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Atari Mega ST Capacitor Replacement Guide: Difference between revisions

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Rebuild from Atari's Mega service manual parts list and ASP34-1 schematic: main-board values checked (4.7 uF parts are 25 V axial), supply list replaced (the Mitsumi SR98/SR118 tables were 520/1040ST supplies); remove uncited rail targets; add board image and categories
 
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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.<ref name="console5">{{cite web |url=https://console5.com/store/atari-mega-st-computer-cap-kit-c100167-main-pcb.html |title=Atari Mega ST Computer Cap Kit — C100167 Main PCB |publisher=Console5 |access-date=2026-03-27}}</ref>
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{| class="wikitable styled-table"
[[File:Atari Mega ST2 board layout C100167.png|thumb|right|360px|Mega ST2 main board C100167, labelled in German (atari-computermuseum.de). The power supply connector and the large reservoir capacitor C4 are at the right-hand edge.]]
|-
! 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.'''
The '''[[Atari Mega ST]]''' has twelve electrolytic capacitors on the main board and nine in its internal ASP34-1 power supply. Both lists below come from Atari's own ''Mega Service Manual'', which prints a parts list for the main board and a schematic for the supply with a value against every capacitor.<ref name="sm-parts">Atari Corporation, ''Mega Service Manual'' (Mega 2 and Mega 4), undated, section 7, "Parts list", main-board capacitors (PDF p. 62). Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref><ref name="sm-psu">Atari Corporation, ''Mega Service Manual'', section 8: "Schematic diagram (ASP34-1), Computer", Atari (Japan) Corporation, production release A (PDF p. 73), and "Power supply assy ASP34-1 (117V)" wiring diagram, sheet 5 of 5 (PDF p. 81). Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref>


=== Important notes ===
== Signs that a recap is due ==


* All capacitors are radial through-hole types.
* The machine needs the reset button, or several power cycles, before it starts.
* 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.
* The picture dims or wavers when the floppy drive starts.
* 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.
* Random crashes, floppy errors or hard disk errors that come and go with temperature.
* '''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.
* A ticking or whining supply, a bulging can, or residue under a capacitor.


== Power supply capacitor list ==
These symptoms have other causes too; measure the +5 V and +12 V rails under load before replacing anything. See [[Capacitor Failure Symptoms]].


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


A pre-assembled capacitor kit is available from Console5.<ref name="console5psu">{{cite web |url=https://console5.com/store/atari-1040-computer-power-supply-cap-kit-mitsumi-68-4242a-1040stf.html |title=Atari 1040 Computer Power Supply Cap Kit — Mitsumi 68-4242A |publisher=Console5 |access-date=2026-03-27}}</ref>
The service manual's silkscreen is of PCB C100167 Rev 5.0 (assembly CA200019).<ref name="sm-silk">Atari Corporation, ''Mega Service Manual'', section 8, main-board silkscreen (PDF p. 77). Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref> Console5 sells a main-board kit for boards marked C100167-001 Rev B and CA200019 whose list matches Atari's positions and values, except that Console5 supplies 35 V parts for the 4.7 µF positions.<ref name="c5wiki">Console5 TechWiki, [https://wiki.console5.com/wiki/Atari_ST "Atari ST"], section "Mega ST Cap Lists: C100167-001 REV.B / CA2000019" and section "ASTEC ASP34 / ASPF34-1", page last edited 20 May 2024. Secondary source.</ref>


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{| class="wikitable styled-table" style="width:100%; text-align:center;"
{| class="wikitable styled-table"
|+'''Mega ST main-board electrolytic capacitors (Atari parts list)'''<ref name="sm-parts" />
! Location !! Capacitance !! Voltage !! Lead style !! Notes
|-
|-
! Designator !! Capacitance !! Voltage !! Recommended replacement
| C3 || 10 µF || 16 V || Axial ||
|-
|-
| C104 || 220 µF || 200V || 220 µF 200V
| C4 || 4,700 µF || 16 V || Radial || Large can beside the power connector J1
|-
|-
| C201 || 2,200 µF || 16V || 4,700 µF 16V
| C8, C37, C51 || 100 µF || 16 V || Axial ||
|-
|-
| C202 || 2,200 µF || 10V || 4,700 µF 16V
| C22, C24, C25 || 4.7 µF || 25 V || Axial || Console5 supplies 35 V
|-
|-
| C203 || 1,000 µF || 16V || 1,500 µF 16V
| C34, C35 || 47 µF || 16 V || Radial ||
|-
|-
| C204 || 2,200 µF || 10V || 4,700 µF 16V
| C49 || 100 µF || 16 V || Radial || Printed "1001 µF" in Atari's list; Console5 gives 100 µF
|-
|-
| C205 || 1,000 µF || 10V || 1,500 µF 16V
| C140 || 1,000 µF || 16 V || Axial ||
|-
| C206 || 1 µF || 50V || 1 µF 50V
|}
|}


=== Mitsumi SR98 / SR118 / 68-4231A (220–240V, European/UK market) ===
Radial replacements can be fitted in the axial positions with one lead folded back, provided the part lies flat enough for the metal shield to go back on.
 
