Dissertation on HP DeskJet repair

(From: Paul Grohe (grohe@galaxy.nsc.com).)

I've dealt with a few of these buggers before! ;^)

Is yours a DeskJet, DeskJet Plus, or any of the DeskJet 500 or 600 series?

I believe it to be an original DeskJet.

If not, then ...well....then I may as well tell you this anyways because you will probably run into these some day....(some of the below can apply to other ink-jet printers).

Now, how to do a self test? Fortunately, the complete HP users' manual was part of the deal. "Hold the FONT button while turning on power". OK, no problem.

Try holding down 'FF' during power-up, you may get a different test page. One of the pages should have a jet-test pattern (A slanted diagonal line separated by 11 vertical marker lines and little jet ID numbers).

The first page of the standard self test results in that pattern.

I first tried the cartridge that looks exactly the same as the one that came with the printer (though the part number is different). Then, I tried another somewhat larger one that apparently has identical connections.

The different part numbers are the 'standard capacity' and the 'high capacity' cartridges. They're interchangeable.

Going back and forth, they are consistent. I am not sure if one of the two missing lines are the same on both cartridges. Could something be marginal or is the priming not working? However, all other nozzles seem to be rock solid. Reseating the connectors to the print head makes no difference. If I knew which drivers were involved I could look at the signals but it will be difficult to trace the circuitry from the driver board to the actual nozzle.

I assume you have cleaned the contacts (with a Q-tip, on both cartridge and socket). Use a magnifying glass and check EACH of the gold 'bump' contacts. Repeated cartridge swapping, or improper insertion, can cause a crack to form around the base of the 'bump' and the pad (or the pad and the trace). The 'bumps' can also be 'flattened' by cartridges that were forced in at too much of an angle. There should also be some 'give' or 'sponginess' to the contact area to assure even contact with the cartridge.

Check for broken/bad traces in the flex-cable that goes from the driver board to the cartridge. Ohm out the cable between the supply commons and the individual driver lines (at the PCB) with the cartridge in place. I think the jet resistance was about 50 ohms (It's been a while). There were four separate jet sections (commons). All four commons were tied to the +20V supply through four separate (12 ohm?) series current limiting resistors. The driver outputs seemed to be grounded emitter, open collector (w/clamp diode?). The jets themselves are driven individually and are not multiplexed.

To test, I printed an all-black page (with an empty, but installed cartridge) and watched for activity on each of the lines at the PCB end. Good pulses are 'bi-levelish'. Normally negative going 20V, with pulses down around 15 V, and going all the way near ground for that particular jet. The commons 'bounce' because of the shared series current limiting resistor, causing the numerous smaller pulses around 15 V (caused by the firing of other jets sharing that common). A bad connection will show up as a weak or distorted pulse. An open or broken line will show up as 0 V. I theorize that a bad driver would show just the smaller 'line bounce' 15 V pulses and a shorted driver would show 'GND' (and also would blow out that jet!).

The current involved to drive the 'jets' is a pulse of short duration and pretty high current. Any poor connections will cause excessive I/R drop and the jet may not fire hard enough. A sign of this is drops (dribbles) of ink that form on the head during printing.

While you are in there, check and clean the rectangular rubber seat that the cartridge rests on in the 'parked' position. Dry ink can cake up on it, causing a faulty seal and resulting in dried-up cartridges ($$$!). The rubber seat pulls off and is easily cleaned with a wet paper towel (wear gloves, or you will suffer the dreaded 'black finger syndrome'). Also clean the 'nose wiper' that sticks up about a centimeter to the left of the cartridge seat. This always cakes up and can cause printing problems.

To manually prime an uncooperative cartridge, you do not have to suck on the business end. You can gently blow into the top vent (located on the top of the cartridge, inside the green arrow) to prime it. But be careful!; If the jets are severely plugged, ink may blow out the check-valve on the bottom (under the plastic 'flap' with the 'maze-like' area). Very messy! Have a towel ready!

