Electronics

VISA'

11/85

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Photo 3. The Star's paper feed gears. The arrow marks the idler gear, which must be moved before you remove the platen.

screwdriver. Here's a step-by-step for the MX- and RX-80s:

1. Slide the print head carriage all the way to the left.

2. Remove the C washer retaining clip from each end.

3. Push in the brass bearing on the end of the shaft until it clears the hole and slips inside the frame.

4. Lift out the platen by lifting the left side out first.

The only difference with the Star Gemini printers is that the idler gear sits in the way (see Photo 3). A small C clip holds it on. Just be careful when you remove the C clips—they have a habit of shooting across the room and getting lost in the rug.

After you remove the C ring, slide the idler gear out of the way and remove the C rings on the platen shaft: remove the wavy washer, plastic washer, and bushing on the left end of the platen. Push the roller to the left until the bushing on the right side clears the hole, then lift out the roller (see Photos 4. 5. and 6).

All printers are a variation on this theme. If I haven't covered your printer, look over your printing mechanism and make a drawing of where the parts go when you take it apart, so you'll be able to put it back together.

If you have a Radio Shack Daisy Wheel II, you're lucky; all you have to do is push down the levers and lift the platen roller out.

Epsons without a friction feed platen have a solid platen held in place with two screws on each end of the printer mechanism. It's easy to remove, but you have to get a lot of stuff out of the way first. Here's how:

1. Remove the two screws that hold the paper feed motor on the printer mechanism.

2. Remove the C clip and the idler gear.

3. Remove the two screws on each end and lift the platen out.

Now that you've got the printer apart, how do you get the labels out? There's only one way—soak them off. Peel off the bulk of the debris by hand, then dissolve the glue with Dr. Scat or some other solvent that eats glue but not on plastic. Alcohol works. So does soap and water, given enough time. Don't ever scrape off a label with a knife or sharp object. Scratches In the paper guides make the paper catch, or cause other problems. Take your time and do it right: a platen is expensive to replace and metal paper guides can be hard to find.

Be sure you get all the glue and label off, then put the printer back together. Assembling it is just like taking it apart, only backwards. Consult the photos or the drawing you made. When you put the motor back on an Epson, start the screws in their holes, push the motor as far away from the idler gear as possible, and tighten the screws. This gives the proper amount of free play and clearance between the motor and the gears.

Photo 4. Bushings on the carriage's left side. Note which way the wavy washer is bent.

C clips can be tricky to put back on. The best way is to line them up in the groove and press them in place with a pair of pliers.

Ribbon Blues

You can trace most other mechanical problems to worn or bent parts. When working with a printer, it's important not to force things; printers have many plastic and sheet-metal parts that can break if you're not careful.

When the ribbon starts to bind, it's usually telling you that it's over the age of retirement. A worn-out or defective ribbon can prevent the printer from working properly. On Epsons and Stars, the motor that moves the carriage feeds the ribbon, too, so a Jammed ribbon can keep the print head from moving. To And out if the problem is with the printer or the ribbon, remove the ribbon cartridge and see if the ribbon drive works by moving the print head back and forth. Or put paper in the printer and run the self-test without the ribbon in place. The ribbon drive should move freely.

If the ribbon is motor-driven and the motor doesn't seem to be working, it's probably time for a trip to the service shop. Most printers with separate ribbon feed motors use stepper motors—motors that require special electronics to drive them. When a motor goes, it usually takes the electronics with it, so putting in a new motor won't solve the problem.

When a printer with a mechanical ribbon drive stops working, it's usually because dirt and inky goo gums up the works. Careful cleaning and light oiling should fix the printer. The best cleaner for this job is a solvent in a spray can. like Tune-O-Wash from Chemtronics (681 Old Willets Path. Hauppauge. NY 11788). or a plain, non-silicone tuner cleaner (available from Radio Shack).

When using this cleaner, put newspaper under the printer to protect your work surface and make sure you put clear plastic case parts out of the way, as these solvents will fog them. Direct the spray into the gears where the gunk is. Wear old clothes when working on dirty printers, as spray solvents can spatter ink all over the place.

If the ribbon drive doesn't work after cleaning and oiling, the gears probably need replacing. As strange as it might seem, many plastic mechanical parts tighten up when they wear.