Leave these alone: C53 is a 5–30 pF trimmer that sets the real-time clock frequency, and the remaining capacitors in the list (C1–C2, C5–C7, C9–C21, C80–C95 and others) are ceramic parts of 30 pF to 0.47 µF, which do not dry out.<ref name="sm-parts" /><ref name="sm-rtc">Atari Corporation, ''Mega Service Manual'', section 2, "Real Time Clock with Battery Backup", p. 2.7: the 32.768 kHz oscillator is adjusted by a trimmer capacitor to give 16.384 kHz at CLKOUT. Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref>
 
== Power supply (ASP34-1) ==


A pre-assembled capacitor kit is available from Console5.<ref name="console5psueu">{{cite web |url=https://console5.com/store/atari-mega-st-computer-power-supply-cap-kit-mitsumi-sr118.html |title=Atari Mega ST Computer Power Supply Cap Kit — Mitsumi SR118 |publisher=Console5 |access-date=2026-03-27}}</ref>
The Mega ST's supply is an internal unit with its own fan. Atari's schematic is of the ASP34-1, an Atari (Japan) design built around a BU508 switching transistor, with a TL431 reference and a PC817 optocoupler in the feedback loop. It has no adjustment potentiometer. The same drawing is used for 110 V and 220 V supplies: a wire link, W1, is fitted across the midpoint of the two reservoir capacitors for 110 V operation and left out for 220 V.<ref name="sm-psu" /> atari-computermuseum.de records four versions in Atari's documentation, ASPF34-1 to ASPF34-4, two of them with extra power outlets for a hard disk.<ref name="acm-megast">"Desktop-Modelle: Der Atari Mega ST", atari-computermuseum.de, last edited 13 July 2024, https://www.atari-computermuseum.de/megast.htm. Secondary source.</ref>


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{| class="wikitable styled-table" style="width:100%; text-align:center;"
{| class="wikitable styled-table"
|+'''ASP34-1 electrolytic capacitors (Atari schematic)'''<ref name="sm-psu" />
! Location !! Capacitance !! Voltage !! Side !! Notes
|-
|-
! Designator !! Capacitance !! Voltage !! Recommended replacement
| C4, C5 || 47 µF || 250 V || Primary || In series across the rectified mains. Replace both, with the same value
|-
|-
| C104 || 47 µF || 400V || 68 µF 400V
| C7 || 330 µF || 25 V || Primary ||
|-
|-
| C201 || 2,200 µF || 10V || 4,700 µF 16V
| C8 || 1 µF || 16 V || Primary ||
|-
|-
| C202 || 2,200 µF || 16V || 4,700 µF 16V
| C9 || 100 µF || 25 V || Primary ||
|-
|-
| C203 || 2,200 µF || 10V || 4,700 µF 16V
| C12 || 2,200 µF || 25 V || Secondary || +5 V output
|-
|-
| C204 || 1,000 µF || 16V || 1,500 µF 16V
| C13 || 4,700 µF || 16 V || Secondary || +5 V output
|-
|-
| C205 || 1,000 µF || 10V || 1,500 µF 16V
| C14, C15 || 330 µF || 25 V || Secondary || +12 V output, either side of L2
|-
| 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.<ref name="exxospsu">{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=Atari PSU Repair — The LaST Upgrade |publisher=exxos |access-date=2026-03-27}}</ref>
Console5's kit for the "ASTEC ASP34 / ASPF34-1" lists the same positions and values except C8, which it omits, and C12, which it gives as 16 V; it supplies 35 V parts for C7, C14 and C15.<ref name="c5wiki" /> exxos's notes on an ASP34-2 describe the same set (two 250 V reservoirs of 33 or 47 µF, three 330 µF and one each of 4,700 µF and 2,200 µF) and fitted 47 or 68 µF reservoirs, 470 or 820 µF 35 V parts and a second 4,700 µF in place of the 2,200 µF.<ref name="exxos-psu">exxos, [https://www.exxosforum.co.uk/atari/last/psu/index.htm "ATARI PSU REPAIR"], The LaST Upgrade, part 6, section "ASP34-2", 2012, updated 2026. Community source.</ref> The supply's other capacitors (C1, C1A, C2, C3, C6, C10, C11) are film and ceramic parts, and C1, C1A and C3 are mains-rated safety capacitors.
 
