Atari 850 Maintenance Guide
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The Atari 850 Interface Module needs little routine care. Atari's 850 Interface Module Service Manual (CS 400/800-S004-B, April 1981) gives a short preventive maintenance routine, a cable check, a way to read the ROM revision and a loopback test for the serial ports.[1] Faults are diagnosed in the Atari 850 Troubleshooting guide.
Power adaptor
[edit | edit source]The 850 takes 9 V AC from Atari's power adaptor and makes +5 V, +10 V and -8 V on its own board.[1] The service manual names adaptor CA014748; the FAQ lists the CA017964 as an equivalent, and gives the module's consumption as 17 W.[1][2] With 120 V AC into the adaptor, Atari expects 9-10 V AC RMS (about 20-25 V peak to peak) at the input.[1] Do not use the 830 modem's adaptor on the 850. Atari warns that it can seriously damage any other Atari unit.[3]
Work area
[edit | edit source]Atari states that by design there are no high voltages in the 850.[1] Its chips are static-sensitive. Atari asks for a grounded, anti-static work surface, a wrist strap, and no plastic sheets on the bench, warning that laying a board on a Mylar page protector can destroy several chips.[1]
Opening the module
[edit | edit source]- Turn the 850 upside down and remove the four screws in the base. This separates the two halves of the shell.
- Hold the halves together, turn the module the right way up and lift off the top half. All the voltage checks can be made at this point.
- To take the board out, lift it from the shell at the serial port edge first, bending the shell very slightly if needed.
- To reach the ROM alone, slide off the small metal window in the RF shield. To reach the chips under the shield, straighten the tabs of the bottom half that pass through the board and the top half, and lift the top half off.
- Leave any serial number sticker in place; the factory needs it.
To refit the board, put in the edge with the protruding switch and LED first, and the board drops into place.[1]
Preventive maintenance
[edit | edit source]Atari's routine:[1]
- Separate the shell and the board.
- Clean the shell with a mild soap or detergent and a soft cloth. Most shells are Cycolac plastic; some are anodised aluminium.
- Remove both halves of the RF shield and inspect the board. It should be flat; replace a visibly warped board.
- Look for discoloration, which usually means a component has overheated. Power the board up and feel for a hot component; if a replacement also runs hot, suspect the power supply.
- Remove any solder splashes and refit the RF shield.
- Check the SIO connectors J106 and J107 for broken or bent pins and lost plating, and the serial and parallel connectors J101 to J105 for broken spring contacts inside their Delrin housings.
Check that the heatsink on the +5 V regulator A112 is firmly attached; a loose heatsink lets A112 overheat and lose regulation.[1]
Cables
[edit | edit source]An SIO cable that has been plugged in many times can become unreliable without failing outright, through lost plating, metal fatigue or corrosion. Atari sets a limit of 0.03 Ω for the resistance of each conductor and says to discard any cable that exceeds it or shows lost plating. Check the computer's SIO socket at the same time.[1] The 850's ports use Atari's own pinouts; the CX86, CX87 and CX88 cables adapt them to a Centronics printer, a modem and a terminal.[2]
ROM and board revision
[edit | edit source]The ROM's part number ends in its software revision: a ROM stamped C012099-03 holds revision 3. Read it through the window in the RF shield. The board revision is printed on the solder side, so the board has to come out of the shell to read it.[1]
Port test
[edit | edit source]Atari's port test loops a serial port's output back to its input. Make up a 9-pin plug with pins 3 and 4 joined (send and receive data), or in an emergency join them with hook-up wire, and fit it to the port under test.[1] Boot the 850 (850 first, then the computer), wait for READY, and enter Atari's program, changing "R2:" to the port being tested:[1]
10 DIM IN$(30),OK$(30) 20 BAUD=1:REM Make BAUD 0, 1, 2, ..., 15. See Table A7-1 30 XIO 36,#2,BAUD,0,"R2:":REM or R1 or R3 or R4 40 OPEN #2,13,0,"R2:" 50 XIO 40,#2,0,0,"R2:" :REM No configuration commands after XIO 40 60 INPUT IN$ 70 PRINT #2;IN$: REM Output IN$ though Port 2 80 INPUT #2,OK$: REM Input IN$ through Port 2, and rename it OK$ 90 PRINT OK$ 95 CLOSE #2 99 END
Type a line of text; the program sends it out of the port and prints what comes back, so the two can be compared. The value of BAUD in line 20 selects the speed from Atari's table A7-1, where 0 is 300 bit/s and 14 or 15 is 9,600 bit/s.[1] If a port works at every speed except the highest, replace its buffer chip.[1] The test does not check the handshaking lines.[1]
Capacitors
[edit | edit source]The 850 has nine aluminium electrolytic capacitors, eight of them in the power supply. They are listed on the Atari 850 Capacitor Replacement Guide; see also Capacitor Failure Symptoms.
Related pages
[edit | edit source]- Atari 850
- Atari 850 Troubleshooting
- Atari 850 Capacitor Replacement Guide
- Atari 850 Documentation
- Recommended Tools
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 1.11 1.12 1.13 1.14 1.15 Atari, Inc., 850 Interface Module Service Manual (CS 400/800-S004-B, April 1981): work station and safety pp. 3-2 and 3-3; disassembly and assembly pp. 3-5 and 3-6; preventive maintenance pp. 3-6 and 3-7; 5 V checks p. 3-13; note on cables, appendix 2; power supply, appendix 3; revision level, appendix 6; port test program, appendix 7; parts list, appendix 8. Hosted here as File:Atari 850 Interface Module Service Manual CS400-800-S004-B.pdf.
- ↑ 2.0 2.1 Michael D. Current (ed.), Atari 8-Bit Computers: Frequently Asked Questions, comp.sys.atari.8bit FAQ, last modified 2 August 2026, section 5.2.1. mcurrent.name. Community reference.
- ↑ Atari, Inc., Atari 850 Interface Module Operator's Manual (C017651, 1982), installation. Hosted here as File:Atari 850 Interface Module Operator's Manual C017651.pdf.