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Atari SM124 General Maintenance

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Presets and parts on the MT-9 main board and CRT board (Atari service manual)

Care, safety and adjustment notes for the Atari SM124 monochrome monitor, taken from Atari's service manuals for the MT-9 and MT-21 chassis and the 1985 owner's manual.[1][2][3] The two chassis use different designators for some of the same controls, so identify the chassis before turning anything.

Safety

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The SM124 runs its picture tube at up to 12 kV (Atari's limit is 12 kV +1 −2 kV at zero beam current on 220 V mains), and parts of both chassis carry mains or rectified-mains voltage.[4][5] Atari's manual warns that many B+ and video terminals are exposed with the back off.[4]

  • Discharge the tube before handling the chassis. Atari's method connects a 10 kΩ resistor, in series with an insulated lead such as a test probe, between the tube's outer coating (the dag) and the second anode lead. See CRT Discharge Procedure.[4]
  • Keep the earth leads between the chassis, the tube dag and the power board connected whenever the chassis runs.[4]
  • The tube has integral implosion protection. Do not lift it by the neck, and wear shatter-proof goggles when handling it.[4]
  • Parts marked with a shaded area on the schematic or an "S" in the parts list affect safety, and Atari says to replace them only with the specified part.[4]
  • The MT-21's switch-mode supply rectifies the mains directly into C907 (120 µF 400 V), so its primary side sits at mains potential.[5][6]

Identify the chassis

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MT-9 and MT-21 compared
MT-9 MT-21
Service manual P/N 483-303A[7] P/N 483-351A[8]
Power supply Mains transformer T901 on a separate power board, bridge D901 to D904, regulator IC901 (MC78T12CT)[9][10] Switch-mode supply with a TDA4601 (IC901) and a 2SC3507 switching transistor (Q901)[5][6]
Mains fuse F901 0.5 A time-delay (T0.5A/250V)[1][11] 1.6 A time-lag (1.6A/250V)[2][12]
Horizontal oscillator Discrete transistors, Q702 to Q705[9] AN5790N (IC701)[13]
Mains current 0.26 A[14] 0.3 A[15]

One of oldcrap.org's board photographs shows Atari's caution label for F901, which asks for a 1.6AT 250V fuse.[16]

Cleaning and care

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Atari's owner's manual asks owners to keep the ventilation slots in the housing clear, to keep liquids away from the monitor, to keep it out of direct sunlight, extreme temperatures and high humidity, and to clean the screen with a soft, slightly damp, lint-free cloth. Spray cleaners are ruled out because the spray can reach the vents. It also says not to leave the monitor switched on when it is not in use.[3] The rated operating range is 5 to 50 °C and 8 to 80 % relative humidity.[17]

Fuses

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Both chassis have a single mains fuse, F901, on the AC line.[1][2] The MT-9 takes a 0.5 A time-delay fuse on its power board; the MT-21 a 1.6 A time-lag fuse.[11][12] A fuse that blows again at once on either chassis points at the rectifier: Atari's power charts send a repeated failure to D901 to D904 on the MT-9 and to D901 and IC901 on the MT-21. See Atari SM124 Troubleshooting Guide.[18][19]

User controls

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The brightness, contrast and on/volume knobs sit on the right side panel, in that order from top to bottom.[20] Atari's setting-up advice is to display text, turn brightness fully clockwise, bring it back to a comfortable level, then set contrast until the text is clear and distinct.[20]

Adjustments

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Atari's main troubleshooting chart has the technician leave the monitor switched on for 15 minutes and check it again before moving on to adjustment.[18] The focus procedure uses a test pattern. Both manuals give the same procedures with different designators.[1][2]

Service adjustments
Adjustment MT-9 MT-21 Procedure
Focus VR703 VR703 Best overall focus on the test pattern. Centre and corners do not focus at the same setting, so settle on a compromise.[1][2]
Sub-brightness VR702 (board legend and video chart); the adjustment text names VR301[21][18][1] VR702 in the video chart; the adjustment text names VR701[19][2] With the front brightness control at maximum, set for visual cut-off of the raster.[1]
Vertical size VR602 VR601 Fill the screen vertically.[1][2]
Vertical linearity VR603 VR602 Best overall linearity. Size and linearity do not interact.[1][2]
Vertical hold VR601[18][21] The first step of the video chart when the display is faulty.[18]
Horizontal size L702 L703 Fill the screen horizontally.[1][2]
Horizontal linearity L703 L704 Best overall horizontal linearity.[1][2]
Horizontal centre or hold VR701 (H-CENTER)[18][21] VR701 (H-HOLD in the video chart)[19] The first step of the video chart when the display is faulty.[18][19]
B+ voltage VR901 Set the supply to the flyback to 27 V DC.[2]
Raster centring Centring tabs on the neck behind the deflection yoke Turn the ring assembly to move the raster; spread the two tabs further apart to move it further. Correct a tilted raster by loosening the yoke clamp and rotating the yoke. The tabs are close to the high-voltage yoke terminal.[1][2]

