SDG #351 eBay NAD Hybrid Digital Amplifier D 3020 V1 Repair - Won't power on (Hypex UCD102 Class D)

A £75 eBay NAD D 3020 sold as “parts or not working” turns into a genuinely instructive fault-finding exercise, because the symptom (orange standby LED, five front panel flashes, then red and back to amber) points at a protection event rather than a dead unit. With the NAD service manual to hand, the diagnosis proceeds logically: the 5 V and 3.3 V standby rails are healthy, the 5 V digital rail switches on command, but the plus and minus 7 V, 12 V and 35 V rails are all absent, meaning the main SMPS never starts.

The clever step is proving the microcontroller is actually asking for power. Measuring the drop across the 200 ohm series resistor feeding the optocoupler LED (around 1.1 V forward) confirms the standby enable signal is being asserted, which shifts suspicion firmly onto the full bridge converter itself. Component level checks clear the primary MOSFETs, synchronous rectifier FETs and output diodes, and the culprit turns out to be a pair of current sense resistors that had gone open circuit and were so degraded they physically snapped during removal.

That alone would not explain the failure, and the follow-up on the Hypex UcD102 Class D board is the real story: two output MOSFETs shorted drain to source, most likely from a shorted speaker output. More interesting for anyone who owns one of these amps is the black adhesive used liberally around the output stage, which measures in the low megohms and has electrolytically eaten through several vias and at least one pad. Several link wires were needed to restore the plus and minus 35 V connections. The lesson for buyers is blunt: working D 3020 units are ticking time bombs until that glue is removed.