mike wrote:

I have an HP Laserjet 4MP.

Something went haywire in the toner cartridge
and dumped toner.
I cleaned it out and replaced the toner cartridge
and it sorta worked. Had some background artifacts,
but I chalked that up to 20 year old toner cartridges.

A month later, I tried it and it had severe banding
and the background was almost as dark as the print.
Ran a dozen prints and it got a little better.

I pulled the toner cartridge out of a perfectly working
Laserjet 4L and ran 8 successive prints in the 4MP.
I put the toner back into the 4L and it still prints
perfectly.

A common problem on many of these printers is the wiper blade. it is a
silicone elastomer that wipes excess toner that didn't transfer onto the
page off the photoconductor drum. The blade gest a permanent set after
sitting for a year or so, and won't remove toner effectively. I am not
POSITIVE that is your problem, but it resembles what I've seen. I have a 5m
that I think may use a similar print engine and cartridge. I buy NOS carts
on eBay, and then generally have to replace the wiper blade. There are guys
on eBay that will sell you a new blade. One way to check is look at the
green photoconductor drum, you just have to pull back the protective cover.
If it is strongly streaked with toner, then the wiper blade is not working.
Or, look through the slot on top where the laser shines through, if there is
any toner whatsoever there (it should be glassy clean and shiny) then the
wiper is bad.

If you tear down the cartridge (which you have to do slightly to change the
wiper blade) then inspect and clean the corona wires. if they get a bunch
of toner on them, they either short out the HV power supply or just don't
work.

The cart on the 5m has a cover on each end, held by 1 or 2 screws. After
removeng them, there are two sections to the cart, held together by hinge
pins. it is tricky to work the pins out, but when you do, the cart comes
apart in two pieces. One is the toner applicator, VERY messy, keep from
tilting it so the toner doesn't spill. The other part has the
photoconductor and corona wires, and the wiper blade and spent toner bucket.
When you get it apart, there are bearing pins in each end of the
photoconductor, pull these out, the drum comes out, and you can change the
wiper blade. Then, reassemble.

This all applies to the 5m cart, but I think the 4m either uses the same or
a very similar cart.

Jon