Konica 7033 copier gray background/image quality issue

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Hello,

I just acquired a Konica 7033 copier, with around 215K copies on it.
The print quality was quite bad and streaking originally, so I
installed the Konica PM kit, which primarily consisted of new corona
wires, charging grid, some transfer rollers, and a new drum cleaning
blade. After doing this, the quality improved substantially and the
streaking disappeared.

Now I am left with a gray or "dirty" background on the page. Text,
etc. is clearly reproduced, but the page is dark, similar to what you
see if the darkness control is adjusted way down, or on some printers
when the toner cartridge is near empty. I've performed all the
Konica recommend adjustments in the service manual, with no change to
the print quality. Copying a blank sheet of paper still yields a
blank sheet with a new gray background.

The service manual does not address this problem. I suspect the drum
may need to be replaced, but from researching this type of problem on
the internet, other people have suggested developer or other issues.
I'd appreciate any tips or guidance anyone can offer on how to narrow
down the cause of the problem without replacing things wantonly.

Thanks,
Dale