Axion DKI-N-11: no impedance reading — 'no contact' on every load
What happened
The defibrillator reported 'no contact' and did not measure impedance on any load — neither on a patient nor on the internal discharge test resistor.
What was checked
- Reproduce the symptom on known loads: 'no contact' on both the internal discharge test resistor and a dummy load shifts suspicion from pads and cable to the impedance measurement electronics.
- Test the passive parts in the measurement path (diodes, resistors, zeners) — the engineers found them all good.
- Check the ADC of the impedance channel before ordering anything; if the chip carries only a code marking, decode it first — reported in the case as 'A18', identified as an ADS7818.
Cause
The engineers found the impedance channel returned no data at all while the passives around it were good, pointing to the failed ADC (marked 'A18', identified as an ADS7818): with the converter dead, the unit read an empty measurement channel as 'no contact'.
The cause is probable and was not fully confirmed.
What was done
Replace the ADC chip marked 'A18' (ADS7818) on the impedance measurement board. Reported in the case: the part was identified by its marking code and ordered; the outcome was described as probable, not yet confirmed.
Safety
Before opening a defibrillator, disconnect it from mains and discharge the internal energy-storage capacitors using the manufacturer's approved procedure — stored charge can be lethal even with the unit off; verify discharge function and impedance measurement on test loads before return to use.
Lesson: A measurement channel that fails on every load, including the built-in test resistor, points to the measuring electronics rather than the patient interface — and an unreadable chip marking is worth decoding before swapping parts.
Covered in the course: Fault-Finding Method for Medical Equipment →
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.