Light coating of black toner on pages Xerox DC 242

I have a problem with a very light coverage of toner on all prints and its so light that if I were to scan the page it wouldn't show up but hold the page next to a unprinted page and you can see the difference.

I have swapped black drums, its a recent black dev, new cleaning blade, recent transfer belt and I have just swapped the 105K20980 Developer Bias (K) HVPS board all to no avail.

Reading the service manual all the relevant settings are within spec - where do I look next?

Thanks
Chris

 

Did you find a solution? I have similar problem