Electro-Harmonix EH4800 Small Stone Nano not working

Symptom

The pedal works well when powered with internal 9V battery, but nothing when powered with external PSU.

Component side

Back side

Possible cause

1. Check the fuse labelled FB3 (red box above) is passing; replace if not.
2. Check the diode labelled D3 (yellow box, a MMBD4148SE from OnSemi) with a diode tester or adequate DMM
   • Setting the + probe on pin 3 and - on pin 2 should be passing with ~0.5V voltage drop. Must be blocking when reversing the probes.
   • Setting the - probe on pin 3 and + on pin 1 should be passing with ~0.5V voltage drop. Must be blocking when reversing the probes. On my unit, this junction is always blocking hence possibly defective.