Atari ST Capacitor Replacement Guide

This guide provides complete capacitor lists and replacement procedures for the Atari 520ST, 520STM, 520STF, 520STFM, 1040ST, 1040STF, and 1040STFM computers. These machines share common motherboard designs — notably the C070789-001 board which is used in both the 520STFM and 1040STFM. Capacitor lists are organised by board revision, as the component count and values vary between revisions.

For the Atari STE range (520STE / 1040STE), see Atari STE Capacitor Replacement Guide.

When to Recap

With these machines now 35–40 years old, electrolytic capacitors are the most common point of failure. Symptoms of failing capacitors include:

• Unstable or low +5V supply voltage (PSU caps)
• Random crashes or freezes
• Screen dimming when the floppy drive is accessed (PSU peak power)
• Audio distortion, hum, or crackling
• Floppy read/write errors
• Video noise, ghosting, or wavy lines

The PSU should be recapped as a first priority, as failing PSU capacitors can cause symptoms that mimic motherboard faults. Motherboard recapping is a secondary priority — the axial electrolytic capacitors used on ST motherboards (often Wang's brand) have generally proven durable, but replacement is recommended as preventive maintenance.

Safety Warning

The internal PSU (1040ST/STF/STFM models) contains mains-voltage components. Capacitors on the high-voltage side of the PSU can retain lethal charge even after the machine is unplugged. Only experienced individuals should service the PSU. Allow at least 5 minutes after unplugging before touching any PSU components.

Tools Required

• Temperature-controlled soldering iron (chisel tip recommended, 350–370°C)
• Solder sucker and/or desoldering braid
• Quality replacement capacitors (Nichicon, Rubycon, Panasonic recommended; low-ESR types for PSU)
• Digital multimeter for post-recap voltage verification
• Isopropyl alcohol (99%+) and brush for flux cleanup

Motherboard Capacitor Lists

Identify your motherboard revision by the part number printed on the PCB. The following lists cover all known ST/STF/STFM motherboard revisions.

C070243 Rev B (520ST / 520STM)

Early short-case board used in the 520ST and 520STM.

Ref. Designator	Capacitance	Voltage	Type	Notes
C31	1 µF	50V	Electrolytic
C33	47 µF	25V	Electrolytic
C34	1 µF	50V	Electrolytic
C50	22 µF	16V	Electrolytic
C55	22 µF	16V	Electrolytic
—	470 µF	16V	Electrolytic	Not designated on silkscreen
KBD	100 µF	25V	Electrolytic	Keyboard controller board

C070243 Rev C (520ST)

Ref. Designator	Capacitance	Voltage	Type	Notes
C31	100 µF	25V	Electrolytic
C33	47 µF	25V	Electrolytic
C34	47 µF	16V	Electrolytic
C50	22 µF	16V	Electrolytic
C55	22 µF	16V	Electrolytic
C??	470 µF	16V	Electrolytic	Designator unclear on PCB
C105	22 µF	16V	Electrolytic
—	470 µF	16V	Electrolytic	Not designated
KBD	100 µF	25V	Electrolytic, axial	Keyboard controller board

C070243 Rev H (520ST / 520STM)

Ref. Designator	Capacitance	Voltage	Type	Notes
C31	100 µF	16V	Electrolytic
C33	47 µF	16V	Electrolytic
C34	47 µF	16V	Electrolytic
C50	22 µF	16V	Electrolytic
C55	22 µF	16V	Electrolytic
C56	470 µF	16V	Electrolytic	May be marked C56 or unmarked
C104	470 µF	16V	Electrolytic
C105	22 µF	16V	Electrolytic
KBD	100 µF	25V	Electrolytic, axial	Keyboard controller board

C070523-001 Rev B / Rev C (1040STF)

Earlier 1040STF-specific board design.

