printing dark and light bars

[smallprintds]

Hi,

I've got a GR 3770 that has begun to print dark and light bars across the width of my 11x17 paper on areas of dense black. The worst place seems to be right in the middle of the paper, kind of looks like I need to adjust the vertical hold knob on an old black and white tv, if you get the idea.

I've checked the pressure roller, even traded the rollers out with a machine that I have for parts, but the bars are exactly the same, regardless of which roller I use. (I thought maybe there was a flat spot on the pressure roller)

I also have three black HD drums, one I use for parts, and two that print ok, or used to. I've now tried both of my working black drums, and each appears to create the same issue.

I can fix the problem by increasing my print density to the darkest and the print speed to almost the slowest setting, which will give a pretty crisp looking black across the page, but oh, so slow. I used to be able to set my density to the lightest setting and the speed to max, and still have nice blacks.

Using my red drum on the same machine, I don't seem to notice the same problem, in fact it prints super nice.

Sorry about the long post, I figure it's better to throw everything I know into the pile... with that in mind;

I wonder if the problem is maybe that I got a bad batch of ink... or maybe because it's now colder the ink isn't flowing a well? Yesterday I tried sticking a tube in a nice warm spot for a while, and this didn't seem to make any difference, but I didn't put the drum in the same spot, so maybe it's too cold?

I'm sure my space is at least 70 degrees, but maybe that's too cold. I guess I can try putting the whole drum in the sun to warm it up nice and see if this doesn't help while I wait for a reply.

Thanks a lot!
Dave

[TECH BOY]

Hi there,
If you have a red drum that seems to work then it is probably the two black drum out of adjustment.Black drums usually get used much more and can fall out of ajustment.If you can correct the image by running it slow with the preasure up then I think the black drum is starving for ink.Check the doctor gap inside the drum to ensure the proper ink bead is being maintained.You can also force the ink pump on in service mode but this won't reall solve the problem.You will need your feeler gauges for this ajustment.Good luck!

Tech boy

[smallprintds]

Tech boy,

Thanks... I was expecting the doctor roller to be the problem, but I had hoped someone would have a magical cure, other than stripping down a drum. Ugh!

I taken a screen off a drum already, that's not too bad, but it looks like a pain in the hindquarters to pull the drum apart. I guess that will wait for next week.

And forcing ink doesn't sound like a very good option, but it might come in handy in an emergency. I'll try and remember that trick, but most days it's enough to remember my name.

Thanks again and have a nice night.

Dave

[smallprintds]

I rebuilt one of the GRHD drums today. Not nearly as bad as I thought it was going to be. Amazing machines.

Rebuilding was messy however. I found the doctor roller was almost tight against the ink roller, not much hope for ink passing unless it was 120f degrees outside and I wasn't printing much more than text. I adjusted the screws to set it open to .8mm, as my manual suggested, then put the whole mess back together.

I have a couple of extra drums lurking around so I robbed a sweet master assembly and screen off a newer red drum, put all the pieces together, ran the inking test mode, and now I have a sweet black drum again.

I'll probably be rebuilding my other two this week while I can still remember how they come apart and go together.

Kudos to you Tech Boy... You rock! Thanks again for the help.

Dave

[TECH BOY]

You are very welcome.Have a good night!