Atari 1040STF Power Supply Repair

A Mitsumi SR98 supply out of its ST, before recapping. The mains inlet and switch are at the top and the output cable at the right (photo: exxos).

The Atari 1040STF has a mains switching power supply inside its case, which provides +5 V and +12 V for the whole machine. The same long case and the same supplies are used in the 520STF, the 520STFM and the 1040STFM, and everything below applies to all four. Atari 1040ST describes the 1040 pair. Atari rates the supply at 3 A on +5 V and 0.9 A on +12 V, 33.5 W maximum input, with a single adjustment for both outputs.

Several makers built it. exxos has documented the Mitsumi SR98, DVE DSP-508A, Astec ASP34-2, Tokin 4501E1, Liton PE-2350-2, Phihong PSM2512-2 and Mitsumi SR151 in ST machines. Atari's specification for the 220 V Mitsumi SR98, drawing C103594, includes its parts list and schematic, and the SR98 is the supply worked through below. The capacitor lists for the other supplies are on the Atari ST Capacitor Replacement Guide.

The failures are those of age. The electrolytic capacitors lose capacitance and their ripple filtering, the switching transistor can fail short, and joints on parts that run hot crack.

Symptoms

• Random crashes, poor video, unreliable floppy or hard disk access and noise in the audio. exxos gives these as the signs of a failing supply, and measured about 1 V of ripple on the +5 V output of an unrecapped SR98.
• The picture dims for a moment when the floppy drive motor starts. On the SR98 exxos found that the regulation loop responds too slowly to the sudden load; new capacitors reduce the dimming but do not remove it.
• No power LED and no drive activity. Check the mains lead, switch and fuse. Atari's manual says that because the supply has overcurrent protection, a blown fuse probably means a catastrophic failure such as a shorted primary, so find that fault before fitting a new fuse.
• A machine that starts cold but crashes once warm. One owner's STFM did this; its Astec ASP34-2 (revision 5) had an output capacitor fitted the wrong way round at the factory, with its top starting to bulge.

• Horizontal lines across a colour monitor, or changes in brightness on a monochrome monitor, especially during disk access. Atari Benelux found this mainly on 1040STfs with the Astec ASP34-1 supply, and fixed it by raising the 5 V reservoir C35 on the main board from 470 µF to 2200 µF 16 V and, in the ASP34-1 itself, raising its C35 to 3900 µF 16 V. Atari's 1 May 1986 parts list confirms C35 on the main board as 470 µF 16 V radial.
• A hum at 50–100 Hz from the supply. Atari Benelux traced this in Tokin and Atari supplies to capacitor C1 vibrating when the mains waveform has a dip, and says it does not affect operation or the life of the machine. Atari supplied a mechanically stronger replacement as order number 10510 CO.
• A faint whistle at about 12 kHz. This comes from the 12 V converter on the main board, not from this supply; see Atari 1040STF 12 V Converter Whistle Repair.

A machine that stays blank until the reset button is pressed, or that fails only with a cartridge plugged in, has other causes; work through the Atari ST Troubleshooting Guide once the supply is proved good.

Safety

Warning: the supply's primary side runs from the mains, and its reservoir capacitor (C104 on the SR98, 47 µF 400 V) holds a dangerous charge after the machine is unplugged. Unplug the machine, measure across C104 and discharge it before touching the primary side. exxos's own warning on his supply page is that mains voltages can kill and that capacitors can explode.

Run the computer and every peripheral connected to it either all earthed or all unearthed. The 1040STf's supply has capacitors between the mains and earth; Atari Benelux warns that an unearthed 1040STf connected to an earthed peripheral discharges through it, which can damage the interface of either.

Do not measure the supply on the bench without a load. exxos warns that an ST supply disconnected from the computer will probably malfunction, and loads it with a 2.2–4.7 Ω resistor of at least 10 W on +5 V and a 22 Ω, 10 W resistor on +12 V (or 5–10 W lamps of the right voltage).

Tools and parts

• Soldering iron, desoldering tool and solder; a 1 mm drill if fitting the larger rectifier on an SR98.
• Multimeter and, for ripple, an oscilloscope.
• Load resistors as above for bench tests.
• An insulated trimming tool for the 5 V preset (VR201 on the SR98).

Mitsumi SR98: original parts (Atari parts list) and exxos's replacements

Position	Original	exxos's replacement	Notes
C104	47 µF 400 V	68 µF 400 V (Panasonic ECOS2GP680BA or EPCOS B43305A9686M000)	Primary reservoir
C201, C203	2200 µF 10 V	4700 µF 16 V (Panasonic EEUFR1C472)	+5 V rectifier output
C202	2200 µF 16 V	4700 µF 16 V (Panasonic EEUFR1C472)	+12 V rectifier output
C204	1000 µF 16 V	1500 µF 16 V (Panasonic EEUFR1C152L)	+12 V after L201
C205	1000 µF 10 V	1500 µF 16 V (Panasonic EEUFR1C152L)	+5 V after L202
C206	1 µF 50 V	Not in exxos's list	In the regulation loop; see the regulation fix below
D101	RB155 bridge	KBU1005, 10 A 600 V	exxos suggests no less than 5 A, 600 V
D202	HRW34-01 or ESAC82-004 Schottky	STPS10H100CFP	+5 V output rectifier
R201	2.7 kΩ	1.2 kΩ	Regulation fix
R202	2.4 kΩ	1 kΩ	Regulation fix

Sources: the original values are from Atari's parts list and schematic; the replacements are exxos's. exxos chooses low-ESR parts with a high ripple-current rating and warns that cheap capacitors are not suitable for a switching supply and can overheat or explode. He has since stopped listing part numbers because they go out of production, and generally uses Panasonic's FR series.