Ref. Designator	Capacitance	Voltage	Type	Notes
C7	22 µF	16V	Electrolytic
C8	22 µF	16V	Electrolytic
C9	47 µF	16V	Electrolytic
C12	1 µF	50V	Non-polarised	25V replacement acceptable
C14	1 µF	50V	Non-polarised	25V replacement acceptable
C25	4.7 µF	25V	Electrolytic
C27	4.7 µF	25V	Electrolytic
C29	4.7 µF	35V	Non-polarised	25V replacement acceptable
C30	4.7 µF	25V	Electrolytic
C35	470 µF	16V	Electrolytic
C48	100 µF	25V	Electrolytic
C71	22 µF	16V	Electrolytic
C105	470 µF	16V	Electrolytic

C070523-001 Rev D (1040STF / 520STFM)

Applies to Rev D boards with a large 2200 µF cap at C35.

Ref. Designator	Capacitance	Voltage	Type	Notes
C7	22 µF	16V	Electrolytic
C8	22 µF	16V	Electrolytic
C9	47 µF	16V	Electrolytic
C12	100 µF	16V	Electrolytic
C14	47 µF	16V	Electrolytic
C25	4.7 µF	25V	Electrolytic
C27	4.7 µF	25V	Electrolytic
C29	4.7 µF	35V	Non-polarised	25V replacement acceptable
C30	4.7 µF	25V	Electrolytic
C35	2200 µF	16V	Electrolytic	Large axial or radial cap
C71	22 µF	16V	Electrolytic
C105	470 µF	16V	Electrolytic
—	100 µF	16V	Electrolytic	Add-on above C41 (not soldered to C41 pads)

C070789-001 Rev C / Rev F (520STFM / 1040STFM)

This is the most commonly encountered board revision. It is shared between the 520STFM and 1040STFM — the only difference is RAM population (512 KB vs 1 MB). Also compatible with Rev B, Rev D, and Rev F2 boards based on board photos.

Ref. Designator	Capacitance	Voltage	Type	Notes
C2	22 µF	16V	Electrolytic
C7	22 µF	16V	Electrolytic
C9	47 µF	16V	Electrolytic
C11	100 µF	16V	Electrolytic
C12	47 µF	16V	Electrolytic
C22	100 µF	16V	Electrolytic
C49	1000 µF	16V	Electrolytic	Main bulk decoupling
C70	100 µF	16V	Electrolytic
C73	100 µF	16V	Electrolytic
C74	4.7 µF	35V	Non-polarised	Radial replacement provided in kits
C75	4.7 µF	35V	Electrolytic
C92	100 µF	16V	Electrolytic
C148	100 µF	16V	Electrolytic
C149	4700 µF	16V	Electrolytic	Large main filter capacitor
C215	1 µF	50V	Electrolytic	Audio path
C216	10 µF	16V	Electrolytic	Audio path
C217	10 µF	16V	Electrolytic	Audio path
C218	10 µF	16V	Electrolytic	Audio path
C219	33 µF	16V	Electrolytic	Audio path
C220	100 µF	16V	Electrolytic	Audio path
C221	100 µF	16V	Electrolytic	Audio path
C223	470 µF	16V	Electrolytic

Total: 22 electrolytic capacitors on the motherboard.

C103225 Rev 1 / Rev 2 (1040STFM)

Late-production board.

Ref. Designator	Capacitance	Voltage	Type	Notes
C2	22 µF	16V	Electrolytic
C7	22 µF	16V	Electrolytic
C9	47 µF	16V	Electrolytic
C11	100 µF	16V	Electrolytic
C12	47 µF	16V	Electrolytic
C49	1000 µF	16V	Electrolytic
C70	100 µF	16V	Electrolytic
C73	100 µF	16V	Electrolytic
C74	4.7 µF	35V	Non-polarised
C75	4.7 µF	35V	Electrolytic
C92	100 µF	16V	Electrolytic
C148	100 µF	16V	Electrolytic
C149	4700 µF	16V	Electrolytic

C103572 Rev 1 (1040STFM)

Final production board. Identical to C103225 but with one additional cap:

