Two symbol families show up on the schematics you will repair from. US / ANSI-IEEE (Y32.2 / IEEE 315) style uses the zigzag resistor and distinctive curved logic-gate shapes, and dominates US aerospace and military documentation. International / IEC 60617 style uses a plain rectangle for resistors and rectangular logic gates labeled "&" or "≥1". You must be able to read both — modern avionics drawings frequently mix them, and older military prints add their own quirks (for example, diodes called out as CR instead of D). The function of the part is identical regardless of which symbol style is used.
R
Limits current and drops voltage. On the bench: measure resistance in-circuit only with power off, and expect parallel paths to read low — lift one leg to be sure. Open resistors usually show a burn mark or read OL.R
Same component, international drawing style. Don't confuse the empty rectangle with a fuse or jumper — check the reference designator (R vs F vs W/JP) before assuming.R or VR
Three-terminal adjustable divider; the arrow is the wiper. Bench gotcha: end-to-end resistance is fixed — sweep wiper-to-end with an ohmmeter and watch for dead spots or jumps, the classic dirty-pot failure.R or VR
Two-terminal adjustable resistance (a diagonal arrow through the body means "adjustable"). Often a pot wired with wiper tied to one end — verify on the board before measuring.RT
Temperature-dependent resistor (NTC drops resistance as it heats; PTC rises). Bench note: its reading drifts while you handle it — your fingers are a heat source. Compare against the datasheet value at 25 °C.RV
Voltage-dependent resistor that clamps transients across power inputs. Common failure: shorted or cracked after a surge — a dead-short reading across the input often traces to a blown MOV, not the supply itself.C
Two equal parallel plates — ceramic or film, install either way. On a DMM capacitance range it should charge and settle; in-circuit a shorted ceramic decoupling cap is a top cause of a dead supply rail.C
Electrolytic/tantalum: straight plate is positive (+), curved plate negative. Install backwards and it fails — sometimes violently. Bench note: bulging, leaking, or high ESR electrolytics are the most common age-related CCA failure.C
Adjustable capacitance, usually a few pF for tuning oscillators or RF stages. Bench gotcha: factory-set trimmers are often sealed with paint — do not "re-tune" one unless the repair procedure calls for it.L
Series of arcs; stores energy in a magnetic field, passes DC, opposes changes in current. On a DMM it reads near 0 Ω — that is normal, not a short. An open winding (OL) means a failed inductor.L
The parallel lines indicate an iron or ferrite core (higher inductance). Dashed lines mean powdered/ferrite core on some drawings. Same DMM behavior: low DC resistance is healthy.FB
Drawn as a slanted bead (or a plain inductor) on the line; absorbs high-frequency noise as heat. It should read near 0 Ω on a DMM. An open bead silently kills a supply rail — easy to overlook because it "isn't a real component."Y or X
Quartz resonator (rectangle between two capacitor-like plates) that sets clock frequency. A DMM tells you nothing useful — verify with a scope on the oscillator pins. Crystals are shock-sensitive; suspect them after a hard-landing or drop event.F
Rectangle-with-line (IEC) and the older S-curve both mean fuse. Bench rule: a good fuse drops ~0 V and reads <1 Ω. Always ask why it blew — a fuse is a symptom, not usually the fault.T
Two windings coupled through a core (parallel lines). The dots mark same-polarity winding ends — instantaneous voltage at both dotted ends moves together. Bench: each winding should show low resistance end-to-end and OL between windings.D or CR
Conducts anode→cathode only; the bar is the cathode (matches the band on the part). DMM diode test: ~0.5–0.7 V forward, OL reversed. Reads both ways = shorted; OL both ways = open.D / CR / VR
Cathode bar has bent "Z" wings. Operated in reverse breakdown at its rated voltage as a reference or clamp. Diode test looks like a normal diode forward; verify zener voltage only with the circuit powered (or a curve tracer).D / CR
