Atari 820 Troubleshooting

The 820 taken apart: top housing cover, top housing, front cover, the base plate assembly and the base, from figure 5-10 of the field service manual

Fault-finding on the Atari 820 printer follows the troubleshooting guide in Atari's ATARI 820 Printer Field Service Manual (FD100048). The guide works to the replaceable chip, the main board, the transformer and the main mechanical parts. Cleaning, lubrication, the head gap and the belt tension are on the Atari 820 Maintenance Guide.

The 820 runs from 120 V AC. Mains is present at the power cord, the fuse holders, the switch, the transformer and the AC motor. Unplug it before opening the case or changing any part.

First checks

1. Run the operator interactive test, which needs no computer. Switch on and check that the power light comes on. Press the paper advance button: the head should travel across and back to home, and the paper should advance one line for each press.
2. If that works, connect the printer to a 400 or 800 and run Atari's Printer Quick Check program in BASIC (listed as figure 5-2 in the manual). Check the printout for both cases of the alphabet, the symbols, a full 40-character line with an automatic line feed, straight columns, single and multiple line feeds, even repeated characters and missing dots.

The printer stops for about three seconds at intervals during long print runs to keep the head from overheating. This is normal and does not need repair.

Troubleshooting guide

Atari's guidelines: substitute only known-good parts, retest after each change, keep the power off and the cord unplugged while changing anything, and leave substituted chips in place until all the suspect chips have been tried.

The procedure steps referred to in the table are:

1. Check both fuses on the back of the printer and replace either or both if they have blown. Use only 2 A type 3AG fuses.
2. Remove the top housing.
3. Inspect all the connectors for open or partial contacts, and the fuse holders for breaks or damage.
4. Replace the chips on the main board one at a time (see below).
5. Replace the transformer.
6. Replace the main board.
7. Replace the switch board and harness.
8. Adjust the drive belt tension.
9. Adjust the print head to platen gap.
10. Replace the electromechanical part named in the table: print head, Print OK switch or drive belt.

Troubleshooting guide (FD100048 table 5-1)

Symptom	Possible causes, in order	Procedure steps
No power light; will not paper advance	Blown fuse; broken fuse holder; bad transformer	1; 2, 3; 2, 3, 5
Will not paper advance	6532 PIA; solid-state relay; power switch board; Print OK switch	2, 3, 4; 2, 3, 6; 2, 3, 7; 2, 3, 10
Paper advances continuously from power-up	6532 PIA; transistor array; solid-state relay; Print OK switch	2, 3, 4; 2, 3, 4; 2, 3, 6; 2, 3, 10
Printer does not respond to computer commands (error 138)	6532 PIA; 6507 MPU; transistor array	2, 3, 4 for each
Print head moves but will not print	Blown fuse; broken fuse holder; bad transformer; main board	1; 2, 3; 2, 3, 5; 2, 3, 6
Wrong characters printed	ROM; 6532 PIA	2, 3, 4 for each
Print head vibrates in the home position	Solid-state relay; transistor arrays; 6532 PIA	2, 3, 6; 2, 3, 4; 2, 3, 4
Data transmission errors	Transistor arrays; 6532 PIA	2, 3, 4 for each
Dot missing	6532 PIA; transistor array; driver; print head	2, 3, 4; 2, 3, 4; 2, 3, 6; 2, 3, 10
No power light but paper advances	Power switch board	2, 3, 7
Print motor stalls	Drive belt too tight; defective motor	2, 3, 8; replace the motor, then 8
Uneven or missed line feeds	Belt tension wrong; paper slipping; defective paper feed part	2, 3, 8; clean the platen; check the paper feed mechanism
Too much variation in character width	Drive belt tension; print head binding on the guide bar; 6532 PIA; ROM	2, 3, 8; clean and lubricate, then 9; 2, 3, 4; 2, 3, 4
Ribbon catches or snags on the print head	Wrong head-to-platen gap; driver failure	2, 3, 9; 2, 3, 6

Power supply

The transformer has two secondaries. One feeds the bridge CR109 to CR112 (MR501 diodes) and the reservoir capacitor C140 for the unregulated +40 V that fires the print head. The 2 A fuse F101 sits between C140 and a second 4,700 µF capacitor, C141, at the +40 V output. Atari's guide lists a blown fuse first when the head moves but does not print. The other secondary feeds the bridge CR113 to CR116 (1N4001) and C142 for an unregulated +12 V, which supplies the solid-state relay and the paper advance switch. The 78M05 regulator A108 makes +5 V for the logic from the +12 V. The second fuse protects the 120 V AC input. Atari gives no tolerances for these rails. The electrolytic capacitors are listed in the Atari 820 Capacitor Replacement Guide.

