Siemens Acuson X300 Premium Edition: 3.3 V rail collapses under load and the unit shuts down
What happened
As reported in the case, the unit came up briefly and then shut down with an overcurrent flag from the lower power supply: the P3.3A rail read about 0.6 V instead of 3.3 V. With the RC board disconnected, the unit started and all rails read normal — which pointed to either the rail or one of its loads.
What was checked
- Continuity check of the 3.3 V rail with the unit unplugged, to look for a short on the bus.
- Repeat of the rail check with the RC board removed, as a diagnostic step only, to separate the supply from its loads.
- Load test of the 3.3 V rail with a power resistor, 10 ohm / about 300 mA as reported in the case, to see whether the PSU holds regulation under load.
Cause
The engineers found a degraded MOSFET (FDD8444) in the P3.3A circuit on the lower power supply: the supply could not hold the rail once a real load was applied.
What was done
Replacement of FDD8444 in the P3.3A circuit on the vertical board of the lower power supply. After the replacement the unit started normally with all rails in spec; the outcome in this case was a solved fault.
Safety
Before opening the power supply, disconnect the unit from mains and verify the bulk capacitors are discharged; after the repair, confirm all rail voltages under load and complete the electrical safety test before returning the unit to use.
Lesson: A rail that measures fine with no load can still collapse under load — test the PSU with a dummy load before condemning the boards it feeds.
Covered in the course: Ultrasound Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.