Could be anything from a faulty esv to the cleaner not cleaning. Things to check
1 - Cleaner is cleaning OK. Brushes OK, Flicker bars not built up with compacted toner. Cleaner vacuums Good (3.5 inches of water)
2 - All grid shields (inboard and outboard) are still in place on all stations inc Pre - transfer
3 - All grids are good and none bent
4 - all charge corotrons are seated fully and the connectors are secure at the rear
5 - Faulty ESV - Service rep is needed to check readings and replace
This fault can sometimes be declared on one station, but the root cause can be occurring on a different station.
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Mark