Procedure

Identify the supply

1. Read the label on the supply and the number printed on its board. The SR98 board in the photographs on exxos's page is marked 68-4231A.
2. If it is not an SR98, use the parts for that supply from the Atari ST Capacitor Replacement Guide and the notes under other supplies below. The 110–120 V Mitsumi SR97 is a different board from the SR98, with different values.

Remove the supply

Atari's sequence for the 1040ST:

1. Unplug the mains lead and every cable. Turn the machine upside down.
2. Remove the seven screws in the square holes, which hold the top case to the bottom. The three screws in the round holes hold the disk drive and can stay in.
3. Turn the machine upright and lift off the top case.
4. Unplug the keyboard harness at the right front corner and lift the keyboard away.
5. Remove the screw on the right side of the power supply shield.
6. Straighten the two twist tabs and lift the shield off from the rear.
7. Unplug the supply's harness at its right front corner.
8. Remove the two screws at the front corners of the supply.
9. Lift the supply up out of the main assembly.

Atari notes that the supply can be left in place with the upper shield removed for troubleshooting.

Atari's schematic of the Mitsumi SR98, drawing C103594 sheet 7. Mains enters at the left through the fuse, line filter L101 and bridge D101 to the reservoir C104; Q101 switches the transformer T101. On the secondary side D202 feeds the +5 V filter (C201, C203, L202, C205) and D201 the +12 V filter (C202, L201, C204); IC201 compares the +5 V output, through R201, R202 and VR201, with its reference and drives the optocoupler PC101.

Check the primary side

1. Discharge C104 and confirm with a meter that it is below a few volts.
2. Check the fuse. Atari's parts list for the SR98 gives a 250 V T1A fuse and its schematic agrees; Mitsumi's own diagram of the SR98 marks it 250 V 2 A, and Atari's field service manual gives 2 A for the 1040ST supply it describes. Fit the rating printed on your board. Atari Benelux found that some of Atari's own supplies with a green circuit board had been fitted with a 1 A fuse that blew without apparent cause, and specified a 1.5 A fast-acting fuse for that supply.
3. If the fuse has blown, measure the switching transistor Q101 (2SC2979 on the SR98) for a short between collector and emitter. One of exxos's SR98s died with this transistor short-circuited.
4. Measure the other parts in the path from the fuse to Q101 on the schematic: the line filter L101, the bridge D101, the 8.2 Ω 3 W wirewound resistor R101 in series with the bridge output, and C104. Replace anything shorted or open before powering the supply again.
5. Inspect every solder joint. Resolder the switching transistor's legs and anything else that runs hot or looks suspect; exxos counts bad joints among the main failures of these supplies and resolders them even when they look good.

Recap

1. Work on one capacitor at a time. Before removing each, note the polarity printed on the board, and check that the old part was fitted that way round; one ASP34-2 had a factory-reversed capacitor. If the print is in doubt, check continuity from the black 0 V output wire to the pad you believe is negative.
2. Replace C104 with a 400 V part of at least the original 47 µF.
3. Replace the output capacitors C201 to C205 using the table above. exxos replaced the 10 V parts with 16 V ones and raised the values to carry the extra current drawn by add-ons. His old SR98 parts measured 42 µF (C104), 1868–1951 µF (the 2200 µF parts) and 887–975 µF (the 1000 µF parts).
4. Replace the bridge rectifier D101 if fitting larger, lower-ESR capacitors: the extra capacitance puts more strain on the rectifier at switch-on. The legs of the KBU1005 were slightly too big for the SR98's holes, which exxos opened carefully with a 1 mm drill without damaging the pads. Match the + and − markings on the rectifier to the board.

A replacement bridge rectifier at D101 on an SR98 board (68-4231A), beside C104 and the 8.2 Ω resistor R101 (photo: Jonathan Whiteside, via exxos).

Output diode

The +5 V rectifier D202 (HRW34-01) sits on a heatsink beside the output capacitors. After recapping, exxos measured about 49 °C on that heatsink after ten minutes. He replaced the diode with an STPS10H100CFP, which has a lower voltage drop and an insulated package, and measured about 39 °C, with noise on the +5 V rail falling from about 500 mV to about 200 mV.

1. Desolder D202 from the board and remove it from the heatsink.
2. Fit the STPS10H100CFP in the same orientation, checking the pin-out of both parts against their data sheets.
3. Refit it to the heatsink and resolder.

