Atari SM146 Capacitor Replacement Guide

Electrolytic capacitors in the Atari SM146 monochrome monitor, read off the two-sheet SM146 schematic, the only document found for the monitor.^[1] The schematic has no parts list, so the values are those printed beside each part on the drawing, and some of them carry no voltage rating.

The SM146 rectifies the mains directly into C907 (47 µF 400 V). Unplug the monitor, check that C907 has discharged and discharge the picture tube before working on the board (CRT Discharge Procedure).^[1]

Capacitor list

The table lists every capacitor drawn with an electrolytic symbol. "Not printed" means the schematic gives no voltage for that part; read the rating from the part itself before ordering a replacement. Parts marked "hatched" are drawn with a symbol hatched on both plates instead of a "+" mark, which on this schematic may mean a bipolar part; check the old part before fitting a polarised one.^[1]

**SM146 electrolytic capacitors (28)**^[1]
Ref	Capacitance (µF)	Voltage (V)	Where it is drawn
C502	10	Not printed	Sound, beside the volume control
C503	100	16	Sound
C504	470	16	Sound
C506	100	16	Sound; hatched
C508	0.22	Not printed	Sound
C509	1000	16	Speaker coupling, IC501 output
C510	47	16	Sound
C603	470	16	12 V supply to the vertical stage
C604	100	16	Vertical, IC601
C609	22	16	Vertical
C611	1000	16	Vertical
C702	220	16	Video, 12 V
C704	10	Not printed	Video
C705	100	100	Video
C808	1	Not printed	Horizontal, IC801
C813	470	16	Horizontal, IC801 supply
C819	470	Not printed	Horizontal, 12 V
C822	47	100	Flyback secondary
C823	10	160	Flyback secondary
C824	10	Not printed	Horizontal; hatched
C627	4.7	160	Printed "C627" but drawn in the brightness circuit beside VR804; hatched
C907	47	400	Mains reservoir after the bridge D901 to D904
C910	100	25	Power supply primary side, beside ZD901
C921	1000	Not printed	35 V output
C922	220	Not printed	35 V output
C923	1000	16	12 V output
C924	470	16	12 V output
C926	1000	16	Power supply, secondary winding through D923

C921 and C922 filter the 35 V output, so their replacements need a rating well above 35 V.^[1] C818 (1.5 µF 200 V) in the horizontal circuit is marked MPP, a metallised polypropylene film part, and is not an electrolytic.^[1]

Procedure

Unplug the monitor, open the case and discharge the picture tube (CRT Discharge Procedure).
Check that C907 has discharged before touching the power supply.
Photograph the board and record the polarity of each capacitor before removing it.
Replace the capacitors one at a time with the same capacitance and the same or a higher voltage rating. Where the schematic prints no voltage, use the rating on the part removed.
After reassembly, check the 12 V and 35 V outputs of the power supply, then the picture.

Related pages

References

↑ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 Atari, SM146 schematic diagram (CMPC-146), two sheets, revision dates 20 June 1991 to 25 February 1992. File:Atari SM146 Schematic.pdf. Values as printed beside each part; there is no parts list.

[sch-1] 1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 Atari, SM146 schematic diagram (CMPC-146), two sheets, revision dates 20 June 1991 to 25 February 1992. File:Atari SM146 Schematic.pdf. Values as printed beside each part; there is no parts list.

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