Ref. Designator	Capacitance	Voltage	Type	Notes
C2	22 µF	16V	Electrolytic
C7	22 µF	16V	Electrolytic
C9	47 µF	16V	Electrolytic
C11	100 µF	16V	Electrolytic
C12	47 µF	16V	Electrolytic
C49	1000 µF	16V	Electrolytic
C70	100 µF	16V	Electrolytic
C73	100 µF	16V	Electrolytic
C74	4.7 µF	35V	Non-polarised
C75	4.7 µF	35V	Electrolytic
C92	100 µF	16V	Electrolytic
C148	100 µF	16V	Electrolytic
C149	4700 µF	16V	Electrolytic
C190	100 µF	16V	Electrolytic	Additional cap not present on C103225

PSU Capacitor Lists

The 1040ST's internal PSU is the most failure-prone component. The 520ST uses an external PSU that is generally more reliable but should still be checked. Below are lists for the most common internal PSU models found in the 1040STF and 1040STFM.

Mitsumi SR98 / SR118 / 68-4231A (220–240V)

The most common PSU in European 1040STs. Main switching transistor: 2SC2979 (800V, 3A, 40W).

Ref. Designator	Original Value	Voltage	Recommended Replacement	Notes
C104	47 µF	400V	68 µF 400V	High-voltage primary filter
C201	2200 µF	10V	4700 µF 16V	Secondary filter
C202	2200 µF	16V	4700 µF 16V	Secondary filter
C203	2200 µF	10V	4700 µF 16V	Secondary filter
C204	1000 µF	16V	1500 µF 16V	Secondary filter
C205	1000 µF	10V	1500 µF 16V	Secondary filter
C206	1 µF	50V	1 µF 50V ceramic	Feedback loop; consider ceramic replacement

Also recommended: Replace the bridge rectifier with a KBU1005 (10A, 600V) or similar higher-rated part. The original 2A rectifier is marginal.

Mitsumi SR97 / 68-4242A (110–120V)

US market PSU.

Ref. Designator	Original Value	Voltage	Recommended Replacement	Notes
C104	220 µF	200V	220 µF 200V	Primary filter
C201	2200 µF	16V	4700 µF 16V
C202	2200 µF	10V	4700 µF 16V
C203	1000 µF	16V	1500 µF 16V
C204	2200 µF	10V	4700 µF 16V
C205	1000 µF	10V	1500 µF 16V
C206	1 µF	50V	1 µF 50V

ASTEC ASP34 / ASPF34 (220–240V)

Common European PSU with BU508A main transistor. Multiple sub-revisions (Rev 1–6) exist with minor differences.

Ref. Designator	Original Value	Voltage	Recommended Replacement	Notes
C4	47 µF	250V	47 µF 250V	Primary filter
C5	47 µF	250V	47 µF 250V	Primary filter
C7	330 µF	25V (or 35V)	680 µF 35V	Secondary
C9	100 µF	25V	100 µF 25V
C12	2200 µF	16V	4700 µF 16V
C13	4700 µF	16V	4700 µF 16V
C14	330 µF	25V (or 35V)	680 µF 35V
C15	330 µF	25V (or 35V)	680 µF 35V

DVE DSP-508A / DSP-508AA (220–240V)

Uses BU508A transistor. Generally good regulation even with aged caps.

Ref. Designator	Original Value	Voltage	Recommended Replacement	Notes
C3	47 µF	250V	47 µF 250V	Primary
C4	47 µF	250V	47 µF 250V	Primary
C8	330 µF	25V	680 µF 25V
C9	1 µF	50V	1 µF 50V
C??	100 µF	25V	100 µF 25V	In shrinkwrap near safety cap at board top
C20	330 µF	25V	680 µF 25V
C22	2200 µF	16V	4700 µF 16V
C23	330 µF	25V	680 µF 25V
C25	2200 µF	16V	4700 µF 16V

FPK-N3 5657-B001-1

Ref. Designator	Capacitance	Voltage	Notes
C5	220 µF	200V	Primary
C8	47 µF	16V
C9	47 µF	16V
C101	470 µF	25V
C102	470 µF	25V
C103	100 µF	25V
C201	2200 µF	10V
C202	2200 µF	10V
C203	470 µF	10V
C205	—	—	Empty position on PCB
C303	1 µF	50V

C070091-014 / Model 1103 (520ST External PSU)

