Atari Mega STE Capacitor Replacement Guide

Mega STE main board C301564, labelled in German (atari-computermuseum.de). The supply connector (Netzteil) is at the top left; the DMA sound circuit is at the far left.

The Atari Mega STE has electrolytic capacitors in its internal power supply and about 25 on the main board, most of them in the DMA sound circuit. The supply is the part that fails with age, and exxos's list of recommended fixes puts recapping it first for every Atari ST.

Signs that a recap is due

• The picture dims or wavers when the floppy or hard disk starts.
• Random crashes, especially at 16 MHz with the cache on, or a machine that will not start with a full 4 MB fitted.
• A ticking or whining supply, a bulging can, or residue under a capacitor.

Measure the rails under load before replacing anything. See Capacitor Failure Symptoms and Atari Mega STE Troubleshooting Guide.

Power supply

exxos documents two Phihong supplies in the Mega STE: the PSM-5341, which is also the Atari TT030's supply, and the PSM-8012-2. The list below is for the PSM-5341; read the label on the supply before ordering. exxos has not published a capacitor list for the PSM-8012-2.

Phihong PSM-5341 electrolytic capacitors

Location	Original	exxos kit value	Notes
C6	120 µF 400 V	120 µF or 150 µF 400 V	Primary reservoir
C7	10 µF 400 V	10 µF 400 V or 450 V	Primary side
C8	100 µF 25 V	220 µF 50 V	Mount higher than the original; a resistor is in the way
C13	4,700 µF 16 V	4,700 µF 16 V
C14	2,200 µF 10 V	4,700 µF 16 V
C15, C16	1,000 µF 16 V	1,500 µF 16 V
C17, C18	680 µF	1,500 µF 16 V	Console5 gives 16 V, exxos 10 V for the originals
C19	100 µF 10 V	100 µF 10 V or 16 V
C20	3.3 µF 50 V	10 µF 50 V
C23	22 µF 25 V	10 µF 50 V (later kits 22 µF)
C24	220 µF 25 V	220 µF 50 V
C26	4.7 µF 50 V	10 µF 50 V

Sources: the original values and locations are from Console5's list, a community list read from supplies; exxos's page gives the same set of values without locations, with its replacement values; and a step-by-step recap on the exxos forum gives the replacement values against each location, with part numbers and sizes. C6 is 18 mm across, 30 mm high, with 17.5 mm lead spacing in the kit that forum post used.

On a Mega STE without a hard disk, exxos measured the supply delivering 2.90 A on +5 V and 0.11 A on +12 V, with 176 mV of noise on +5 V before the recap and 116 mV after.

Main board

The values below are read from Atari's revision 2 schematic, which gives capacitance but no voltage ratings. Production boards may differ from revision 2, so check each fitted part before ordering, and never fit a lower voltage rating than the original. The board's highest supply is +12 V; the DMA sound circuit runs from its own 8.2 V regulator.

Mega STE main-board electrolytic capacitors (Rev 2 schematic)

Location	Value	Circuit	Sheet
C208, C218	10 µF	Timing capacitors for the power-on reset and reset button (LM556, U201)	2
C227	470 µF	+5 V at the power connector J201	2
C230	100 µF	+5 V to the cartridge port, after fuse F21	2
C402	100 µF	Clock supply (VBAT)	4
C509	47 µF	Audio to the monitor socket	5
C601	100 µF	+5 V to the sound DACs	6
C611	100 µF	−5 V to the sound DACs; positive lead to ground	6
C631	100 µF	8.2 V regulator output (U609)	6
C613, C614, C626, C627, C633, C636, C639, C641, C643, C644, C646, C647, C648	47 µF	DMA sound filters, mixer and outputs	6
C640, C645	10 µF	LMC1992 mixer	6
C702	100 µF	+5 V at the SIMM sockets	7
C511	470 µF	RF modulator (modulator machines only)	5
C512, C518	100 µF	RF modulator (modulator machines only)	5
C514, C519, C521	15 µF	Colour encoder inputs (modulator machines only)	5
C528	1 µF	Colour encoder (modulator machines only)	5

Console5's list with locations C100 to C433, including four non-polarised 4.7 µF parts, is for the 520STE/1040STE main board (CA300779), not the Mega STE.

Replacement procedure

Warning: the supply's primary side runs at rectified mains voltage, over 300 V DC on a 230 V supply, and C6 holds that charge after the plug is pulled.

1. Unplug the machine. Remove the hard disk, if one is fitted, and the top cover.
2. Unplug the supply from J201 and lift it out, taking care not to knock components on the main board beside it.
3. Before opening the supply, test it into dummy loads and note the readings: a 2.2 Ω resistor rated 17 W between +5 V and ground, and a 10 Ω 17 W resistor between +12 V and ground, run for under 30 seconds at a time.
4. Open the supply and measure across C6 and C7 with a meter on the DC range. Do not touch the primary side until both read close to zero.
5. Photograph the board and note the polarity of every capacitor before removing it. Replace one at a time.
6. Retest into the dummy loads and compare with the earlier readings before refitting the supply.

For the main board, remove the board from the case, replace one capacitor at a time, and observe the reversed orientation of C611 on the −5 V rail.

After recapping

The supply's output leads are colour-coded: red +5 V, yellow +12 V, blue −12 V, white −5 V, orange Power Good (about +5 V when the supply is healthy) and black ground. At the main board, J201 carries Power Good on pin 1, +5 V on pins 2 and 10–12, +12 V on pin 3, −12 V on pin 4, ground on pins 5–8 and −5 V on pin 9.

1. Check all four rails and Power Good with the board connected.
2. Start the machine and run it at 16 MHz with the cache on for an extended period.
3. Watch the picture while the floppy and hard disk start.
4. Play DMA sound through the RCA outputs if any audio capacitors were replaced.

Related pages

• Atari Mega STE
• Atari Mega STE General Maintenance
• Atari Mega STE Troubleshooting Guide
• Atari Mega STE Schematic
• Atari TT Capacitor Replacement Guide
• Capacitor Failure Symptoms