HP-97 Printer Paper Advance

[Mike DiGirolamo]

When printing, my HP-97 sometimes doesn't fully advance the paper which results in an overlapping of print lines. Can someone tell me what's wrong and how to fix this problem? Is it a clutch, lube or ?? problem?

Thanks - Mike

[Katie Wasserman]

There are several things that can cause this that I've come across:

- The little rubber (soft plastic of some type, actually) drive rollers on the extreme right and left side of the paper advance bar are dirty.

- The springs that push this bar against the paper are week.

- A bad idler gear (the middle of the three drive gears).

- Bad paper -- old paper seems to slip more often for some reason.

- There's excess friction in the paper path -- something else is dirty in there.

[David Smith]

The usual cause is flat spots on the printer rollers. These are caused by not using the printer. The rollers are constantly pushed against the platen rollers which causes a flat spot on the rubber. This shows up as a hiccup every four lines (assuming the paper moves at all). Mint-in-box never used machines are almost always severly affected.

Cures:
1) Clean the rollers and printer mechanism. If you can find it, apply some rubber belt conditioner to the rollers (like slot car tire traction liquid or MG Chemicals sells somthing called Rubber ReNew. These are usually sodium salicylate based solvents that disolve rubber and make it tacky).

2) Clamp a large paper clamp to the end of the roll of paper, hang it over the edge of a table, print some long listings.

3) If this does not get things working you can try removing the printer drive cam and sanding the rollers round again. Removing the drive cam and reinstalling it is not for the mechaincally challenged. You have to remove and reinstall the printhead... klutzes will crack their heads.

4) If this does not work replace the drive cam with one from the HP41 82143A printer.

[Tony Duell]

If the problem is caused by the pressure of the rear rollers against the rubber drive rollers, would it be worth unhooking the 2 little tension springs at the sides of the printer in any machine that you're going to store rather than use? It's easy enough to do, and a Topcat can be opened without disturbing any labels (for those who care about such things).

[Tony Duell]

I've fixed several machines with this problem, and every time it's been a problem with the actual rubber rollers. Unlike the one in the card reader, I've yet to have one turn to goo, thankfully.

The main problem is getting the roller assembly out. I would seriously recomend getting the service manual that's on the MoHPC CD-ROMs. Then :

Take out the battery, take off the bottom cover (6 screws), slide out the printer paper window, take out the 3 screws, washers, and grommets that hold the printer in place. Unplug the printer PCB from the logic PCB and take the whole lot out.

Unplug the 6 single wires from the PCB, noting their positions (2 white -- home position reed swtich, 2 red -- out-of-paper contact, red and black -- motor). It's difficult to unplug the printhead tapewire from the PCB without damaging it -- one trick that helps is to push a couple of old magnetic cards between the tapewire and the socket connects, then pull the wire out. If you're not happy doing that, leave the printhead connected to the PCB.

Undo the 2 screws holding the metal 'tab' clamp that holds the printhead in place. Slide out the printhead (with the PCB still attached if you've not disconnected it). When reassembling, make sure the printhead is properly seated before fitting the clamp, and tighten the screws carefully and evenly. There is a real risk of cracking the ceramic head if you're not careful.

Mark the position of the home switch, take out the screws and take it off. Take out the 3 screws holding the motor in place and remove that. Take off the E-circlip and remove the idler gear.

Take off the E-circlip and the gear on the end of the leadscrew. Turn the leadscrew to move the carriage away from the RHS of the frame, then press on the leadscrew to force out the RH leadscrew bearing. Slide the carriage and leadscrew towards the right, Push the LH leadscrew bearing to the right (into the frame) and remove that too. Then 'unscrew' the leadscrew from the carriage and remove it through the left side of the printer frame.

Remove the 2 E-clips on the carriage rails _inside the frame_. Take out the rails and remove the carriage. Notice how the ridge on the paper feed roller fits into the carriage. The platten strip will probably fall out at this point.

Unhook the tension springs on the sides of the printer that pull the paper feed pressure shaft forwards. You can now push the feed assembly rearwards and unclip it from the slots at the side.

I've found that carefully rubbing the rubber tyres with 1000-grit wet-n-dry paper (used dry) will greatly improve their grip.

When reassembling, put a smear of plastic grease on the leadscrew, and a bit of dry-film (teflon) lubricant on the carriage rails, the idler gear spindle and the gear teeth.

[Tony Duell]

There's a printer feature of topcat models that might cause you to spend a couple of hours looking for a non-existant fault. If you use the printer paper feed button _without paper loaded_, the head will go back and forth as it should do, but it will not be correctly braked when you release the button. It will just coast to a stop, and will not necessarily stop at the right position.

I spent a bit of time looking at the printer schematics and testing transistors (this is made harder by 2 facts, firstly the drive signals go straight from the PIK chip to the bases of transistors, so there's almost no voltage change there to check with a logic analyser, and secondly, the HP schematic doesn't show how the transistors in the 3 quad transistor arrays are allocated. I have my own schematic that does (on the HPCC CD-ROM), but...)

Only after a lot of thought did I realise that the way the out-of-paper circuit works (it grounds the FWD line, preventing the head from moving forward, if printer is out of paper and the feed button is not pressed) would also affect the brake circuit.

Disconnecting one of the red wires from the out-of-paper contact to the printer PCB proved the point. As did putting some paper in the thing.