Atari SM125 General Maintenance

SM125 main board with the presets and, along the bottom, the power supply (Atari service manual)

Safety, power and adjustment notes for the Atari SM125 monochrome monitor, from Atari's SM125 Service's Manual (P/N C026408). The manual has no parts list or schematic, so the designators below are those in its text, flowcharts and board layout.

Safety

• The picture tube's anode supply is "over several thousand volts", in the manual's words. Discharge the tube before handling the chassis (CRT Discharge Procedure), and remember the outer coating (aquadag) holds a charge too.
• The power supply rectifies the mains directly into C710 and C712, which charge to about 300 V DC on either mains setting.
• Atari's manual carries an X-radiation warning and refers the technician to the safety notes inside the cabinet and on the chassis.

Mains voltage setting

The SM125 runs on 230 V 50 Hz or 115 V 60 Hz, chosen by a jumper. The jumper is J701 on the power supply: closed for 120 V, where the rectifier D701 to D704 works as a voltage doubler, and open for 240 V, where it works as a bridge. The supply accepts 90 to 130 V or 180 to 260 V at 47 to 63 Hz. Check J701 before connecting a monitor that has come from another country.

Fuses

The board layout marks the mains fuse 1 A 250 V. The service notes say a completely dead monitor has probably blown one or both fuses. The manual does not give the second fuse's rating.

Adjustments

SM125 adjustments

Adjustment	Control	Procedure
Sub-brightness	VR302 (internal), with the brightness control VR303	Connect the signal cable. Turn the external brightness control VR303 to maximum. Turn VR302 to the threshold of the raster. Then set the external brightness to taste.
Vertical size and linearity	VR202 (size), VR203 (linearity)	With a test pattern, set VR202 for the size wanted, then VR203 until the top and bottom characters are the same height as those in the centre. Re-adjust VR202.
Focus	VR304	Best focus at a point near the centre, about one third of the way down the screen.
Raster centring	Vertical size VR202 and the deflection yoke	Turn VR202 down until all four edges of the raster show. Slide the yoke on the tube neck for best centring, restore the size with VR202, then fix the yoke to the neck with hot-melt adhesive.

The board layout also shows VR201 (V-hold) and VR301 (H-phase) next to VR202, and VR701 sets the power supply's 12 V output.

Mechanical adjustments

• Raster centring: two levers on the back of the deflection yoke move the picture. Turn them alternately until it is centred.
• Tilted raster or shaded corners: loosen the yoke clamp screw, press the yoke firmly forward against the flare of the tube, and turn it left or right until the scan lines are horizontal.

Removing the picture tube

Atari's order, after discharging the tube:

1. Remove the anode cap from the tube.
2. Pull the CRT socket off the tube base.
3. Loosen the deflection yoke screw and slide the yoke off the tube.
4. Remove the four mounting screws.
5. Disconnect the CRT earth lead.
6. Lift the tube out of the cabinet.

The manual notes that the components are mounted on one side of the board with straight, uncrimped leads, which makes desoldering easier.

Capacitors

The hosted manual has no capacitor values, so no list can be given from it. Read the values from the parts on the board, and see Capacitor Failure Symptoms for what to look for.

Related pages

• Atari SM125
• Atari SM125 Troubleshooting Guide
• Atari SM125 Documentation
• Atari SM124 General Maintenance
• CRT Discharge Procedure