Zebra (UPS) LP 2844 thermal printer problem

[gxti]

I made the mistake of buying a "for parts or not working" thermal printer, gambling that it would be something simple like a power supply issue. Unfortunately the power supply works fine, and the problem is with the printy bits. Ugh.

The issue is that as the label feeds, it seems to stick or stutter and it seems almost as if the label is briefly fusing to the print head then breaking loose. Slower speeds or higher darkness setting exacerbates the problem, as does larger areas of black in the printed document, and there's no setting I can find that results in acceptable print quality without any sticking. Is the print head toast? Crappy media? (these came with the printer, which could be a blessing or a curse) Or perhaps something else entirely? I don't want to spend another $100 on a print head only to find out it still doesn't work right.

[george graves]

Did you clean the print head/strip?

[gxti]

I swabbed it with isopropyl but there's no change.

[george graves]

That's odd.  I have one of these printers, and use it all the time.  Never had an issue with it (other then finding drivers for it 

Only thing I can think of is when printing large dark areas, the print head is using much more current, and the paper drive motor is getting robbed of some.  Bad power supply?  Bad connection somewhere?

[SeanB]

Try using another power supply. If the original one is failing it does that, as the ripple is increasing.

Take the feed roller out ( 2 small clips to release, then rotate the 2 plastic bushes to allow it to lift up out of the slots) and clean it, and check the hollow is clean, and the gear and drive gear does not have junk on them. Careful as the lot will fall apart on removal.

I replaced the power supply on one with a universal laptop power supply that gasve 18V at 3A, and it works well now. Careful cleaning the head, you only touch the ridge at the front, not the gold coloured mylar flex board as that will tear loose.

[sunnyhighway]

Sounds like a media transport problem to me.

Have you cleaned the platen roller?
Use isopropyl alcohol and a cleaning swab or lint-free cloth
This has to be done every 5 rolls or so.

[gxti]

The power supply was physically damaged (broken screw tower) when I received it but I peeked inside and there was nothing visibly wrong, no bulging caps etc. I checked Zebra's knowledgebase and they actually have pictures of the factory original power supplies, and it looks like the 1.5A one so it's not like someone put on the wrong supply. I'll put an oscilloscope on it and see if it's cutting out, and maybe try running the printer off my bench supply instead. It's certainly a sound theory -- too much current to the head might cause the motor to stall if the caps were failing. It must not be too severe though as the logic board has not reset on me yet.

I'll also check the roller. I swabbed the visible portion but it wasn't immediately obvious how to remove it so I didn't clean the whole thing.

[SeanB]

The logic board runs off a 5V rail with a linear regulator. It needs a pretty big drop for that to brown out, the motor and the head will stop long before that.

[gxti]

Alright, here's what I did. First I cleaned the roller with 91% IPA. This helped a little, but not enough to keep it from stalling. You can see in the first picture how the "USPS Priority Mail" is half missing, and at the bottom there's a bunch of whitespace that got squashed. The bottom of the image printed first, so it looks like whenever there's a higher density of print the motor stalls afterwards (just above the lines).

Then I put my oscilloscope on the 20V supply during use. There's a horrible drop when the printer switches on but that might be normal. During printing there is nasty ripple but it stays above 18V. I tried using my 18V 3A bench supply and that seemed to feed better but the printing faded very badly. Finally I brought out the big guns -- a 20A industrial power supply, adjusted to its lowest setting of 23V. This seems to work perfectly -- no spotting or skipped lines at all.

So the question is -- is the power brick busted? Are the printer's own bulk caps dried out? And/or is the print head worn enough that it needs more current to do anything? If the stock supply was only 1.5A, and SeanB replaced his with a 3A 18V supply, but my 3A 18V bench supply produces awful results, it seems like the issue is the printer demanding too much current, not that the supply isn't putting it out. Unless anyone is completely convinced that isn't the problem, I will try replacing the bulk caps next. It looked like there were 3x 1mF 25V caps inside, also not visibly bulging. I happen to have some very nice UCC caps with the same specs to drop in.

[gxti]

Scratch that, even with the 23V supply it is stalling, although the print quality seems otherwise improved. So it's not (just) the PSU brick.

[SeanB]

Then the most likely candidate then is the drive motor is dying. The head has no moving parts but the motor has brushes that wear. You probably can find a replacement that has the same mounting and similar voltage and try it.