MT4 MT4 Plus (Lilium base): low-voltage and monitoring-dropout errors
What happened
The unit periodically reported monitoring-dropout and low-voltage errors. The engineers found the mains outlet voltage normal and, as reported in the case, about 460 V on the inverter capacitors, but one of the two backup batteries was fully discharged.
What was checked
- Mains voltage at the wall outlet — within normal limits.
- DC voltage on the inverter capacitors — reported at about 460 V.
- Both backup batteries checked — one of the two fully discharged.
- Charging resistor measured cold and warm: around 2 Ω cold, with resistance rising into the kiloohms as it heated (PTC-type behaviour reported in the case).
Cause
A fully discharged battery overloaded the battery charging resistor, a small SMD part in a DPAK package. The burnt-out resistor lost conductivity as it warmed, its resistance climbing into the kiloohms, which took the charging and monitoring path down and produced the low-voltage and monitoring-dropout errors.
What was done
Both batteries and the DPAK charging resistor were replaced, and the errors cleared. The engineers also recommended replacing the backup batteries on time and switching the unit off with the rear-panel power switch before unplugging it from the mains, so the batteries are not drained while the unit sits without AC power.
Safety
Before working on the power section, isolate the unit from the mains and verify that the inverter capacitors are fully discharged, and complete the manufacturer's electrical safety and functional checks, including X-ray output verification, before returning the mammography unit to clinical use.
Lesson: Backup batteries left discharged can quietly destroy their own charging components, so battery condition and correct power-down habits belong in routine preventive maintenance, not in reaction to faults.
Covered in the course: Mammography 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.