SDG #353 eBay Cambridge Audio DacMagic Plus Repair - High pitched whining noise from output

This is a proper fault-finding exercise rather than a parts-swap, and that is what makes it useful for anyone holding a dead or noisy Cambridge Audio DacMagic Plus. The unit was bought cheaply as faulty with the classic symptom: a loud high pitched tone on the analogue outputs at power-up that sometimes clears after several power cycles. Forum wisdom points at C148, the output capacitor on the 1.2 V DC-DC converter feeding the Analog Devices DSP, and previous owners had already attacked that part, lifting a pad in the process.

The diagnostic step worth copying is the signal-path split. Because the CS8416 digital audio interface receiver has its own S/PDIF output, feeding optical in and monitoring the digital out proves the front end is healthy while the analogue side screams. That isolates the fault to the DSP supply rails immediately, with no schematic required. Scope work on either side of the ferrite beads then shows a very noisy input node to the ST1S09 buck converter while the upstream rail stays clean, which is a textbook sign of a high ESR input capacitor drawing spiky current through the bead.

ESR measurements confirm it: the input electrolytic reads around 22 ohms, with several nearby parts in the 10 to 24 ohm range against sub-ohm replacements. Replacing C179 rather than just C148 addresses the cause instead of masking it. The follow-up work is equally instructive, including swapping selected digital-section electrolytics for 1210 X7R ceramics, restoring the output relay stage that a previous owner had bypassed, using leaded solder at 200 C to avoid melting the temperature-sensitive film capacitors, and a thermal survey showing charge pumps near 72 C that likely accelerated the original capacitor’s ageing.