== Recapping procedure ==
 
=== PSU recapping (safety-critical) ===
 
{{Warning|'''The power supply contains mains voltage components that can cause fatal electric shock.''' Only qualified individuals should perform PSU repairs. Capacitors inside the PSU can retain lethal charge even when disconnected from mains. Allow at least 24 hours for capacitors to discharge before opening the PSU.}}
 
# '''Disconnect''' the PSU from the computer and mains supply. Allow capacitors to discharge for at least 24 hours.
# '''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.
# '''Photograph''' the original capacitor placement, noting polarity markings and designator labels on the PCB.
# '''Desolder''' each capacitor using solder wick, desoldering pump, or desoldering iron. Work carefully to avoid lifting PCB pads.
# '''Clean''' each pad with IPA and inspect for trace damage.
# '''Install''' replacement capacitors, verifying correct polarity for each. The negative stripe on the capacitor body must align with the negative marking on the PCB.
# '''Inspect''' all solder joints under magnification. Ensure no solder bridges exist.
# '''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 ===
 
# '''Remove''' the motherboard from the case and RF shielding.
# '''Photograph''' original capacitor positions and polarity.
# '''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.
# '''Clean''' pads with IPA.
# '''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.
# '''Inspect''' all joints. Verify no solder bridges to adjacent traces.
# '''Reassemble''' and test.
 
== Post-recap testing ==


After recapping, verify the following:
Console5's Mitsumi SR98/SR118 kits are for the supplies used in the 520ST and 1040ST family. Read the label on the supply before ordering parts.<ref name="c5wiki" />


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


== Capacitor sourcing ==
'''Warning: the supply's primary side carries rectified mains, over 300 V DC on a 230 V supply, and C4 and C5 hold that charge after the plug is pulled.'''


Pre-assembled capacitor kits are available from:
# Unplug the machine. Remove the nine screws in the square holes in the base, and the three in the round holes if the board or drive is coming out. Lift the top cover at the back and unplug the clock battery lead from under its left rear corner before taking the cover off.<ref name="sm-disasm">Atari Corporation, ''Mega Service Manual'', section 4, "Disassembly/assembly", pp. 4.1–4.3. Hosted as [[:File:Atari Mega ST Service Manual.pdf]].</ref>
* '''Console5''' — [https://console5.com/ console5.com] (motherboard and PSU kits)
# Straighten the six twist tabs and lift off the upper shield. Remove the two screws at the front corners of the supply, unplug its harness from the right front corner and lift it out.<ref name="sm-disasm" />
* '''exxos''' — [https://www.exxosforum.co.uk/atari/store2/ exxosforum.co.uk/atari/store2/] (PSU recap kits with upgraded values)
# With the supply on the bench, measure across C4 and across C5 with a meter on the DC range. Do not touch the primary side until both read close to zero.
# Photograph both sides of each board and note the polarity of every electrolytic before removing it.
# Replace one capacitor at a time. Match capacitance, match or exceed the voltage rating, and use 105 °C parts in the supply.
# To reach the main board, remove the six studs holding the I/O shield to the case, lift the board assembly out, straighten the six twist tabs of the lower shield and lift the board off it.<ref name="sm-disasm" />
# Reassemble with the insulation panel under the board. Atari's own advice is to retest the machine once it is back in its case, to catch shorts to the shield.<ref name="sm-disasm" />


Individual components can be sourced from:
== After recapping ==
* '''Mouser Electronics''' — [https://mouser.com mouser.com]
* '''Farnell/element14''' — [https://farnell.com farnell.com]
* '''Digi-Key''' — [https://digikey.com 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.
# Check the supply connector for shorts to ground before first power-up.
# Power up with the board connected and measure +5 V and +12 V at the supply connector: +5 V on pins 1 and 2, ground on pins 3 to 5, +12 V on pin 6.<ref name="sm-psu" /> Atari rates the supply at 3 A on +5 V and 1 A on +12 V.<ref name="om-spec">Atari Corporation, ''Computer MEGA ST Owner's Manual'', 1987, appendix C, "MEGA ST Computer Specifications", pp. 109–110. Hosted as [[:File:Atari Mega ST Owner's Manual (1987).pdf]].</ref>
# Watch the screen while the floppy drive starts: a picture that dims on drive access points back to the supply.
# If a diagnostic cartridge is available, run the full test set. See [[Atari Mega ST Troubleshooting Guide]].


== See also ==
== Related pages ==


* [[Atari Mega ST]]
* [[Atari Mega ST]]
* [[Atari Mega ST General Maintenance]]
* [[Atari Mega ST General Maintenance]]
* [[Atari Mega ST Troubleshooting Guide]]
* [[Atari Mega ST Troubleshooting Guide]]
* [[Capacitor Failure Symptoms]]
* [[Recommended Tools]]


== References ==
== References ==
<references />
<references />


{{Navbox-AtariVintageComputers}}
{{Navbox-AtariVintageComputers|state=collapsed}}


[[Category:Atari Systems]]
[[Category:Capacitor Replacement Guides]]
[[Category:Atari Capacitor Replacement Guides]]
[[Category:Atari Capacitor Replacement Guides]]
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