Cathode bar with S-hooks. Low forward drop, fast switching — common in power and clamp duty. DMM diode test reads noticeably lower than silicon: ~0.15–0.35 V. Don't mistake that low reading for a leaky junction.D / CR
Diode with two arrows pointing out (emits light). Diode test: ~1.6–3.3 V forward depending on color; most DMMs will faintly light it — a free functional test. Higher Vf than a signal diode is normal.D / CR
Arrows point in (light in, current out); usually operated reverse-biased as a light sensor. Bench note: ambient light shifts its readings — shade it or test in consistent lighting.D / CR
Back-to-back zeners that clamp transients of either polarity — standard on aerospace I/O lines for lightning/EMI protection. Common failure is a short after absorbing a big hit; a shorted data line often traces here.D / CR / BR
Four diodes in a diamond: AC in at the ~ corners, DC out at + and −. All cathodes steer toward +. Bench: diode-test each leg through the AC pins; one shorted diode blows the input fuse every time.Q
Emitter arrow points OUT — mnemonic: NPN = Not Pointing iN. Diode test: B–E and B–C each read ~0.6 V (red probe on base), OL in reverse and C–E both ways. Anything else, replace it.Q
Emitter arrow points IN toward the base. Same diode-test logic as NPN but reversed: black probe on base gives ~0.6 V to E and C. Watch supply orientation — PNPs typically switch the high side.Q
Body arrow points IN toward the gate for N-channel. The body diode (S→D) reads ~0.5 V on diode test — that is normal, not a fault. Gate gotcha: discharge the gate (G–S short) before testing or it may stay turned on.Q
Body arrow points OUT (away from the gate). Body diode conducts D→S. P-FETs usually sit on the high side of a rail — a shorted one leaves the rail permanently on; an open one looks like a dead rail with good input power.U
Amplifies the difference between + and − inputs. Repair rule: with negative feedback both inputs should measure nearly equal voltage; a large difference means the stage (or its feedback path) is broken. Supply pins are often omitted on the drawing — find them in the power table.U
Same triangle as an op-amp — only the part number tells you which it is. Gotcha: many comparators (LM339/LM393) have open-drain outputs and need an external pull-up. An output stuck low may just be a missing or open pull-up resistor.U or VR
7805/LM317 style: IN, GND (or ADJ), OUT. Bench check: input must exceed output by the dropout voltage (~2 V for a 7805). Good IN + bad OUT = check for a shorted load before condemning the regulator.U
An LED faces a phototransistor inside one package — electrical isolation across the dashed boundary. Bench: diode-test the LED side (~1.2 V); the output side only switches when the LED is driven. Degraded LEDs (low CTR) cause flaky, temperature-dependent faults.U
Output high only when ALL inputs are high. Flat back, round nose (ANSI). IEC version: rectangle labeled "&". Logic-probe the inputs first — a stuck input upstream looks exactly like a bad gate.U
AND with an inversion bubble: output low only when all inputs high. The bubble always means logical inversion, on any pin of any symbol — learn to spot it instantly.U
Output high when ANY input is high. Curved back, pointed nose. IEC version: rectangle labeled "≥1".U
OR with an inversion bubble: output high only when all inputs are low.U
Output high when inputs DIFFER. The extra curve behind the OR shape is the tell. IEC label: "=1". Used in parity, comparison, and adder circuits.U
Output is the logical opposite of the input. Quick scope check: input and output should always be complementary; both stuck at the same level means a dead gate (or a shorted output net).U
Triangle with no bubble: output equals input, but with restored drive strength. Used to fan a signal out to many loads or drive long traces. Logically transparent, so probe both sides — a failure here just looks like the signal "stops".U
The enable pin adds a third output state: high-impedance (Hi-Z, disconnected). Gotcha: a disabled output floats — a "weird" intermediate voltage on a bus line may be a tri-stated net, not a fault. A bubble on EN means active-low.U
The hysteresis mark inside means the switching thresholds for rising and falling inputs differ — it cleans up slow or noisy edges. Used on switch inputs and reset lines; if a slow signal causes multiple output transitions, the Schmitt stage is suspect.U
