FONA X70: mains overvoltage error A03 while the mains is normal
What happened
A dental X-ray unit reported error A03: the built-in mains voltmeter showed 308 V as reported in the case, while the actual mains measured about 235 V. The unit only powered on without the error when fed from a variac below 200 V.
What was checked
- Compared the built-in voltmeter reading with an independent meter at the mains input.
- Checked the position of the mains-range selection switches on the control board.
- Measured the two auxiliary transformer secondaries — as reported, nominally 9 V and 18 V AC versus 11 V and 24 V found; the engineers took these as acceptable, no anomaly indicated.
- Measured after the rectifier bridge — 14 V and 30 V, consistent with the transformer readings.
Cause
As found in the case, the over-read came from the mains-sensing measurement chain — the voltage divider feeding the built-in ADC/processor — not from the real mains: the transformers and rectifier outputs matched the actual supply. The behaviour resembled a drifted or miscalibrated sensing divider rather than a genuine mains problem.
What was done
The fix reported was matching the divider ratio of the mains-measurement channel until the internal reading agreed with the actual mains — a calibration procedure from the manufacturer's service documentation, done by authorised personnel. After the correction the unit reportedly powered on without error at normal mains, and the case was closed as solved.
Safety
Isolate the unit from the mains and prove the circuit dead before working on the control board — capacitors can hold a charge — and after any change to the mains-measurement chain run the electrical safety tests and confirm error-free operation at normal mains before returning the unit to clinical use.
Lesson: When an internal mains reading disagrees with a trusted external meter, suspect the sensing divider and its calibration first — and correct the measurement so the protection keeps seeing real values, never the other way round.
Covered in the course: X-ray 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.