SDG #368 Tricky Repair: Cambridge Audio Bluetone 100 - No Bluetooth

This is a long form fault finding exercise on a mains powered Cambridge Audio Bluetone 100 bought as faulty because it refused to advertise or pair. The useful diagnostic lesson comes early: the 3.5mm and line inputs both play correctly, which immediately clears the class D amplifier, the Nuvoton NPCA110 codec/DSP and the PIC24FJ64 host microcontroller, narrowing the fault to the CSR8645 Bluetooth module and its support parts.

The oscilloscope work is the most instructive part. A multimeter shows healthy 3.3V rails on the module, but the oscilloscope reveals the 26MHz crystal starting and then shutting down, and the I2C EEPROM being re-read roughly every 11 seconds. That is a classic watchdog reboot loop signature. The reset line is ruled out, the UART pins are dead, and IO14/IO15 (the only lines actually going back to the host microcontroller) show no traffic. Reflowing the module changes the oscillator behaviour but not the outcome, a reminder that a change in behaviour is not the same as a fix.

With an AliExpress CSR8645 development board and a USB to SPI programmer, the original PS keys are dumped and compared side by side with the demo board, confirming that the pairing scheme uses just two GPIO inputs and two status outputs, with no UART. The obstacle is firmware: the stock ROM on the replacement will not expose I2S. The pragmatic fix is a CSR8675 module wired in by hand with the GPIO and LED states cloned from the original configuration, the PCB trace antenna cut and the factory internal antenna reconnected via U.FL. Range and volume are excellent as a result. Anyone repairing older CSR based audio equipment should expect firmware availability, not hardware, to be the real obstacle.