Regulation fix for the floppy dimming

On the SR98 the +5 V output is divided by R201 (2.7 kΩ), R202 (2.4 kΩ) and the preset VR201 (1 kΩ) and compared by IC201, a TL431-type shunt reference, which drives the optocoupler PC101; C206 (1 µF 50 V) is part of the same loop. exxos found that the loop was too slow to follow the sudden load when the floppy motor started, and that lowering the divider resistors cured the dimming:

1. Replace R202 with 1 kΩ.
2. Replace R201 with 1.2 kΩ.
3. Reset VR201 for 5.00 V on the +5 V rail (see testing below).

exxos arrived at these values by fitting resistors in parallel with the originals (1.8 kΩ across R202 and 2.2 kΩ across R201). He also tried a faster optocoupler and a 2 µF part at C206, and saw little or no change from either.

Refit the supply

Atari's reassembly steps for the supply:

1. Plug in the supply connector and position the supply with its tabs in the slots.
2. Fasten the supply to the bottom case at both front corners with the two screws. Atari suggests a magnetised screwdriver through the shield, or removing the shield.
3. Refit the supply shield: slide its front tabs under the slots, twist the rear tabs and refit the screw.
4. Refit the keyboard harness, the top case and the seven screws.

The same SR98 after exxos's recap (photo: exxos).

Testing

1. Before refitting, test the supply on the bench into the load resistors, or test it in the machine with the shield off.
2. Measure +5 V at the supply connector with the machine running. Atari's instruction is to adjust it to within ±5 %, 4.75–5.25 V; the single preset sets both outputs.
3. Measure +12 V.
4. Measure the ripple on +5 V with an oscilloscope. A ×10 probe changes the reading by a factor of ten; exxos's published figures are corrected for it.
5. Access the floppy drive with a white screen showing and watch for the picture dimming as the motor starts.
6. Retest the machine once it is back in its plastic case; Atari recommends this to make sure the reassembly was done correctly and nothing shorts to the shield.

Expected values

Item	Specified	Measured by exxos (SR98)
+5 V output	5.1 V nominal (SR98 specification); 4.75–5.25 V (Atari manual)	Set to 5.00 V after the regulation fix
+5 V current	1.5–3.0 A (SR98); typical 2.2 A, 2.3 A during disk access (Atari manual)
+12 V current	0–0.9 A (SR98); typical 160 mA, 340 mA during disk access (Atari manual)
+5 V ripple and noise	100 mV peak to peak (SR98)	About 1 V before recapping, 400–500 mV after, about 200 mV after the diode change
+12 V ripple and noise	150 mV peak to peak (SR98)
Load regulation	±0.255 V on +5 V, ±1.2 V on +12 V (SR98)

Sources: SR98 figures from Atari's specification; manual figures from Atari's field service manual; measurements from exxos.

Other supplies

exxos's notes on the other supplies

Supply	Switching transistor	Notes
DVE DSP-508A	BU508A	Good regulation as found. exxos replaced the three 330 µF 35 V parts with 680 µF 25 V, the two 2200 µF 16 V with 4700 µF 16 V and the two 47 µF 250 V with new 47 µF 250 V, fitted a KBU1005 without drilling, and resoldered the BU508A's legs
Astec ASP34-2	BU508	exxos's sample ticked loudly, four or five times a second. Recap the two 250 V parts (33 or 47 µF), three 330 µF 35 V, one 4700 µF 16 V and one 2200 µF 16 V. Revisions 5 and 6 omit C3
Tokin 4501E1	2SC3460	Dims by about half on floppy access, and an extra 4700 µF on +5 V did not help. exxos does not upgrade it because the capacitors are packed too tightly
Liton PE-2350-2	2SC4233	About 200 mV ripple as found and no visible dimming; output capacitors 2 × 220 µF 10 V and 2 × 220 µF 25 V
Phihong PSM2512-2	2SC3150	Recap 47 µF 400 V, 2 × 2200 µF 10 V, 1000 µF 25 V and a 460 µF 25 V part; about 160 mV of noise afterwards
Mitsumi SR151 (240 V)	2SC3459	Recap 100 µF 400 V, 3 × 3300 µF 10 V, 2200 µF 16 V and 1000 µF 16 V; drill the rectifier holes to about 1.2 mm

Source: exxos's supply page, where the replacement values for each are given.

Replacing the supply

Where a supply is beyond economic repair, exxos sells replacement supplies and publishes a DIY board for the old SR98 frame. The board carries two Mean Well modules, MPM-20-5 or IRM-20-5 for +5 V and MPM-20-12 or IRM-20-12 for +12 V, which accept 100–240 V AC, with its own output ripple filter and mains noise filtering; exxos measured about 30 mV of ripple with the MPM modules.

Related pages

• Atari 1040STF, Atari 1040STFM, Atari 520STFM and Atari 1040ST
• Atari ST Capacitor Replacement Guide
• Atari ST Troubleshooting Guide
• Atari ST General Maintenance
• Atari 520ST and 1040ST Computer Field Service Manual
• Capacitor Failure Symptoms