HP Laserjet III Stalls

[45Overload]

I've had this Laserjet III since new.  I've done most of the common maintenance/repairs including freeing up sticking solenoids, oiling the scanner motor, etc.  I've installed new pickup rollers at least twice during its lifetime (20 years?) and I think at least 2 fuser assmblies.  And it keeps on working!

Here is one problem I haven't been able to track down.  When the printer is relatively cold (powered on for less than 15 minutes) it will print one page no problem, but the second sheet will often stop feeding about 1/2 way through the fuser.  This symptom does not occurr if the printer has been warmed up for 15 minutes or more.

The second page isn't mechanically stuck - I can pull it out easily.  Successive pages seem to work OK.  I fooled with the fuser a little to make sure that the optical paper feed sensors were working - yes.  The thermistor on the fuser is obviously allowing the printer to get to the "Ready" state.  All the gears in the feed mechanism are in good shape.  The fault might not be associated with the paper path.  Aging power supply?

I've had it all apart, looking for obvious loose connectors, had a look at the control pcb, etc.  I wonder if anyone else has been down this path?

Thanks!
45O

[45Overload]

One interesting reason that the printer stops processing the paper is that the scanner drum motor is not phase locked.  Lots of these motors need to be cleaned and re-oiled as shown on the U toob.  Keep an ear out for the scanner motor making a "hunting" noise.  At least that is one of many reasons for stoppages, and might be why the printer works ok after a good warm up. 

I pulled the scanner motor for cleaning and oiling.  So far so good.  Note that the motor rotor has to be disconnected at the bottom!  Leave the upper part where the mirror is attached alone.  Just a thought.  Yes, I know it is B&W and only 300 dpi. 

[Gyro]

I had a laserjet 3 main motor fail (lost speed control) due to toner build up on its PCB. I cleaned it off and it was good as new.

[drussell]

What error does it display on the LCD when it stops?

[45Overload]

By main motor, did you mean the hefty thing that drives the gears in the  paper path?  That thing is built like a tank.  The printer stops, and displays "Paper Jam".  That's not very helpful and unfortunately does not give a error number.  Cleaning and checking the full paper path, rollers, etc. hasn't turned up anything.  I'll look into whether this printer has a separate main motor drive board.  Some of this stuff is buried pretty deep.
Thanks.

[Gyro]

Ah, small problem, sorry - I just looked in the garage and it's a Laserjet 4 not a 3  but yes, the main motor driving the gears and rollers of the paper path, mounted to the side of the chassis. The motor (on the 4 anyway) has an integrated PCB with driver IC etc. and a crystal. The toner had collected on the PCB and thrown the crystal frequency off.

[edpalmer42]

You mentioned replacing the pickup rollers, but what about other rollers?

A typical problem with old printers is hardened rubber rollers that won't grip the paper.  Sometimes you have to replace them, but often you can renew them by rubbing them with a cloth that's moistened with one of the 'rubber renew' products.

I'd look for one intended for printers rather than an automotive one.  Sorry, don't have any links for you.

[45Overload]

Gyro - thanks for the tip on the motor.  The LJ4 is probably identical to the III.  I'll poke around on the main motor drive.  Never thought of that.  edpalmer - you bet about the rollers.  I've installed 2 roller kits over the years and they are in good shape, especially the pickup roller, then the thin one near the corona wires, new fuser ass'y and so on.  The rollers are not jammed.  It is the drive motor that just stops so I'm thinking that Gyro has a good point about checking the drive pcb to the motor.  This error is not one of the Laserjet common faults, unfortunately.  I'll post after checking the drive board.

[edpalmer42]

The drive motor could be stopping because the paper isn't moving and the system thinks it's jammed.  In fact, it isn't moving because the rubber rollers are just sliding on the paper.

You said that you can easily pull the sheet out.  It shouldn't be easy! 

In my limited experience, dried up, hard rollers are the first thing to check on any paper-handling problems on old printers.

[SeanB]

HP printers of that age are also kind of prone to the LED's in the paper path optosensors going low emission, so the printer thinks there is paper in a place where it should have passed according to the timing, or has reached ahead of the timing. Being it is half way through the fuser, it likely is the feed sensor, output tray sensor, and there are a few solutions for this. One is new sensors, or to find the same leds from a newer printer and rebuild the sensors with new led units, or the bodge is to increase led current slightly.

Chill each sensor down in turn with freezer spray, and see which change state with the light shining on the transistor. That is the failing one.

[Gyro]

HP printers of that age are also kind of prone to the LED's in the paper path optosensors going low emission

Or potentially also getting a dusting of toner, pretty opaque stuff!

[SeanB]

True, but I see many that soldier on while seemingly able to dump half a fill of toner per page on the desk around them from leaking cartridges. HP did at least there make those sensors reasonably immune to toner contamination. Freezer spray will clean the sensor of toner though, just a single blast will do it.

[45Overload]

The drive motor could be stopping because the paper isn't moving and the system thinks it's jammed.  In fact, it isn't moving because the rubber rollers are just sliding on the paper.

You said that you can easily pull the sheet out.  It shouldn't be easy!

True.  I meant that I can grab the paper and pull it out without disassembling the fuser or other items in the paper path.  When the printer stops moving the paper, it is always half way through the fuser.  About 1/2 of the page has appeared in the top exit tray.  It always stops in the same place.  I thought it was a fuser problem, because the fuser has optical shutters and a thermistor, but new fusers didn't change anything.  There is no problem with rubber rollers since paper gets picked up, and makes it half way through a relatively new fuser with rollers in good shape. 

I have checked the opto couplers and shutter mechanisms.  They do swing from 0 to 5 volts when the shutter operates, at least when I'm able to measure them.  You may be right that they are getting weak so I'll re-check the output voltages.  Then we have an aging power supply.  Caps tend to improve a bit when they warm up.  Another thing to look at.

45O