Atari 850 Capacitor Replacement Guide
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The Atari 850 Interface Module has nine aluminium electrolytic capacitors. Eight are in the on-board power supply, which turns the adaptor's 9 V AC into +5 V, +10 V and -8 V, and one is in the power-on reset circuit. Atari's 850 Interface Module Service Manual (CS 400/800-S004-B) lists every one with its value, rating and part number, and its power supply schematic shows where they sit.[1]
Signs of failure
[edit | edit source]- Serial ports that work with some devices and not others, or give intermittent errors. Atari traces these to the +10 V or -8 V rail first.[1]
- Atari's power supply checks name capacitors directly: below 18 V at test point E points to C142 (or CR118, CR119); below 8.8 V at G to C136 or C139 (or CR112, CR113); less than 8.25 V negative at H to C147 (or CR111, Q102).[1]
- A module that fails to come out of reset when the computer is switched on; Atari lists C135 among the parts to check in the power-on reset circuit.[1]
- Bulged, leaking or crusted capacitors.
See Capacitor Failure Symptoms for general signs.
Capacitor list
[edit | edit source]| Ref | Value | Rating | Type | Atari part | Where it sits |
|---|---|---|---|---|---|
| C138 | 2,200 µF | 16 V | Axial | C014373 | Bridge output, input of the +5 V regulator A112 |
| C136 | 2,200 µF | 16 V | Axial | C014373 | -8 V supply input, test point G |
| C139 | 1,000 µF | 16 V | Radial | C015999 | -8 V doubler, with CR112 and CR113 |
| C141 | 470 µF | 16 V | Radial | C014368 | +10 V doubler, from the AC input (test point A) |
| C142 | 470 µF | 16 V | Radial | C014368 | +10 V doubler output, test point E (input of A113) |
| C137 | 10 µF | 16 V | Radial | C014392 | +5 V output |
| C143 | 10 µF | 16 V | Radial | C014392 | +10 V output |
| C147 | 10 µF | 16 V | Radial | C014392 | -8 V output |
| C135 | 1 µF | 50 V | Radial | C014773 | Power-on reset circuit |
The values, ratings, types and part numbers are from the parts list. The positions of the power supply capacitors are from figure A3-1, and C135's from the power-on reset checks on page 3-15.[1] The two 2,200 µF parts are axial and lie flat on the board; the photograph in figure A3-2 shows them with C141 and C142.[1] C140, across the AC input, is a 0.22 µF 100 V film capacitor ("poly film" in the parts list), not an electrolytic.[1]
Procedure
[edit | edit source]- Unplug the adaptor and all cables.
- Open the module and lift the board out of the shell, serial port edge first, as described in the maintenance guide.[1]
- Replace the capacitors one at a time, matching polarity to the board markings and noting the orientation of each before removing it. Axial replacements for C136 and C138 lie flat in their original positions.
- Check that the heatsink on A112 is firmly attached before refitting the board; a loose heatsink lets A112 overheat.[1]
- Refit the board, switch and LED edge first.[1]
Checks after recapping
[edit | edit source]| Point | Expected |
|---|---|
| A-B | 9-10 V AC RMS with 120 V AC mains |
| D (+5 V) | 4.75-5.25 V DC, ripple no more than 0.1 V |
| E | About 18.5 V DC, not below 18 V |
| F (+10 V) | 9.5-10.5 V DC |
| G | Half-wave rectified 9 V, not below 8.8 V |
| H (-8 V) | At least 8.25 V negative |
| J | 8.7-9.1 V (zener CR111) |
The test point positions are in figure A3-2; full checks are in the Atari 850 Troubleshooting guide.[1] Then boot the 850 from a computer and run Atari's port test on each serial port.
Related pages
[edit | edit source]- Atari 850
- Atari 850 Maintenance Guide
- Atari 850 Troubleshooting
- Atari 850 Documentation
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Atari, Inc., 850 Interface Module Service Manual (CS 400/800-S004-B, April 1981): disassembly pp. 3-5 and 3-6; power-on reset p. 3-15; power supply, appendix 3 (schematic figure A3-1 p. A3-2, test points figure A3-2 p. A3-3, circuit description pp. A3-4 and A3-5); complete parts list, appendix 8. Hosted here as File:Atari 850 Interface Module Service Manual CS400-800-S004-B.pdf.