The old DeskJets were (and still are) notorious for paper feed problems as they age. This is caused by the three big paper pick-up rollers drying out and becoming hard and smooth. Roughen them up with some rough sandpaper. The HP FTP site has a article about this in the DeskJet DOC directory. A free kit is available from HP (to qualifying S/N#'s) that 'dresses' the rollers (basically forces the rollers to turn and sandpapers them).

OH! Biggie! Another big 'failure mode' of the early printers is that the paper sensor lever will jump out of position and jam if the printer has suffered some rough handling (especially if it was turned upside-down or on it's side). The paper sensor lever (pivot) is located on top above the middle roller. The other end breaks the beam of a photosensor. The 'interrupter' end will move over just enough to wedge itself above the photosensor. This is cured by simply raising the lid and wiggling it until it drops back into position (I have 'fixed' many an alleged 'broken' printer this way). The 'interrupter' end seems to have been made larger on the later printers to prevent this.

From time-to-time, the cartridge's nose should be wiped clean with a soft, moderately damp cloth (~ every 100 pages). Keep the 'business' end pointed down when handling/cleaning the cartridge (Yes, this means hold it above you and clean it from the bottom!). This keeps the galleys and jets primed.

Comments: I suspect the original problem resulting in the dead printer to have been a cold solder joint on the DC power connector which I repaired. I don't really think that the nozzle problem was caused by the water since the print head driver board was never wet. Since the data connection to the print head driver board is a 20 pin cable, this must be a common bus and thus it is unlikely that any failure on the main logic board could manifest itself as a single or pair of bad nozzles. Stay tuned.

I concur. If just ONE jet is not firing, then it is on the driver/flex- cable/connector/cartridge side. All the nozzle decoding is done on the driver board, so the 20 pin interconnect cable is okay. The DC (well..really 20VAC) power connector does take some abuse in normal service, this could have aggravated the cold joint.

Don't forget to check the buttons for water damage/contamination.

Been there, done that. The FONT button was Coke-logged.

Do you know what the difference is between the DeskJet and PaintJet cartridges? There is at least one contact that is open on a DeskJet cartridge and wired to something on a PaintJet cartridge.

I also have a color PaintJet 300 with a possible "dead" driver line, but I focused my attention to the ailing DJ500, so I did not have a chance to "buzz-out" the PaintJet cartridge. From looking at it, it looks like the PaintJets are multiplexed in some way (there are more jets/contacts than wires in the flex-cable). I never got around to fixing/looking into it (it's still sitting there).

BTW 1; The DeskJet, DeskJet Plus and DeskJet 500 (non 'C' models) are basically the same (except for some internal fonts). The DeskJets speak PCL, so if a driver for a DeskJet is not available, you can use a basic HP LaserJet driver (but the margins may be cut off, as the DeskJets print area is not as big).

The missing line problem turned out to be bad connections between the flex cable and the gold contacts on the print cartridge due to the flex cable shifting position on its indexing pads.

These things are so damn simple that not much can happen to them. I have yet to run across one with a severe electrical problem. They are always minor mechanical failures (or missing power bricks...$35 from HP).

As noted previously, the HP DeskJet series in general is a well engineered design with only a half dozen basic components. While my (DeskJet) printer is one of the oldest models, the fundamental design has not really changed dramatically in the last several years as evidenced by the fact that print cartridges for some much more modern printers work just fine in this old machine.

All the DeskJet/DeskWriter printers, up to and including the 6X0 series, use the same B&W cartridge as the original DeskJet. Those cartridges will still be available for some time. Your printers life is not over any time soon!

The print quality with a new cartridge is nearly laser-quality. Yes, HP seems to come out with a new, faster, cheaper, color. etc. printer every few weeks. But, looking inside newer printers shows that their basic design and construction is quite similar.

