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Bench Skills Cheat-Sheet
Measure, debug, and de-noise any build. Print me; keep it by the breadboard.
From Lessons
30–
33.
Applies to every project — pedals and home automation alike.
Multimeter — the four functions
| Function | Power | How |
|---|
| DC volts | ON | Across two points; black on ground, red on the node. Checks rails & bias. |
| Continuity (beep) | OFF | Two points joined → beep. Finds breaks & shorts. |
| Resistance (Ω) | OFF | Measure a part out of circuit. Confirm values. |
| Diode test | OFF | ~0.5–0.7 V Si / ~0.3 V Ge forward, open reversed. Confirms part & direction. |
Expected DC readings (single 9 V op-amp pedal)
- Supply rail ≈ 9 V · ground = 0 V
- Virtual-ground / bias node ≈ 4.5 V (half supply)
- Op-amp: pin 7 ≈ 9 V · pin 4 ≈ 0 V · + input ≈ 4.5 V · output (no signal) ≈ 4.5 V
Debugging routine
- Look — IC backwards? cap/diode reversed? leg in wrong column? center-gap?
- Power & ground — rail ≈ 9 V; solid common ground everywhere.
- Bias points — compare each node to what it should be (≈4.5 V, op-amp pins).
- Trace the signal — half-split: present at the midpoint → fault downstream; absent → upstream.
Audio probe
0.1 µF cap on a wire to your amp + alligator-clip ground. Touch along the path: clean here, gone next →
fault is between. Change one thing at a time and re-test.
Usual suspects
Missing/uncommon ground · reversed power · op-amp rotated (pin 1) · polarized part backwards (electrolytic, diode band) · leg in wrong breadboard column.
Noise triage (in order)
- Run on a battery — noise gone? → blame the supply.
- Solid common ground, no ground loops (star-ground to one point).
- Add/confirm decoupling caps (0.1 µF + bulk).
- Shorten signal wiring; move away from screens / wall-warts / dimmers.
Hum/buzz = grounding/interference · Hiss = high-gain noise (use only the gain you need).
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