Atari 820 Capacitor Replacement Guide

The 820 power supply: C140 and C141 on the +40 V print head supply either side of fuse F101, and C142 on the +12 V supply, from appendix A of the field service manual

The Atari 820 printer has eleven aluminium electrolytic capacitors on its main board. Three are in the power supply, seven are in the print head driver stages and one is in the circuit round Q102. Atari's ATARI 820 Printer Field Service Manual (FD100048) lists each one with its value, rating and part number, and the schematic and silkscreen in its appendices show where they are.

Signs of failure

The large capacitors smooth the two unregulated supplies. Atari's troubleshooting guide does not name any capacitor as a cause. It does list the supply-side faults: no power light, or a head that moves but does not print, which it traces to the fuses, the fuse holders, the transformer and the main board. Look for bulged, leaking or crusted capacitors and for electrolyte on the board. See Capacitor Failure Symptoms for general signs.

Capacitor list

Atari 820 electrolytic capacitors (FD100048 parts list and schematic)

Ref	Value	Rating	Type	Atari part	Where it sits
C140	4,700 µF	50 V	Radial	C014375	+40 V bridge output, with R133 across it
C141	4,700 µF	50 V	Radial	C014375	+40 V print head supply, after fuse F101
C142	2,200 µF	16 V	Axial	C014373	+12 V bridge output, input of the 78M05 A108
C120 to C126	4.7 µF	16 V	Radial	C014369	One in each of the seven print head driver stages, on the line from the 6532
C109	1 µF	50 V	Radial	C014773	With R107, R108 and CR101 at Q102, near the I/O connectors

The values, ratings, types and part numbers are from the parts list. The positions are from the schematic in appendix A. The schematic marks R133 as 2.2 kΩ 1/2 W, while the parts list gives a 2.2 kΩ 2 W resistor.

The other capacitors on the board are ceramic or film. They include several on the mains side:

• C135 and C137: 0.1 µF, listed as "across the line" types (Atari C014868). C137 is across the mains after the power switch, at the chokes L103 and L104. Any replacement must be a capacitor rated for use across the mains.
• C136: 1.25 µF 400 V poly film (C014774), at the AC motor connector J108.
• C138: 0.1 µF 100 V Mylar, across the input of the +40 V bridge.
• C143: 0.22 µF, beside C142 on the +12 V supply. The parts list gives a 50 V poly film part (C010394) with a 25 V ceramic as the alternative.

Procedure

1. Unplug the printer. The power cord, the fuse holders, the switch, the transformer and the motor all carry mains.
2. Remove the top housing, then take out the main board: unplug the transformer and lift it away, disconnect the harnesses, remove the four screws and lift the board at an angle. The steps are in the troubleshooting guide.
3. Before working on the board, check with a meter that C140 and C141 have discharged. On the schematic the bleeder R133 is across C140, on the bridge side of F101.
4. Replace the capacitors one at a time and note the polarity of each before removing it. The silkscreen in appendix B shows the positions and polarity marks. C142 is an axial part and lies flat.
5. Refit the board at an angle towards the rear until it sits flat, taking care of the two fuse connectors, then refit the four screws, the harnesses and the transformer.

Checks after recapping

1. Switch on and check that the power light comes on.
2. Press paper advance: the head should travel across and back to home, and the paper should advance one line for each press.
3. Run the Printer Quick Check program from a 400 or 800 and check the printout for missing dots and even character width.

Atari gives no voltage tolerances for the +40 V and +12 V supplies. The +5 V comes from the 78M05 A108.

Related pages

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