Atari 820 Maintenance Guide

The 820 print mechanism, from figure 5-5 of the field service manual

The Atari 820 printer has a cam-driven print mechanism that needs cleaning and greasing, and two adjustments, the print head gap and the drive belt tension, that drift with use. Atari's ATARI 820 Printer Field Service Manual (FD100048) gives a yearly cleaning and lubrication routine, a 13-point inspection and the adjustment procedures, all done with ordinary hand tools and a feeler gauge. Faults are covered in the Atari 820 Troubleshooting guide.

Safety

The 820 takes 120 V AC through a built-in cord, and mains is present inside the case at the fuse holders, the power switch, the transformer and the AC motor. Atari's manual points out that skin resistance is normally about 300 Ω, so any voltage over 30 V can be lethal. Unplug the printer before taking it apart at any level.

The chips on the main board are MOS devices that static can destroy. Atari asks for a grounded anti-static bench cover and a wrist strap, and warns against plastic carriers and desoldering tools that build up charge.

Opening the printer

1. Make sure the print head is in its home position at the left. If it is not, turn the drive cam towards the platen until it is.
2. Turn the printer over onto its top housing.
3. Remove the four black feet to expose the case screws, and remove the four screws.
4. Hold the top housing and base together, turn the printer the right way up and lift off the top housing cover.
5. Take out the paper roll and mandrel, then lift the top housing off the base.
6. Lift the front cover up and towards the back, and remove the two screws that hold the power switch board to it.

Refit in the reverse order and put the four feet back over the screws. Removing the main board and the print mechanism is covered in the Atari 820 Troubleshooting guide.

Cleaning and lubrication

Atari sets the interval at 12 months for a printer kept in a clean place, and shorter for heavy use or a dirty environment. Work with the top housing off.

You need a dust-free cloth, isopropyl alcohol, cotton swabs and grease: Atari specifies IBM #23 or Lubriplate #70.

1. Wipe the print mechanism and the top housing with the cloth moistened in alcohol, taking off hardened grease, ink and dirt. Use swabs where the cloth will not reach.
2. Turn the platen and clean it with the alcohol-soaked cloth.
3. Remove the two hex screws, one on each side of the mechanism, that hold the print head and head shaft. Clean the head and the shaft and refit them.
4. Because the head has been off, set the head-to-platen gap afterwards (see below).
5. Grease these points with just enough to leave a film:
   • the head shaft
   • the groove in the main drive cam
   • the guide in the extrusion along which the head's tailpiece travels
   • the paper feed cam and pawl
   • the ribbon feed pawl
   • the platen detent teeth and ratchet teeth
   • the pressure roll release lever

When a print head is replaced, Atari also greases the head's inner sliding bushings, its tailpiece and guide slot, and the elliptical groove in the drive cam. The metal stub under the head must sit in the slot on the drive cam when the head goes back.

Inspection

Atari's visual inspection, done as part of the routine and during repairs:

1. Press paper advance and check that the print wires on all seven solenoids move and return to rest.
2. Check that the head assembly moves easily on its shaft.
3. Work the ribbon feed pawl back and forth by hand to check the ribbon reverse. Both pawl springs must be in place and of equal tension.
4. Look for loose or missing screws.
5. Turn the drive cam towards the platen and listen for the click of the Print OK switch. The switch arm roller should be half-way up the high dwell ramp of the cam when it clicks, and the switch arm must be straight with no kinks.
6. Check that the right end of the platen is not loose on the platen drum.
7. With the paper release lever pressed and the ratchet pawl disengaged, the platen should spin freely.
8. The edge of the plastic paper guide strip should touch the platen drum along its whole length and be secured to the extrusion.
9. Turn the drive cam towards the platen and check that the head moves freely along its shaft and in the cam groove.
10. Fit a ribbon and check that it does not snag anywhere in its path. Check the spool drag.
11. The paper guides should be secure on the main frame extrusion but move under moderate pressure for adjustment.
12. All cables should be dressed and secured, clear of the moving mechanism.

Print head gap

Set the gap whenever the head comes off or either hex head screw on the guide bar is loosened. You need a feeler gauge.

1. Remove the paper roll and the ribbon.
2. With the guide bar screws loose, measure the gap between the print head and the platen. It must be 0.012 to 0.015 in.
3. Move the head across the platen and check that the gap stays within that range all the way.
4. When the gap is even, tighten the hex head screws.

Always turn the drive cam towards the platen. Turning it the other way can damage the Print OK switch. A wrong gap makes the ribbon catch on the head.

Drive belt tension

Atari lists belt tension as a cause of motor stalls, uneven or missed line feeds and uneven character width. Adjust it after any print fault of that kind and whenever the belt has been off its pulleys.

1. Slightly loosen the two bolts that hold the print motor to the right side plate.
2. Put the belt over and round both pulleys.
3. Move the motor forwards or backwards to set the tension, then tighten the bolts.

Atari gives no tension figure. Too tight a belt is listed as a cause of the motor stalling.

Ribbon

The new ribbon comes on the white spool and the empty spool is blue. Put the white spool on the right-hand ribbon post and the blue spool on the left. Run the ribbon in front of the ribbon tension bracket and round the two ribbon guides. Do not run the printer without a ribbon and paper fitted, as this can damage the print head solenoids. Atari's ribbon is part C014854; the FAQ lists Addressograph/Multigraph 116-2395-366 as the same ribbon.

Paper

The printer takes 3 7/8 inch adding machine roll paper. Switch off and check that the head is at home on the left before loading.

1. Take off the top housing cover and unwind enough paper to let the roll sit on the bench.
2. Feed the paper in at the bottom rear of the platen, using the nylon guides.
3. Turn the manual paper knob counterclockwise to bring the paper round the platen until it is about an inch above the tear bar.
4. Roll the paper back onto the roll, insert the mandrel and drop the roll into the recess behind the platen, with the mandrel ends in their slots.

Turn the paper knob only counterclockwise, away from you. Turning it the other way or forcing it can damage it. Pulling the paper release lever forward helps when straightening the paper.

Checking the result

The operator interactive test needs no computer. Plug the printer in, switch it on and check that the power light is on. Then press the paper advance button. The head should travel across and back to home, and the paper should advance one line for each press.

The print test runs Atari's Printer Quick Check program in BASIC from a 400 or 800. Atari printed the listing in the manual (figure 5-2) and suggested saving a copy on tape or disk. The printout should show both cases of the alphabet and the symbols, a full 40-character line ending with an automatic line feed, straight columns, single and multiple line feeds, repeated characters with no loss of quality, and no missing dots.

Related pages

• Atari 820
• Atari 820 Troubleshooting
• Atari 820 Capacitor Replacement Guide
• Atari 820 Documentation
• Recommended Tools