Philips Achieva 8K: MR drops out during scan on -5 V dip
What happened
During a scan the system shut itself down and the log recorded a -5 Volt A (B) dip (PFEI-CFINT, as reported in the case). The engineers measured 4.8 V on 5VA and 4.5 V on 5VB at the test pads, and after the filter-box power supply was swapped for a known-good one, one channel still read about 4.6 V.
What was checked
- Measure both -5 V rails (5VA/5VB) on the test pads under real load and compare against the ±5% window - at -5 V nominal the lower limit sits near -4.5 V, so a reading of 4.5-4.6 V is already at the trip edge.
- Swap the filter-box supply ('кубик') for a known-good one and re-measure both channels; if a rail stays low on a good unit, the fault is downstream or the module itself is marginal.
- Check the -5 V distribution path: lift the black wires off the filters and measure the feed resistance (Z8/Z9 read roughly 915 down to 450 ohms in this case) to spot a branch loading the rail down.
- Load-test the -5 V module, for example with a rheostat, and watch whether the voltage sags under current rather than only at idle.
- Check ripple on all supply rails, not just -5 V - a tired module or capacitor can pull several rails down at once.
Cause
The -5 V rail dipped below the protection threshold (about -4.5 V as discussed in the case) and the system tripped during scanning. The probable cause was a degraded -5 V module or an ageing capacitor on the filter loading the rail - the low reading persisted even after the PSU swap, which points at the module or the filter side rather than a one-off glitch.
The cause is probable and was not fully confirmed.
What was done
Replace the -5 V power module ('кубик') in the filter box rather than only resetting the protection - the whole PSU is not always needed, but a module that has once dipped tends to sag again under load. Afterwards verify both -5 V channels at the test pads and check ripple and the filter capacitors on all rails. As reported in the case this was the recommended repair, and the outcome was probable.
Safety
Before opening the filter box, de-energise and lock out the supplies, let the capacitors discharge and confirm zero volts with a meter, and use only non-magnetic tools inside the MR suite, where the magnet field is always present.
Lesson: When a supply rail sits at the edge of its protection limit and stays low after a PSU swap, chase the whole path - module, filter and load - and replace the degraded module instead of trusting a reset.
Covered in the course: MRI 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.