Atari's geometry limits, measured on the screen, are 1.5 mm for pincushion or barrel on each edge, 3.0 mm (top and bottom) and 2.0 mm (sides) for trapezium distortion, 2.0 mm for parallelogram distortion and 1° for raster tilt.[22]

Safety checks after a repair

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Atari's safety inspection before handing a monitor back:[4]

  1. Check that no wiring is pinched and that no loose hardware is lodged between the chassis and other metal parts.
  2. Refit every protective part: non-metallic knobs, insulating fishpaper, the cabinet back, covers and shields, and insulators.
  3. Measure leakage current at every exposed metal part with a path to the chassis (input terminals, screw heads, knobs and shafts), with the mains lead plugged straight into a 220 V outlet and no isolation transformer, then repeat with the plug reversed.

Without a leakage tester, the manual connects a 1.5 kΩ 10 W resistor in parallel with a 0.15 µF capacitor between the metal part and a known earth and measures the AC voltage across it; 0.35 V rms or more is excessive. The manual prints the tester limit as "0.26 A", which cannot be meant literally: 0.35 V across 1.5 kΩ is about 0.23 mA.[4]

Capacitors

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Both chassis use aluminium electrolytic capacitors in the power supply, deflection, video and sound circuits.[10][6] The full lists, and the warning about the bipolar capacitor in the horizontal deflection circuit, are on the Atari SM124 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 SM124 Service Manual (MT-9), Adjustment and Maintenance, p. 3.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 SM124 Service Manual (MT-21), Adjustment and Maintenance, p. 3.
  3. ↑ 3.0 3.1 SM124 Owner's Manual, p. 5, "Care and Safety".
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 SM124 Service Manual (MT-9), Safety Precautions and Safety Inspection, p. 2.
  5. ↑ 5.0 5.1 5.2 SM124 Service Manual (MT-21), Theory of Operation, power supply and Fig. 7, Power Circuit, pp. 7-8.
  6. ↑ 6.0 6.1 6.2 SM124 Service Manual (MT-21), Replacement Parts List, p. 21.
  7. ↑ Atari, SM124 High-Resolution Monochrome Monitor Service Manual, chassis MT-9, P/N 483-303A, cover. File:Atari SM124 Service Manual (MT-9 chassis).pdf.
  8. ↑ Atari, SM124 High-Resolution Monochrome Monitor Service Manual, chassis MT-21, P/N 483-351A, cover. File:Atari SM124 Service Manual (MT-21 chassis).pdf.
  9. ↑ 9.0 9.1 SM124 Service Manual (MT-9), Theory of Operation, pp. 5-6.
  10. ↑ 10.0 10.1 SM124 Service Manual (MT-9), Replacement Parts List, pp. 26-27.
  11. ↑ 11.0 11.1 SM124 Service Manual (MT-9), Replacement Parts List, p. 27: F901 T0.5A/250V.
  12. ↑ 12.0 12.1 SM124 Service Manual (MT-21), Replacement Parts List, p. 22: F901 fuse, time lag 1.6A/250V.
  13. ↑ SM124 Service Manual (MT-21), Theory of Operation, horizontal deflection, p. 6.
  14. ↑ SM124 Service Manual (MT-9), Specifications, p. 3.
  15. ↑ SM124 Service Manual (MT-21), Specifications, p. 3.
  16. ↑ Pawel Pieczul, "Atari SM124 Monitor", oldcrap.org, 20 October 2019, https://oldcrap.org/2019/10/20/atari-sm124-monitor/. Board photographs.
  17. ↑ SM124 Owner's Manual, p. 6, "SM124 Monitor Specifications".
  18. ↑ 18.0 18.1 18.2 18.3 18.4 18.5 18.6 SM124 Service Manual (MT-9), Troubleshooting Chart, pp. 16-19 (main, power, video and sound charts).
  19. ↑ 19.0 19.1 19.2 19.3 SM124 Service Manual (MT-21), Troubleshooting Chart, pp. 16-19.
  20. ↑ 20.0 20.1 Atari Corp., SM124 Owner's Manual, 1985, pp. 2-3, "Controlling and Adjusting the Monitor". File:Atari SM124 Owner's Manual (1985).pdf.
  21. ↑ 21.0 21.1 21.2 SM124 Service Manual (MT-9), Component of P.C.B, main board component side, p. 13: control legends VR601 V-HOLD, VR602 V-SIZE, VR603 V-LIN, VR701 H-CENTER, VR702 SUB BRIGHT, VR703 FOCUS. File:Atari SM124 MT-9 main board layout (service manual).png.
  22. ↑ SM124 Service Manual (MT-9), Fig. 2, Geometry Measurements, p. 4.


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