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.

The following image represents 8 successive prints
of the same page in the laserjet 4MP using the known-good
toner cartridge.

http://i.imgur.com/dcuRRlW.jpg

It shows the first page and the left 400 pixels of the
next 7 prints attached to the right to show the banding
in the background.

There are artifacts from the aliasing in the scanner,
jpg and the resampling to reduce the image size to something
manageable to publish on the internet.
The only intention is to show the background
banding differences from print to print. Parts that are supposed
to be black are deep black and clearly defined.

It's the background that is the issue. It's supposed to be white.

Note that the banding varies radically from print to print.
If you look for bands that repeat at the circumferences of
rollers in the print system, you can find some, but even those
don't always repeat on the same page.

If I pull the toner mid-print, the background toner is definitely
on the drum. It appears that the problem is NOT in the fuser
or other rollers in the system. It happens inside the toner
cartridge.

The only thing I can think of is that the power supply that charges the drum
is defective and supplying insufficient voltage.
The service manual is very detailed describing the process.
There's a DC bias plus an AC component. But they stop short
of describing the amplitudes of each. Seems to be about -500V from the
rough graph. I could solder some wires on a toner cartridge
and see what it looks like, but it would be nice to know what
to expect. I don't know the current capacity (impedance) of
the supply. Toner is likely conductive, but it doesn't look
like toner got into the area of the power contacts.
Anybody know what power waveforms/voltages to expect?

The thing is put together like a Chinese Jigsaw puzzle. The
power supply is at the center. I don't have much hope of being
able to run the supply disassembled. About all I can do is
check caps for ESR and look for dirt or bad solder joints.

I like this printer. It does what I need and I have a lifetime
supply of toner cartridges for it. I also have a LJ4L and a
LJ4P. I'd buy an inexpensive color laser printer, but they
all seem to have EXPENSIVE chipped toner
cartridges that expire and refuse to print whether you used them or not.
I don't print much. Inkjet is not an option. My inkjet plugged
up a decade ago from non-use.

I understand that most of you
would not waste time fixing it,
but I have the time and the desire
to learn about how it all works.

Anybody got ideas of what to try?

Thanks, mike