The seven solenoid drivers Q104 to Q110 are TIP122 transistors. Atari's description says the driver circuit sinks the approximately 3.6 A needed to fire the solenoids and quickly removes the stored energy from a solenoid when it turns off.

Chip replacement

Do not change a chip with the power on or the printer plugged in.

1. Remove the noise shield over the chips by straightening its five tabs.
2. Start with the chip the troubleshooting guide suggests, and retest after each change. If you use the print test, load a two-foot length of paper for it.
3. If the suggested chip does not cure the fault, change the others in this order: the 6532 PIA (A103), the 6507 MPU (A102), the ROM (A101), then the 3086 transistor arrays (A104, A105, A107).

All of these are in sockets. The part numbers are on the Atari 820 page.

Taking it apart

Main board

With the top housing off (see the Atari 820 Maintenance Guide):

1. Disconnect the transformer harness from the main board and remove the two bolts that hold the transformer to the base plate. Lift the transformer away.
2. Disconnect the other wires and harnesses from the main board, noting where each goes.
3. Remove the four screws that hold the main board to the base plate.
4. Lift the board at an angle, raising the side nearest the print mechanism first. Take care not to damage the two fuse connectors.

Refit it by lowering the board at an angle towards the rear until it sits flat, then replace the four screws, the harnesses and the transformer.

Main board connections (FD100048 figure 5-11)

Harness	From	To
Switch harness	Switch board	J101 (yellow, orange, white); J109 (black); fuse (red)
Power cord	Base plate	Fuse (black); J110 (black)
Fuse holder	13A, 13B	J106 (purple); J107 (black)
Print head harness	Print head	J102
Motor and microswitch harness	AC motor, Print OK switch	J108
Transformer harness	Transformer	J105

The text of the manual tells you to disconnect the transformer harness from J108, the connector that figure 5-11 and the print mechanism procedures give to the motor and microswitch harness. Check the connector before pulling it.

Print mechanism

1. Remove the two fuses from the back of the base plate.
2. Remove the eight screws that hold the base plate to the plastic base and lift out the base plate with the mechanism on it.
3. Remove the ribbon spools and any paper.
4. Disconnect the print head harness (J102) and the motor and switch harness (J108).
5. Turn the base plate on its side and remove the three screws that hold the mechanism to it.

On reassembly, check that all the rubber mounts are in place on the base before dropping the base plate back in.

Print head

1. Disconnect the print head harness from J102 and free it from the harness clamp on the side plate.
2. Remove the two hex head screws that hold the guide bar to the side plates, and lift the head and guide bar out, easing the head's tailpiece out of its guide slot.
3. Grease the head's inner sliding bushings, the tailpiece and its slot, and the elliptical groove in the drive cam.
4. Slide the head onto the guide bar; it must move freely. Lower it into place with the tailpiece in its slot and the metal stub under the head in the slot of the drive cam.
5. Fit the guide bar screws loosely, set the head gap, then refit the harness clamp and reconnect J102.

Print OK switch and drive belt

Both jobs need the right side plate moved away from the main extrusion. Take the mechanism off the base plate, remove the hex head screw that holds the guide bar to the right side plate and the three screws that hold the side plate to the extrusion, and ease the side plate away.

• For the switch, remove its two leads and fit the new microswitch with its foam pad attached.
• For the belt, open the gap far enough to pass the new belt round the drive cam. Loosen the two motor screws, put the belt over the cam pulley and the motor pulley, and set the tension.

Put the side plate back with the tear bar and drive cam correctly seated, refit the three screws, line up the guide bar and set the head gap before tightening its screw.

Transformer

Remove the two screws and nuts that hold the transformer to the common base, unplug its harness from the main board and plug in the new one.

Related pages

• Atari 820
• Atari 820 Maintenance Guide
• Atari 820 Capacitor Replacement Guide
• Atari 820 Documentation
• Capacitor Failure Symptoms