- Noise floor: with inputs shorted, lamp should remain near‑off; duty < 3/255.
- Hum susceptibility: test with grounded and floating source; check for 50/60 Hz sensitivity.
- Through integrity: null test by splitting the source; ensure buffer is unity and pops are acceptable on power‑cycle.
- PWM leakage: listen for whine; verify Timer1 ~31 kHz on D9 with a scope if possible.