Atari Mega ST Capacitor Replacement Guide

This article provides a complete capacitor replacement (recapping) guide for the Atari Mega ST computer, covering the motherboard and power supply unit. Electrolytic capacitors have a finite lifespan (typically 10,000–15,000 hours or 10–15 years at rated temperature) and will degrade over time, causing a range of symptoms from minor video noise to complete system failure.

When to recap

Recapping is recommended when any of the following are observed:

• Random crashes or freezes during operation
• Video noise, flickering, or screen dimming during floppy access
• Floppy drive read/write errors
• Audio noise or distortion
• PSU emitting unusual sounds (ticking, whining)
• Visible electrolyte leakage or capacitor bulging
• The machine is over 25 years old and has never been recapped (preventive maintenance)

Tools and materials

• Temperature-controlled soldering station (recommended 350–370°C for lead-free, 300–330°C for leaded solder)
• Solder wick (desolder braid) and/or desoldering pump
• High-quality replacement electrolytic capacitors (Panasonic FR-series or equivalent low-ESR types recommended)
• Isopropyl alcohol (99%+) and lint-free swabs for cleaning
• Digital multimeter with capacitance measurement
• Flux (no-clean type)
• PCB holder/third hand tool

Motherboard capacitor list

C100167-001 Rev.B / CA2000019

This is the primary Mega ST motherboard. A pre-assembled capacitor kit is available from Console5.

Designator	Capacitance	Voltage	Notes
C3	10 µF	16V
C4	4,700 µF	16V	Main bulk decoupling capacitor
C8	100 µF	16V
C22	4.7 µF	35V
C24	4.7 µF	35V
C25	4.7 µF	35V
C34	47 µF	16V
C35	47 µF	16V
C37	100 µF	16V
C49	100 µF	16V
C51	100 µF	16V
C140	1,000 µF	16V

Total: 12 electrolytic capacitors on motherboard.

Important notes

• All capacitors are radial through-hole types.
• Replacement capacitors should match or exceed the original voltage rating. Using a higher voltage rating is acceptable and may improve longevity. Do not use a lower voltage rating.
• Low-ESR capacitors (such as the Panasonic FR-series) are recommended for all positions, particularly C4 and C140 which are on the main power rails.
• Observe polarity markings carefully. Electrolytic capacitors installed with reversed polarity can explode violently. The negative (−) stripe on the capacitor body corresponds to the negative pad on the PCB. Double-check against original markings before soldering.

Power supply capacitor list

Mitsumi SR98 / 68-4242A (110V–120V, North American market)

A pre-assembled capacitor kit is available from Console5.

Designator	Capacitance	Voltage	Recommended replacement
C104	220 µF	200V	220 µF 200V
C201	2,200 µF	16V	4,700 µF 16V
C202	2,200 µF	10V	4,700 µF 16V
C203	1,000 µF	16V	1,500 µF 16V
C204	2,200 µF	10V	4,700 µF 16V
C205	1,000 µF	10V	1,500 µF 16V
C206	1 µF	50V	1 µF 50V

Mitsumi SR98 / SR118 / 68-4231A (220–240V, European/UK market)

A pre-assembled capacitor kit is available from Console5.

Designator	Capacitance	Voltage	Recommended replacement
C104	47 µF	400V	68 µF 400V
C201	2,200 µF	10V	4,700 µF 16V
C202	2,200 µF	16V	4,700 µF 16V
C203	2,200 µF	10V	4,700 µF 16V
C204	1,000 µF	16V	1,500 µF 16V
C205	1,000 µF	10V	1,500 µF 16V
C206	1 µF	50V	1 µF 50V

Note: The recommended replacement values above follow the exxos upgrade specifications, which use higher capacitance values than the originals for improved regulation and peak current handling.

Recapping procedure

PSU recapping (safety-critical)

1. Disconnect the PSU from the computer and mains supply. Allow capacitors to discharge for at least 24 hours.
2. Verify discharge with a multimeter set to DC voltage. Check across each large capacitor's terminals. Any reading above 1V indicates residual charge — do not proceed until fully discharged.
3. Photograph the original capacitor placement, noting polarity markings and designator labels on the PCB.
4. Desolder each capacitor using solder wick, desoldering pump, or desoldering iron. Work carefully to avoid lifting PCB pads.
5. Clean each pad with IPA and inspect for trace damage.
6. Install replacement capacitors, verifying correct polarity for each. The negative stripe on the capacitor body must align with the negative marking on the PCB.
7. Inspect all solder joints under magnification. Ensure no solder bridges exist.
8. Test the PSU independently using dummy load resistors (2.2Ω 17W on +5V rail, 10Ω 17W on +12V rail) before connecting to the computer. Measure all output voltages.

Motherboard recapping

1. Remove the motherboard from the case and RF shielding.
2. Photograph original capacitor positions and polarity.
3. Desolder each electrolytic capacitor. Take particular care with C4 (4,700 µF) and C140 (1,000 µF) — these are large capacitors with substantial thermal mass and require longer heating times.
4. Clean pads with IPA.
5. Install replacements with correct polarity. For positions C22, C24, and C25 (4.7 µF 35V), verify these are not non-polarised types in your specific board revision — some revisions use non-polarised (NP/bipolar) capacitors in audio-related circuits.
6. Inspect all joints. Verify no solder bridges to adjacent traces.
7. Reassemble and test.

Post-recap testing

After recapping, verify the following:

1. PSU voltages — All rails within specification (+5V: 4.95–5.10V; +12V: 11.8–12.2V).
2. Boot to GEM desktop — System should boot cleanly without requiring reset button.
3. Floppy operation — Read and write to a known-good floppy disk.
4. Video quality — Display should be stable, with no noise, dimming during floppy access, or flickering.
5. Audio — Test YM2149 output with a sound-producing application.
6. MIDI — If used, verify MIDI In/Out with an external device.
7. Hard disk — If an ACSI hard disk is connected, verify it boots and operates correctly.

Capacitor sourcing

Pre-assembled capacitor kits are available from:

• Console5 — console5.com (motherboard and PSU kits)
• exxos — exxosforum.co.uk/atari/store2/ (PSU recap kits with upgraded values)

Individual components can be sourced from:

• Mouser Electronics — mouser.com
• Farnell/element14 — farnell.com
• Digi-Key — digikey.com

Recommended brands: Panasonic (FR-series), Nichicon (HE/HV series), Rubycon (ZLH series). Avoid unbranded or very low-cost capacitors from marketplaces, as counterfeit and substandard components are common.

See also

• Atari Mega ST
• Atari Mega ST General Maintenance
• Atari Mega ST Troubleshooting Guide