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Atari 2600 Static Protection Modification

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Atari 2600 six-switch motherboard 4050 buffer A203 area
The 4050 hex buffer at A203 on a six-switch motherboard, soldered in (photo: Evan-Amos, public domain)

On the six-switch Atari 2600 (CX2600) the joystick fire buttons reach the TIA through a 4050 hex buffer at A203, which also buffers the luminance and RF output lines.[1] Static discharge through the controller ports damages that buffer and other parts. Atari's service bulletin of 18 February 1982 answered it with a zener diode between each trigger line and ground and static strips on the switchboard switches, and the field service manual makes the modification compulsory on every CX2600 opened for service.[2][3] Tech tip 12 names the 4050 as one of the first parts to check for any trigger fault, loss of sync or loss of the luminance lines, and the reason for a high percentage of returns.[4]

The four-switch CX2600A has no 4050; its trigger lines go straight to the TIA.[1] Atari's only static modification for it is a set of static strips on the front-panel switches of boards of revisions 1 to 13.[3]

Symptoms

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  • A fire button that does not work with a known-good joystick in either port. Atari's checklist for the CX2600 lists A203, the controller ports J202 and J203 and the controller.[5]
  • No sync, or missing luminance (grey levels), on a CX2600, which tech tip 12 also traces to the 4050.[4]
  • Joystick directions that do not work are a different fault. Atari's checklist puts them down to the 6532 (A202), the ports and the joystick.[5] See Atari 2600 Troubleshooting Guide.

Tools and parts

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Parts for the static modification (Atari numbers)
Part Atari number Where
Zener diode 1N4736, 6.2 V C017654 CR202, CR203 (CX2600)
Axial capacitor 200 pF 50 V C014180-05 C236, C237 (CX2600)
Zener diode and axial capacitor assembly CA018263 Replaces C236 and C237
Static strip for three switches C017294 CX2600 switchboard
Static strip for two switches C017297 CX2600 switchboard; CX2600A revisions 1 to 13
Hex buffer 4050B C010816 A203 (CX2600 only)

The part numbers and designators are from Atari's parts lists and bulletin.[6][2] The parts list also allows a 220 pF ceramic disc capacitor at C236 and C237.[6]

You also need a soldering iron, a desoldering tool, a multimeter and a grounding strap; Atari asks for the strap whenever the LSI chips and A203 are handled.[7]

Procedure

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Opening the console

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  1. Open the console and separate the switchboard from the motherboard as described in Atari 2600 Power Supply Circuit Repair#Opening the console.[8]
  2. Turn the motherboard's metal case over and remove the six screws that hold its bottom cover.[8][9]
  3. Remove the two screws that hold the motherboard to the top of the case and lift the board out.[8]

Checking the trigger lines

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Atari 2600 six-switch motherboard controller ports and J201
The controller ports J202 (top) and J203 (bottom) on a six-switch motherboard, with C236 and C237 among the disc capacitors beside them (photo: Evan-Amos, public domain)

Atari's trigger line procedure works from the controller port to the TIA, one side at a time. The left trigger is pin 6 of J202 and the right trigger pin 6 of J203.[10]

  1. With the board powered, measure +5 V at pin 6 of J202 (left) or J203 (right). If it is missing, check the track from R226 to J202 pin 6 (left) or R225 to J203 pin 6 (right), and check for +5 V at the top of R226 or R225.
  2. Measure +5 V at the 4050's inputs: A203 pin 9 for the left trigger, pin 7 for the right. If it is missing, repair the open track from R226 (left) or R225 (right) to A203.
  3. Measure 4.5 to 5 V at the 4050's outputs: A203 pin 10 for the left trigger, pin 6 for the right. If it is missing, replace A203.
  4. If the outputs are correct and the button still does not work, Atari swaps the TIA (A201), then the 6507 (A200) and the 6532 (A202).
  5. If none of that cures it, check and repair C236 and CR202 (left) or C237 and CR203 (right).

Replacing the 4050

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Atari 2600 and 2600A IC pinouts (FSM figure 2-1)
Atari's pin numbering for A203 (16 pins) and the other chips, top view
  1. Note the orientation of A203 before removing it; Atari's figure 2-1 shows its pin 1 next to the notch.[11]
  2. Desolder A203 and fit a new 4050B (Atari C010816) the same way round.[6]
  3. Check the trigger lines again as above. Tech tip 12 also asks for the 4050 to be checked for loss of sync and luminance.[4]

Fitting the zener diodes (CX2600)

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Atari 2600 static modification zener diode (FSM figure 3-2)
The zener lies along the axial capacitor with its leads wrapped one turn and soldered (field service manual figure 3-2)

Atari's procedure:[3][12]

  1. Remove C236 and C237 from the motherboard.
  2. Make two assemblies, each a 1N4736 zener laid along a 200 pF axial capacitor with the zener's leads wrapped one turn round the capacitor's leads and soldered. Atari supplied the finished assembly as CA018263.[2]
  3. Fit the C236/CR202 assembly in place of C236 and the C237/CR203 assembly in place of C237.
  4. Check the polarity: the dark band on each zener must face the J202/J203 connectors.

