Symptoms
- Buzzer is connected to a Flip32
- On connecting a battery, a short click can be heard from the buzzer
- After powering up, there is no way to produce any sound (play_sound, manual trigger from remote control, low battery, …)
Resources
- Schematic used to trace the board: http://uglyduck.ath.cx/PDF/radio-controlled/Flip32/flip32-sch.jpg
- MMBT3906: https://www.fairchildsemi.com/datasheets/2N/2N3906.pdf
- STM32F103 LQFP48: http://electropark.pl/img/cms/mikrokontrolery/STM32F100_opis.jpg
- Board source: http://www.aliexpress.com/item/Flip32-Flight-Controller-With-32-bit-STM32-10Dof/32320642789.html
Buzzer connection schematic (excerpt from [1]):
Traced on the PCB:
Summary
STM32F103’s pin PA12 should drive Q1’s base and pull down ground on the buzzer connector. The trace from PA12 to Q1’s base has infinite resistance, although it should have resistance of R9 (100ohms, but more likely 1kOhm, since, original schematics also suggests 100Ohm resistor for LEDs, which sounds like quite a weak for SMD LEDs).
The only part that was on the path from STM32 to Q1 is a part above dual-schottky package (KL3). Since it is a capacitor, this explains infinite resistance, and also a short ‘click’ on the buzzer when powering on the board.
Workaround
SMD rework; careful removal of 100nF cap and replacing it with a 100Ohm resistor does the trick.