Atari SM124 Capacitor Replacement Guide

The MT-21 switch-mode power supply: C907 across the bridge D901, and the 27 V and 12 V output filters C931 to C935 (Atari service manual)

Electrolytic capacitor lists for the Atari SM124 monochrome monitor, taken from the replacement parts lists in Atari's service manuals for the MT-9 and MT-21 chassis, with a list an owner compiled from the capacitors removed from his own boards. The two chassis share many designators with different values, so identify the chassis first (see Atari SM124 General Maintenance#Identify the chassis).

The MT-21 rectifies the mains straight into C907, a 400 V capacitor. Unplug the monitor, let C907 discharge and check it with a meter, and discharge the picture tube before working on either chassis (CRT Discharge Procedure).

Which list to use

• The MT-9 list is the clearer of the two, from a complete scan of the manual. The MT-9 has a transformer power supply on a separate board, and its reservoir capacitor is C905, 3300 µF 35 V.
• The MT-21 list comes from a poorer scan. Its switch-mode supply puts C907 (120 µF 400 V) across the mains bridge.
• Pawel Pieczul's list at oldcrap.org was read off the parts he removed, and uses the MT-21 numbering. It differs from Atari's MT-21 list in several places (below). He notes that the boards differ from the schematics.

Where the lists disagree, read the value and voltage printed on the part you are replacing.

MT-9 chassis

The parts list marks electrolytic capacitors "CE" and bipolar ones "BP".

MT-9 electrolytic capacitors (31)

Ref	Part number	Capacitance (µF)	Voltage (V)	Notes
C301	08110317	47	16
C302	08110221	220	10
C304	08110221	220	10
C305	08110315	22	16
C306	08110315	22	16
C307	02140321	220	16
C308	02140617	47	100
C401	08110313	10	16
C402	02140323	470	16
C403	02140323	470	16
C404	08110313	10	16
C405	08110511	4.7	50
C406	08110313	10	16
C408	02140321	220	16
C601	08110501	0.1	50
C608	08110313	10	16
C610	02140327	2200	16
C611	02140321	220	16
C612	02140321	220	16
C701	02140321	220	16
C702	02140321	220	16
C711	08110511	4.7	50
C712	08110315	22	16
C714	181-020J	2.2	50	Bipolar (BP); see the warning below
C715	02140421	220	25
C716	02140421	220	25
C717	02140717	47	160
C718	02140619	100	100
C905	181-205A	3300	35	Reservoir after the bridge D901 to D904
C907	02140321	220	16
C909	02140323	470	16

MT-21 chassis

MT-21 electrolytic capacitors (32)

Ref	Part number	Capacitance (µF)	Voltage (V)	Notes
C302	08110219	100	10
C303	08110219	100	10
C304	08110217	47	10
C305	08110317	47	10	The list prints 10 V; the part number is the one the MT-9 list gives for 47 µF 16 V
C306	08110315	22	16
C308	02140617	47	100
C401	08110313	10	16
C402	02140325	1000	16
C403	08110313	10	16
C404	08110317	47	16
C406	02140321	220	16
C601	08110501	0.1	50
C608	08110313	10	16
C610	02140327	2200	16
C611	02140321	220	16
C612	02140321	220	16
C706	08110505	0.47	50
C709	08110317	47	16
C712	08110511	4.7	50
C714	02140522	330	50
C716	181-020J	2.2	50	Bipolar (BP); see the warning below
C717	02140617	47	100
C718	02140619	100	100
C907	181-124M	120	400	Mains reservoir after the bridge D901
C909	02140417	47	25	The power circuit drawing on p. 8 shows 100 µF 25 V
C911	08110219	100	10
C912	08110507	1	50
C914	08110219	100	10
C931	181-254B	470	35	27 V output filter
C932	02140519	100	50
C934	181-254C	1000	25	12 V output filter
C935	02140323	470	16

Owner's list (oldcrap.org)

Capacitors removed from SM124 boards, oldcrap.org (29)

Ref	Capacitance (µF)	Voltage (V)	Ref	Capacitance (µF)	Voltage (V)
C302	100	10	C712	4.7	50
C303	100	10	C713	330	50
C304	47	10	C716	2.2 (bipolar)	50
C305	47	16	C717	47	100
C306	22	16	C718	100	100
C308	47	100	C907	120	400
C402	1000	16	C909	100	25
C404	220	16	C911	100	10
C601	0.1	50	C912	1	50
C608	10	16	C913	100	10
C610	2200	16	C931	470	50
C611	220	16	C932	100	50
C612	220	16	C934	1000	25
C706	0.47	50	C935	470	16
C709	100	16

Compared with Atari's MT-21 list:

• C305 is 47 µF 16 V on the board. Atari's list prints 10 V against a part number that the MT-9 list gives as 47 µF 16 V.
• C404 is 220 µF 16 V on the board (47 µF 16 V in the list), and C709 is 100 µF 16 V (47 µF 16 V in the list).
• C909 is 100 µF 25 V on the board and on Atari's power circuit drawing, but 47 µF 25 V in the parts list.
• C931 is 50 V on the board, 35 V in the list and the drawing.
• The board list has C713 (330 µF 50 V) and C913 (100 µF 10 V) where Atari's list has C714 and C914 with those values; in Atari's list C713 is a ceramic and C913 a film capacitor.
• The board list leaves out C401, C403 (10 µF 16 V each) and C406 (220 µF 16 V).
• C302 and C303 on the CRT board are 100 µF 10 V. An owner who used the list found the page had first given them as 10 µF 100 V, and the author corrected it.

The bipolar capacitor C716

C716 on MT-21 boards (C714 in the MT-9 list) is a 2.2 µF 50 V bipolar capacitor in the horizontal deflection circuit. Pieczul names it as the part most likely to fail; a failed C716 leaves a single vertical line on the screen. He warns that an ordinary electrolytic of the same rating blows up in this position, and fits a polypropylene (MKP) film capacitor instead. A reader who recapped his SM124 with the list fitted a WIMA film capacitor of the same rating there and reported that it works.

Procedure

1. Unplug the monitor, remove the back and discharge the picture tube (CRT Discharge Procedure).
2. Photograph the boards and note the polarity marking of every electrolytic before removing it.
3. Replace the capacitors one at a time, with the same capacitance and the same or a higher voltage rating. Keep 400 V for C907 on the MT-21.
4. Fit a film capacitor, not an electrolytic, for the bipolar C716 (C714 on the MT-9).
5. Refit the boards, connect the earth leads between the chassis, the tube dag and the power board, and run Atari's safety checks before use (see Atari SM124 General Maintenance#Safety checks after a repair).

krs, the reader quoted above, replaced the small 0.1 µF, 0.47 µF and 1 µF 50 V electrolytics with X7R ceramic capacitors of the same values, because good low-value electrolytics had become hard to find.

Checks after recapping

Chassis	Measure	Expected
MT-9	Input of IC901	About 22 V DC
MT-9	Output of IC901 and pin 2 of P901	12 V DC
MT-21	Supply to the flyback (B+)	27 V DC, set with VR901
MT-21	12 V output	12 V DC

Then check the picture and the adjustments in Atari SM124 General Maintenance#Adjustments.

Related pages

• Atari SM124
• Atari SM124 General Maintenance
• Atari SM124 Troubleshooting Guide
• Atari SM124 Documentation
• Capacitor Failure Symptoms
• CRT Discharge Procedure