Feeding Frenzy

When paper doesn't feed, it usually means that you've got jammed paper feed gears or a burned-out paper feed motor. With the power off, see if the paper feed knob turns freely. If it does, you probably have a motor or electrical problem. If you can feel the gears binding, the gears are either worn or dirty.

Spray cleaner comes in handy here, too. Spray the gear train with a degreaser

Photo 7. When unplugging the print head, pinch and pull.

80 Micro, November 1985 • 51

until you get all the gunk out of the gears, then lightly oil them. If the paper feed still doesn't work properly, you probably need a new gear train. Replacing it is easy enough if you can get the parts; or, you might want to have your local service center check it out.

If the paper feed knob turns freely with the power off, and the paper doesn't form feed or line feed properly, you probably have a bad motor or bad driver electronics. If you're handy with electrical stuff and have access to an ohmmeter, trace out the wires from the paper feed motor, unplug the motor from the logic board, and measure the windings' resistance. Most printer stepper motors are split stepper motors, meaning that each half of the motor has two coils sharing a common wire.

The idea is to measure the resistance between each hot wire and the common wires. The four windings should be within a couple of ohms of each other. If one of the windings is shorted or open, the motor is bad. When a motor goes, it almost always takes out the transistors that drive it (or the transistors take out the motor). Since motors are hard to get. you might have to take the printer to a repair shop. If you can get a motor, it's a good idea to replace the four transistors (or the transistor array) that drive it.

Many printers have a transistor that switches the motor voltage up when the printer has to move and back down to a lower holding voltage when paper isn't feeding. If this transistor shorts, the motor voltage is too high and burns out the motor. You can find the transistors related to the paper feed motor if you know how to read the printer's schematic—the printer technical manual comes in handy for this kind of problem.

Fuse and Far Between

Most electronic problems are in driving circuitry (the paper and carriage feeds and the print head drive) or in the computer input buffer. Since the main logic board is fragile and soldering to the PC boards in most printers is difficult, this may be one for the repair shop.

When a printer goes dead, check the fuse; replace It with one of the same size and type if it's blown. If the fuse blows again, don't replace it—take the printer in for service.

Missing dots on your printouts are usually a sign of a dead print head or a blown driver transistor. Most print heads are easy to replace, but If you don't check out the electronics, too, chances are you'll just burn out another print head.

Replacing a print head with the cable attached is easy. The Epson's and Star's print heads plug into a socket on the base of the printer. Unplug the old print head-grasp the flat cable by pinching it near the connector, and pull it straight out (see Photo 7). Two screws hold the Star's print

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Requirements IBM PC or compatible riming PC/MS-POS lx-3«, mmura of 128K memory, and (t least on* disk drive

PC-DOS may (X required lor use eu PC. XT. and AT are r»giii«r«d I'adxnarVs oi n*m«enal BvHrwti MacMnai Corp US Ii ar*gi«»rad iraaxnartxn Mcroeon head in; remove them and pull the print head straight up. Epson secures its print heads with a lever: flip the lever to the left and pull the print head straight up.

Plug the new print head into the connector where the old print head was. Don't mount the print head in the carriage; set it on the table so you can see the print wires. Watch the print wires and turn the printer's power on, but only for about a second. If any of the print wires pushes out. turn the power off fast. Leaving the power on longer could burn out your new print head. When a wire pushes out. it indicates a shorted driving transistor for that dot and you should replace it. It's a good idea to have a technician do this: you should also check out the other parts in the print head's driving circuit.

If no wires push out. chances are the electronics are OK. Repeat the test, this time for a few seconds. If everything still looks OK, install the print head in the carriage and fasten it in place with the screws, clips, or whatever and test out the printer.

What if you replace the ribbon and clean the print head and ribbon guides, and you still don't get clear, clean letters on your printouts? You need to adjust the print head-to-paper distance or replace the print head. Try backing off the print head-to-paper clearance with the paper thickness adjustment lever and print out something. If the printout looks smudged, it's time for a new print head.

Dot's All, Folks

The basic procedures I've outlined will help you keep your printer in top condition. If you do routine maintenance twice a year and keep your printer clean and properly oiled, it will probably last forever. ■

Vincent E. Meyer is a freelance technical writer and president of The Micro Clinic, formerly Wildwood Data Systems, a computer service and support company. You can reach him at 105 State St.. Schenectady. NY 12305.

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