Atari Mega STE Power Supply Repair

The primary side of the PSM-5341 on Phihong's schematic, from a copy marked "Mega STE": fuse F1, bridge D1–D4, reservoir C6, the UC3844 controller IC1 and the MOSFET Q1. The handwritten notes are voltages measured on a working supply.

The Atari Mega STE runs from an internal switching power supply made by Phihong. exxos has documented two models in the machine, the PSM-5341 and the PSM-8012-2; the PSM-5341 is also the Atari TT030's supply. Phihong's specification and schematic for the PSM-5341 survive, and the procedure below works from them.

It is a flyback converter. A UC3844 controller, IC1, switches the MOSFET Q1 across the transformer; the +5 V output is regulated by a TL431 reference and an optocoupler feeding back to IC1, and −5 V comes from a 7905 regulator, IC2. The supply also gives a Power Good signal and drives the fan.

Symptoms

Supply faults

Symptom	Where to look	Source
Dead: no fan, no LED	Fuse F1, the primary side and the controller; see below	Phihong schematic
Supply starts, then cuts out, or ticks	Overcurrent protection: a short or overload on an output	Phihong specification
+5 V low, picture dims on drive access, random crashes	Aged output capacitors	exxos
+5 V out of range after a recap	The +5 V setting, R23 and R24	Phihong schematic
Flickering desktop background	The optocoupler; reported fixed by fitting a PC111	Pacman, exxos forum
−12 V or −5 V missing	That output's rectifier, filter or the 7905	Phihong schematic

Sources: Phihong's documents, exxos and the exxos forum. A missing −12 V stops the serial ports and a missing −5 V the DMA sound, while the rest of the machine works; see the Atari Mega STE Troubleshooting Guide.

Safety

Warning: the supply's primary side runs from the mains, and the reservoir C6 (120 µF 400 V) charges to about 300 V DC; it holds that charge after the supply is unplugged. The handwritten note on the schematic copy reads "300V Gleichspannung" (300 V DC) at C6. Pacman leaves the supply unplugged for 24 hours before opening it and then measures every capacitor, especially the two 400 V ones, before touching anything.

Never run the supply without a load. Phihong specifies minimum loads of 2 A on +5 V and 0.2 A on +12 V.

Tools and parts

• A multimeter and, for ripple, an oscilloscope.
• Dummy loads: a 2.2 Ω 17 W resistor for +5 V and a 10 Ω 17 W resistor for +12 V, as used on the exxos forum. Run the supply for less than 30 seconds at a time, because the resistors get very hot.
• For a recap, the capacitor list on Atari Mega STE Capacitor Replacement Guide#Power supply.
• A PC111 optocoupler if the flicker fault applies.

PSM-5341 outputs (Phihong specification)

Output	Minimum load	Maximum load	Regulation	Ripple and noise
+5 V	2 A	7 A	+5 % / −4 %	100 mV peak to peak
+12 V	0.2 A	2 A (2.5 A peak for 10 s at power-up)	±5 %	120 mV peak to peak
−12 V	0 A	0.3 A	±10 %	120 mV peak to peak
−5 V	0 A	0.4 A	±5 %	120 mV peak to peak

Phihong also specifies 100–240 V AC input at 50–60 Hz, 64.6 W total output, Power Good coming on between 100 ms and 500 ms after all outputs are within limits, a TTL power-fail signal going low at least 1 ms before +5 V falls below 4.8 V, and short-circuit protection on every output that switches the supply off within 20 ms of an overload.

12-way main connector (Phihong wire list)

Pin	Colour	Signal
1	Orange	Power Good
2	–	Open
3	Yellow	+12 V
4	Blue	−12 V
5, 6, 7, 8	Black	Ground
9	White	−5 V
10, 11, 12	Red	+5 V

The two four-way drive connectors carry +12 V (yellow), two grounds (black) and +5 V (red).

Procedure

Get to the supply

1. Switch off and disconnect every cable from the sides and back. Atari's upgrade manuals for the Mega STE start by removing the hard disk, if one is fitted, and the accessory and top covers.
2. Unplug the supply's main connector from J201 on the main board and its drive connectors, and lift the supply out, taking care not to knock the components close to it.
3. Identify the model from the label: PSM-5341 or PSM-8012-2.