The DeskJets are good, sturdy and reliable printers (as long as they are well maintained) You did clean the rubber cartridge seat and flap. Right?.

BTW 2: For maximum cartridge life, make liberal use of the "draft" setting for "not-so-important" printouts (or, er, um, drafts!). It also prints faster because it "swipes" once per line instead of twice.

BTW 3: Use the cheap 'Shark' brand inkjet paper for best results. Pretty near laser quality! Regular copy paper tends to bleed, but is fine for general use.

Can you tell I have a 'few' of these printers around???? ;^)

Just when I thought all was well....

At random times, the print will fade out and require priming by mouth to restore operation. This can be anywhere from a few lines to a few pages. Until it quits there is no evidence of a problem. Blowing into the vent hole will restore operation. This happens with more than one cartridge. It appears as though the ink is just not refilling after being vaporized.

Is the cartridge full? As you get down to the last 20% or so of the cartridges capacity, it tends to start doing this. I guess there is not enough pressure "from above" to force the ink down. If you can start seeing through the cartridge, you are probably near this point.

CAUTION: I found out the hard way that you really do not want to stick anything into the vent hole - ink all over the place as the vent valve must have been damaged by this mischief. I 'salvaged' the cartridge with a blob of silicone sealer. I don't know what the long terms implications of this 'repair' will be.

In troubleshooting the printer, you tend to "burn up" cartridges a LOT faster than in normal use.

I cleaned out the priming tube which was TOTALLY clogged with dried ink and it seems to be much happier now.

This can be a symptom of the print head not seating firmly when in the "parked" position. Use a dental mirror and make sure the seat presses firmly against the head. One cause of this can be turning off the printer before it has a chance to run through all of it's "housekeeping" cycles at power-up, reset (re-boot), or after printing. During certain parts of the cycle, the head is moved slightly, or the cover is moved. Turning off the printer too soon may leave the head exposed. Always let it finish, then turn it off (warn others about this).

If you haven't already, just pull the thing apart and give it a good overhaul (get your favorite pair of Torx bits ready!). Clean all the rubber tires, seals and "nose wipers". Wipe off the slider bar to remove any old lubrication. If there was a serious ink leak and the printer was involved in some "circus acrobatics", some of the ink can get on the slider bar and contaminate the factory lubrication, causing it to become "pasty". I wipe the slider bar clean with a cloth then apply a LIGHT coating of a light, teflon-type machine oil with a cloth (I use "Tri-Flow", a spray-on type usually found in bike shops).

BTW 4: In the winter months, with it's low humidity, the rollers will shrink even more, causing even more paper feed problems. This is also compounded by the fact that the paper sometimes develops a static charge and tends to "stick" together. Sometimes it pulls two or three sheets in at once, or the paper sticks firmly together in the tray and the weak, dry rollers cannot pull the paper in. Just remove the stack of paper and "fan" it out to loosen it (especially if it has been sitting there unused for a couple of weeks).

These printers are, like some other things we won't mention, 'Use `em or loose `em'! They work best with frequent use. They do not like sitting around for months unused. Three months seems to be the limit before a 'good' printer will start to dry up from no use.

Before I 'discovered' the priming problem, I has visions of a serious electronic problem like an intermittent resulting in the nozzle drive pulses getting messed up at random times.

How is pulse width determined in these things?

I never really investigated the timing of the pulses. I'm not sure how they vary the pulse width. I looked at the pulses when it was doing the first page of the self test, which is mostly text, and all the pulses seemed to be the same width.

Happy printing!

The conclusion: After several years of faithful service (the most serious problem being that I really used the printer so infrequently that the cartridges invariably dried up!), I was given an HP DJ1000C with a couple of broken plastic tangs to which the pressure roller springs were attached. Some overzealous paper unjamming had caused thses to snap - not really a quality problem. Thanks to some stiff wire and Duco(tm) cement, the printer is better than new forcing retirement of my trusty $5 original DeskJet. :)