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

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

On 06/11/2016 02:26, Jon Elson wrote:
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


Is there charge neutralising linear lamp/s/LEDS for HP lasers, if so the
cover glass could be dirty. Or problem with the reclaim/retraction (? I
forget the term) "corona" voltage to assist in wiping the drum , not
just the wiper blade to do that

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

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.

mike wrote:


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.

With those sorts of voltages running around, any kind of probing carries a
LOT of risks! So, you are wise.


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.

Well, you did mention a big toner spill. That stuff is conductive, and so
you have to get it nearly all cleaned up, or it will get in somewhere and
load down a corona supply. I'm guessing somewhere along the trail you
cleaned off the magic spot where the toner had shorted something out.


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.
Well, a weak corona voltage or dirty corona wire could give poor charge, so
the toner didn't all transfer to the paper. Then, the weak wiper blade
didn't clean the drum, so the toner accumulated as it rotated. But, I'm
just guessing. My experience has mostly been with the bad wiper blades.
Once the drum is covered in toner, the laser beam can't write the pattern of
charged and discharged areas, so the whole page comes out grey. But, you
may have actually had TWO problems going on at the same time, much harder to
diagnose.

Jon