | 0.5–0.7 V |
| Schottky forward drop | 0.15–0.45 V |
| Germanium forward drop | 0.2–0.3 V |
| LED forward drop | 1.6–3.3 V |
| TTL / LVTTL input thresholds | LOW < 0.8 V, HIGH > 2.0 V |
| 5 V CMOS input thresholds | LOW < ~1.5 V, HIGH > ~3.5 V |
| 3.3 V LVCMOS input thresholds | LOW < 0.8 V, HIGH > 2.0 V |
| Typical logic decoupling cap | 100 nF (0.1 µF) per IC supply pin |
| Linear regulator dropout (classic) | ~2 V |
| LDO dropout | ~0.1–0.5 V |
| Default rail tolerance | ±5% of nominal (unless spec is tighter) |
| Continuity beeper threshold | ~30–50 Ω (meter-dependent) |