Fitting the static strips

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Atari 2600 switchboard static modification (FSM figure 3-4)
Static strips over the CX2600 switchboard switches (field service manual figure 3-4)
Atari 2600A static modifications (FSM figure 3-5)
Static strips on the front-panel switches of a CX2600A, revisions 1 to 13 (field service manual figure 3-5)
  • CX2600: fit the static strips over the switchboard switches as shown in figure 3-4. Atari's strips are C017294 (three switches) and C017297 (two switches).[3][2]
  • CX2600A revisions 1 to 13: mount the strips on the front-panel switches as shown in figure 3-5. Fold the tabs under before pressing the end of each strip onto the shield.[3][13]

Reassembly

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Atari's tech tips list the parts that short against the six-switch casting if they are not placed correctly:[4]

  1. Put L200 and the "chicklet" capacitors back under the casting's shroud (tip 15).
  2. Push C220 and C239 away from the cartridge connector's shroud (tip 17).
  3. On Taiwan-built consoles, trim long leads under the motherboard (tip 18).
  4. On motherboards of revision 8 and lower, check that the coloured insulating dot covers the trace under the upper left corner of J200 (tip 6).[14]
  5. Refit the two board screws, the bottom cover and its six screws, and reconnect the switchboard.[8]

Testing

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  • Plug a known-good joystick into each port in turn and check the fire button in a joystick game such as Missile Command, as Best Electronics does.[15]
  • With Atari's Diagnostic Test Cartridge, run the diagnostic matrix test with the shorting plugs fitted.[16]
  • Measure the trigger lines with the buttons released: +5 V at J202 and J203 pin 6 and at A203 pins 7 and 9, and 4.5 to 5 V at A203 pins 6 and 10.[10]
  • Atari burns every repaired console in for at least two hours.[7]
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References

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  1. ↑ 1.0 1.1 Atari, Inc., Video Computer System Field Service Manual, Domestic Model 2600/2600A, FD100133 Rev. 02, Section 1, p. 1-5: on the 2600A the luminance and RF output buffers and the two TIA input buffers, all in A203 on the 2600, are gone. Hosted as File:Atari 2600 and 2600A Field Service Manual.pdf; see Atari 2600 and 2600A Field Service Manual.
  2. ↑ 2.0 2.1 2.2 2.3 Atari, service bulletin, 18 February 1982, "Damage to Hex Buffer, and other components", in Field Service Manual FD100133 Rev. 02 (PDF p. 156): zener diode between the trigger lines and ground, static strips on the switchboard switches, assembly CA018263 and static strips C017294 and C017297.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Atari, Field Service Manual FD100133 Rev. 02, "2600 Model Modifications", pp. 3-2 and 3-3 (every 2600 opened must be modified; remove C236 and C237, insert the C236/CR202 and C237/CR203 assemblies, dark band towards J202/J203; static strips as in figure 3-4), and "2600A Model Modifications", p. 3-6 (static strips on the front-panel switches of revisions 1 to 13).
  4. ↑ 4.0 4.1 4.2 4.3 Atari, Field Service Manual FD100133 Rev. 02, NTSC technical tips at the back of the manual, pp. 7-2 to 7-4: tips 6, 12, 15, 17 and 18.
  5. ↑ 5.0 5.1 Atari, Field Service Manual FD100133 Rev. 02, Section 5, 2600 symptom checklist, controller failures, p. 5-4.
  6. ↑ 6.0 6.1 6.2 Atari, Field Service Manual FD100133 Rev. 02, Section 8, CX2600 parts list, pp. 8-1 to 8-3: C236 and C237 (220 pF ceramic disc or 200 pF axial C014180-05), zener diode/axial wrap assembly CA018263, 1N4736 6.2 V zener C017654 at CR202 and CR203, 4050B hex buffer C010816 at A203, static strips C017294 and C017297.
  7. ↑ 7.0 7.1 Atari, Field Service Manual FD100133 Rev. 02, Section 4 introduction, p. 4-1: handling A200 to A203 with a grounding band and a chip puller, and a two-hour burn-in after repairs.
  8. ↑ 8.0 8.1 8.2 8.3 Vintage Gaming and More, "Atari 2600 6 Switch Tune Up Kit Installation Guide": separating the switchboard, the six screws of the bottom cover and the two screws that hold the motherboard to the top of the case.
  9. ↑ Dave Farquhar, "How to disassemble an Atari 2600 VCS", The Silicon Underground, 20 February 2020: the six-switch can is held together by six screws on the underside.
  10. ↑ 10.0 10.1 Atari, Field Service Manual FD100133 Rev. 02, "Trigger Line Troubleshooting Procedure", p. 4-27.
  11. ↑ Atari, Field Service Manual FD100133 Rev. 02, figure 2-1, "2600/2600A IC Pinouts", p. 2-3.
  12. ↑ Atari, Field Service Manual FD100133 Rev. 02, figure 3-2, "2600 Static Modification Zener Diode", p. 3-4.
  13. ↑ Atari, Field Service Manual FD100133 Rev. 02, figure 3-5, "2600A (Revs. 1-13) Static Modifications", p. 3-6.
  14. ↑ Atari, Field Service Manual FD100133 Rev. 02, figure 3-3, "Location of Colored Dot Over Trace", p. 3-4.
  15. ↑ Best Electronics, "What to look for when buying a used or third party reconditioned Atari 2600 / 2600A / 2600Jr console", tip 8: testing the ports and the fire button with a known-good joystick.
  16. ↑ Atari, Field Service Manual FD100133 Rev. 02, "Diagnostic Matrix Test", p. 3-11.


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