Atari Mega ST Capacitor Replacement Guide

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.

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.

Signs that a recap is due

• The machine needs the reset button, or several power cycles, before it starts.
• The picture dims or wavers when the floppy drive starts.
• Random crashes, floppy errors or hard disk errors that come and go with temperature.
• A ticking or whining supply, a bulging can, or residue under a capacitor.

These symptoms have other causes too; measure the +5 V and +12 V rails under load before replacing anything. See Capacitor Failure Symptoms.

Main board

The service manual's silkscreen is of PCB C100167 Rev 5.0 (assembly CA200019). 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.

Mega ST main-board electrolytic capacitors (Atari parts list)

Location	Capacitance	Voltage	Lead style	Notes
C3	10 µF	16 V	Axial
C4	4,700 µF	16 V	Radial	Large can beside the power connector J1
C8, C37, C51	100 µF	16 V	Axial
C22, C24, C25	4.7 µF	25 V	Axial	Console5 supplies 35 V
C34, C35	47 µF	16 V	Radial
C49	100 µF	16 V	Radial	Printed "1001 µF" in Atari's list; Console5 gives 100 µF
C140	1,000 µF	16 V	Axial

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.

Power supply (ASP34-1)

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. 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.

ASP34-1 electrolytic capacitors (Atari schematic)

Location	Capacitance	Voltage	Side	Notes
C4, C5	47 µF	250 V	Primary	In series across the rectified mains. Replace both, with the same value
C7	330 µF	25 V	Primary
C8	1 µF	16 V	Primary
C9	100 µF	25 V	Primary
C12	2,200 µF	25 V	Secondary	+5 V output
C13	4,700 µF	16 V	Secondary	+5 V output
C14, C15	330 µF	25 V	Secondary	+12 V output, either side of L2

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. 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. 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.

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.

Replacement procedure

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.

1. 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.
2. 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.
3. 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.
4. Photograph both sides of each board and note the polarity of every electrolytic before removing it.
5. Replace one capacitor at a time. Match capacitance, match or exceed the voltage rating, and use 105 °C parts in the supply.
6. 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.
7. 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.

After recapping

1. Check the supply connector for shorts to ground before first power-up.
2. 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. Atari rates the supply at 3 A on +5 V and 1 A on +12 V.
3. Watch the screen while the floppy drive starts: a picture that dims on drive access points back to the supply.
4. If a diagnostic cartridge is available, run the full test set. See Atari Mega ST Troubleshooting Guide.

Related pages

• Atari Mega ST
• Atari Mega ST General Maintenance
• Atari Mega ST Troubleshooting Guide
• Capacitor Failure Symptoms
• Recommended Tools