Captures D on the clock edge (the > wedge marks an edge-triggered input) and holds it on Q; Q̄ is the complement. Scope tip: verify the clock is actually toggling before suspecting the flip-flop — a dead clock mimics a dead FF.U
Rectangle with numbered pins. Pin 1 sits at the notch/dot, and numbering runs counterclockwise viewed from the top. Schematic pin order rarely matches physical order — always cross-reference the datasheet pinout before probing.GND
Three shrinking horizontal bars. Reference point for the circuit; on equipment, often tied to the chassis/airframe at defined points. Your DMM's black probe lives here for most voltage measurements.GND
Slanted hatch lines: connected to the metal chassis/frame, not necessarily to circuit common. Bench check: continuity from chassis lug to airframe bonding point — corroded bonds cause EMI and intermittent faults.GND
Downward triangle, often labeled AGND or DGND. Marks a quiet reference net for analog circuitry, kept separate from noisy power returns.VCC VDD
Both styles mark a connection to a named supply rail. Every symbol with the same rail name is the same net. First measurement on any dead board: are all rails present and at the right voltage?BT
Long line = positive plate, short line = negative; multiple pairs indicate multiple cells. Bench note: measure a battery under load — an aging cell can read full voltage open-circuit and collapse the moment current flows.S or SW
Single pole, single throw: simple open/close. Bench: closed contacts should read <1 Ω; worn switch contacts read ohms to tens of ohms and cause heat and voltage drop under load.S / SW
Single pole, double throw: the common selects one of two contacts. Note which position is drawn — schematics show the de-energized/normal state. Verify both throws with continuity, not just the one you care about.S / SW
Normally open: the bar bridges the contacts only while pressed. NC variant shows the bar already touching. Bench: intermittent buttons often fail only under fast or partial presses — exercise them while watching continuity.K
Coil plus mechanically linked contacts (dashed line / dashed box). Coil and contacts are electrically separate circuits. Bench: coil should read its rated DC resistance (often 50–500 Ω); listen for the click, then verify the contacts actually closed — burned contacts click but don't conduct.J (jack/fixed) or P (plug)
Numbered pin row. J is the board-mounted side, P the mating cable plug. Bench reality: connectors cause a huge share of avionics faults — wiggle-test, inspect for bent/recessed pins, and check crimp continuity before suspecting the board.TP
A deliberate probe pad or pin, drawn as a small circle with a flag. These are your best friends in repair: the designer chose them because the voltage or signal there tells you something. Find every TP on the drawing before you start probing IC pins.| Designator | Component type | Example |
|---|---|---|
| R | Resistor | R12 — 10 kΩ pull-up |
| C | Capacitor | C4 — 0.1 µF decoupling |
| L | Inductor | L2 — 10 µH filter choke |
| D / CR | Diode (any type: rectifier, zener, LED, TVS) | D3 — 1N4148; CR7 — 1N5811 on older prints |
| Q | Transistor (BJT, MOSFET, JFET) | Q1 — 2N2222A |
| U | Integrated circuit | U5 — LM358 dual op-amp |
| Y / X | Crystal / oscillator | Y1 — 16 MHz crystal |
| F | Fuse | F1 — 1 A fast-blow |
| K | Relay | K2 — DPDT signal relay |
| J | Jack / fixed (board-side) connector | J3 — 9-pin D-sub receptacle |
| P | Plug (cable-side mating connector) | P3 — mates with J3 |
| S / SW | Switch | S1 — panel toggle |
| T | Transformer | T1 — 400 Hz power transformer |
| TP | Test point | TP4 — +3.3V rail |
| FB | Ferrite bead | FB1 — power entry filter |
| VR / RV | Variable resistor, or varistor (MOV) | RV1 — 14 V MOV clamp |
| BT | Battery | BT1 — 3 V lithium keep-alive cell |
| FL | Filter | FL1 — EMI feedthrough filter |
Note: "CR" for diode is common on older aerospace and military schematics (MIL-STD-16/ASME Y14.44 lineage). If a drawing mixes D and CR numbers, they are still all diodes — check the parts list to match each designator to its part number.