On 11/6/2016 10:36 AM, Michael Black wrote:
On Sat, 5 Nov 2016, mike wrote:

I have an HP Laserjet 4MP.

I have no idea. I got my HP 4P about 2004, $15 at a Rotary Club
garage sale, and it's worked fine ever since. It even had a low
page count, I have no idea why it got donated in the first place.

But I know back when I got it, I was able to download a service manual
for it, so at least do a search now. Maybe that gives hints.

Michael
You really have to read the whole thread.
By the time the mad snippers get through with the initial
question, you miss the part that says I referred to the manual.
The input so far is discussed in the original question
that I spent hours to carefully configure for maximum communication...
only to have it snipped and ignored. Recommending I do a search
is not very helpful. It's not like we don't search before posting.
What WOULD be helpful is a recommendation for keywords to search.
If we search for the wrong keywords, we won't find it. A quick
look at the google hit page is misleading. Many of the "hits"
found by google don't exist. And many of those that do exist link
to "information" on other pages that no longer exist.
End of rant...

These are the current image references for the test pages and the
two toner cartridge voltages for the precharge roller and developer
roller.

http://imgur.com/dcuRRlW
http://imgur.com/E1BSKZc
http://imgur.com/sRf3DfH

Just realized that the image links aren't permanent...
Voltage on the developer cylinder is about 1300V P-P with an offset
of about -450V. Sorta square wave with ~550uS period.
Voltage on the precharge cylinder is about 2100V P-P sine wave about
4.2 mS period.
Starts out at 0V offset, then shifts to -400V offset before the
paper gets there.

These voltages are for a WORKING system, see below.
I thought about putting the "wired up" cartridge in a different working
printer,
but I have a long history of using one good system to try to fix
a bad one and ending up with two bad ones.

I verified that the printer elements that are supposed to be grounded
actually seem to be grounded. The resistance to ground for the precharge
power supply in the Laserjet 4MP was 2.5Meg compared to 5Meg in a
Laserjet 4P. You'd think the power supplies were the same, but
dunno...
I attached wires to the toner cartridge contacts and scoped the signals
during printing.

I was very surprised to learn that the AC component of the signal
was several times as big as the DC offset.

I was also very surprised to see that ALL the banding had disappeared.
Both cartridges that had severe banding were now free from that banding.
I had cleaned the contacts and swapped cartridges dozens of time.
Now, it's all good! I didn't actually fix anything, so I'm expecting
it to fail again.

Both my test cartridges were old and had vertical stripes due to
the wiper blade not cleaning properly.
I stuck in a NOS refilled cartridge. At power on, it made a loud
"snap" and dumped toner. I can't catch a break...

I cleaned all that up and tried a NOS genuine HP toner cart.
Before I installed it, I gently rocked the drum back and forth
to make sure that it wasn't stuck to any of the wiper blades.
That cartridge seems to work and the printer is now back in service.

I spent a week messing with this. I learned a lot, but
still have no idea what caused the banding.
Maybe my saga can help someone else in the future.

I'd still be interested in an explanation of how the banding
shown in the picture could possibly happen. I can't come up with
a failure mode that would produce such a varied banding pattern
on successive printouts of the same page.