Bench test

1. Connect the 2.2 Ω load between +5 V (red) and ground (black) and the 10 Ω load between +12 V (yellow) and ground.
2. Switch on from a switched outlet at a distance, wait about two seconds, read one output, and switch off within 30 seconds. Repeat for each output.
3. Compare the readings with the regulation limits in the table above. Power Good (orange) should read about +5 V.

Open the supply

1. Leave the supply unplugged, then remove the screws of its metal housing and lift the top off without touching the components.
2. Remove the circuit board's screws and turn the board over.
3. Measure across C6 and C7 (the 400 V capacitors) and every other electrolytic; continue only when they read close to zero.

The outputs and regulation on Phihong's schematic: the +5 V divider R17, R18, R23 and R24 around the TL431 SHR1, the optocoupler PC111, the 7905 for −5 V and the Power Good circuit. The factory alignment table is at the top.

Dead supply: follow the primary side

Work along Phihong's schematic from the mains inlet, with the supply unplugged and C6 discharged:

1. Check the fuse F1 (2 A, 250 V). If it has blown, look for a short further on before replacing it.
2. Check PT (10SP005L) in the mains feed and the line filter NF.
3. Check the bridge diodes D1–D4 (1N5395).
4. Check C6 (120 µF 400 V) and C7 (10 µF 400 V).
5. Check the start-up resistors R1 and R2 (50 kΩ, 2 W), which feed the controller's supply pin and its capacitor C8 (100 µF 25 V) from the rectified mains.
6. Check the MOSFET Q1 for a short between drain and source, and the current-sense resistor R13 (0.33 Ω, 2 W) in its source. Phihong's schematic shows an MTP6N60 (the clearer of the two copies; the Mega STE copy is hard to read at this point); exxos found an SSP6N60 (600 V, 6 A) in the supply he examined.
7. Power up again into the dummy loads and measure the controller IC1 (UC3844) against pin 5 (ground). The copy of the schematic carries these readings from a working supply: pin 7, 12.8 V; pin 2, 2.5 V; pin 1, 2.24 V; pin 3, 0.03 V; pin 4, 1.95 V; pin 8, 4.98 V.

Recap

Replace the electrolytics as listed on the Atari Mega STE Capacitor Replacement Guide, one at a time, noting each part's position, value and polarity first. exxos's kit raises several values; C8 is replaced by a taller 220 µF 50 V part that must be mounted higher because a resistor is in the way.

Set +5 V

Phihong set the +5 V output at the factory by fitting one or both of R23 (68 kΩ) and R24 (91 kΩ) in the divider of the TL431 reference. Its alignment table, for 220 V AC input and minimum load, is:

+5 V factory alignment (Phihong schematic)

+5 V reading	R23	R24
5.00–5.15 V	Fitted	Fitted
Below 5 V	Fitted	Removed
Above 5.15 V	Removed	Fitted

Optocoupler

Pacman reported a case, documented by Michael Ruge, of a Mega STE whose desktop background flickered until the optocoupler (marked PC on the board) was changed from a CNY17-3 to a PC111; recapping alone had not cured it in that case. exxos had not seen the fault and pointed out that a faster optocoupler does not always improve regulation; the supply he examined already had a PC111 fitted. Phihong's schematic specifies a PC111.

Refit

Refit the board and the housing, repeat the bench test, then refit the supply and reconnect J201 and the drive connectors.

Testing

• With the machine running, measure all four rails and Power Good at J201 against the limits above.
• On an oscilloscope, +5 V ripple and noise should be within Phihong's 100 mV. exxos measured 176 mV on an unrecapped supply in a Mega STE and 116 mV after recapping, and 2.90 A drawn on +5 V and 0.11 A on +12 V without a hard disk.
• Watch for the picture dimming when the floppy and hard disk start.

Related pages

• Atari Mega STE
• Atari Mega STE Capacitor Replacement Guide
• Atari Mega STE Troubleshooting Guide
• Atari Mega STE General Maintenance
• Atari TT030 Power Supply Repair, the same PSM-5341 in the TT030