Designators not available for this external PSU. Replace by value:

Capacitance	Voltage	Quantity
4700 µF	16V	1
2200 µF	25V	1
100 µF	25V	1
220 µF	25V	1
10 µF	25V	1

Internal Floppy Drive Capacitors (1040ST)

The 1040ST's internal floppy drive (commonly the TDK D357 / T41V) also contains electrolytic capacitors that can fail:

Capacitance	Voltage	Quantity	Notes
22 µF	16V	2
47 µF	16V	2
10 µF	25V	1
4.7 µF	35V	1
2.2 µF	50V	1
0.47 µF	50V	5	May be film or ceramic on some drives
3.3 µF	50V	1

Replacement Procedure

General Tips

1. Always note polarity before removing old caps. The negative stripe on the capacitor body and the polarity markings on the PCB silkscreen should match. If the silkscreen is unclear, photograph the board before starting.
2. Use quality replacement capacitors. Panasonic FR series, Nichicon, and Rubycon are recommended. Avoid cheap unbranded capacitors, particularly for the PSU — poor ESR characteristics can cause the PSU to malfunction or even damage components.
3. Axial to radial conversion: Many ST motherboard caps are axial (leads from each end). Modern replacements are commonly radial (both leads from the same end). Radial caps can be used in axial positions by bending the leads appropriately, or by mounting them upright.
4. Non-polarised (NP) capacitors: Several positions (notably C74 on the C070789-001 board and C29 on the C070523-001 board) require non-polarised electrolytic capacitors. These can be replaced with bipolar/non-polarised electrolytics or with appropriately rated film capacitors.

PSU Recap Procedure (1040ST)

1. Unplug the machine and wait at least 5 minutes.
2. Remove the top case and disconnect the PSU from the motherboard.
3. Remove the PSU from the case (typically held by screws and/or clips).
4. Remove the metal PSU shield (if present) — usually held by bent tabs.
5. Photograph the PCB before starting, noting all capacitor polarities and positions.
6. Desolder each capacitor one at a time, clean the pads, and solder in the replacement.
7. For the bridge rectifier replacement: The replacement may have a different pinout. Triple-check the + and − markings match the PCB before powering on. An incorrectly installed rectifier will destroy the PSU.
8. Reassemble the PSU shield and reinstall in the case.

Motherboard Recap Procedure

1. With the machine open and PSU disconnected, work through each capacitor position.
2. Desolder the old cap, clean the pads with desoldering braid, and solder in the replacement.
3. Pay attention to the large C149 (4700 µF on C070789-001 boards) — this is a physically large cap and removing it cleanly requires adequate desoldering.
4. After completing all replacements, clean the board with IPA to remove flux residue.

Post-Recap Voltage Checks

After recapping, verify the following before reconnecting any peripherals:

Test Point	Expected Value	Tolerance	Notes
+5V at motherboard power header	5.00V	±0.25V	Measure with machine powered on and under normal load. Adjust PSU trim pot if available.
+12V at motherboard power header	12.0V	±1.0V	Needed for floppy and RS-232.
+5V ripple (oscilloscope)	<200 mV pk-pk	—	Indicates healthy PSU caps. >500 mV indicates a problem remains.
+5V at CPU VCC pin	4.90V – 5.10V	—	Check for excessive voltage drop from power header to CPU.

If the PSU has a voltage adjustment trimmer (present on some Mitsumi and ASTEC models), set the +5V output to 5.00V–5.10V under load. Do not exceed 5.25V.

Sourcing Capacitor Kits

Pre-assembled capacitor kits for specific board revisions are available from:

• Console5 (console5.com) — Kits for most ST motherboard and PSU revisions
• exxos (exxosforum.co.uk/atari/store2) — PSU recap kits and replacement PSU boards

When sourcing individual capacitors, use reputable suppliers (Farnell/Element14, Mouser, Digikey, RS Components) to avoid counterfeit or low-quality parts.

Related Pages

• Atari 520ST
• Atari 1040ST
• Atari ST General Maintenance
• Atari ST Troubleshooting Guide
• Atari STE Capacitor Replacement Guide