So I am still fairly confident it's a transport thing but I am happy to be wrong...so long as a fix comes to light one way or another. BTW we have a 7900 that exhibits the black banding issue as well, so it's obviously not that uncommon a problem - two 900 series printers, two for two problems.
It is definitely not a paperfeed tension or transport issue.
When you look at this page
and print the testfile as described you will understand that paperfeet-tension problems would be visible in the colors also.
Futhermore if it was a paperfeed-tension problem one would expect the wavelike patern to be more consitend in its appearance.
So remains inkpressure issues.
Only photo and matblack has, so to say, two dempers for one (printhead) channel.
The first "demper" beeing the switch damper.
Right behind this you find a "regular" demper.
( As I understand..., forgive me if I am wrong..., I am getting tired... )
When I start printing the testfile the first cm or so of the black is flawless...
When the whole black printhead channel is used in laying down ink the wavelike pattern appears.
At that time it looks like the inkflow is not sufficient enough to lay down enough ink.
Strange enough the inkpressure pump does not get a signal inkpressure is insufficient.
Is this maybe also the case when suddenly one cartridge color has disappeared? ( Nozzle check )
If there's a leak in the inkpressure system the inkpressurepump will start trying to lift the lack of pressure.
So it does get a signal ... but maybe not soon enough ... or the pressure is not sufficient enough.
Because the black has "two dempers" maybe thats just enough "obstruction" to frustrate the inkflow without triggering the pressurepump.
The pressure is okay. Only the black inkflow is not enough.
(excuse my english..)
Hartelijke groet van Matthijs