Replacing RISC PSUs with ATX

Overview

Power Supplies for RISC systems like the RS/6000, SPARC, Alpha, etc have an increasing risk of failure due to leaking capacitors. The most common fix is to replace the capacitors in the PSU before they leak and destroy the logic board. This does require good re-soldering skills and being comfortable working with high(110V or 220V) voltage components.

For those who are not comfortable doing that or if the PSU is beyond repair, it is possible to replace the PSU with a modern PC power supply like an ATX, ITX, or STX power supply.

Wikipedia - ATX for a more detailed list of voltages and Molex connector parts.

For this work, it highly recommended that you use an ATX Molex 24pin extender cable to make these changes. This allows you cheaply fix any mistakes and not damage the original supply.

Findings

Each make and model will have its own unique connectors. For example, the RS/6000 Model 43P-150 uses most of the AT and ATX connectors, but needs specific voltages on various pin. Using a electrical pin extractor tool is when you are not splicing and soldering wires together.

Removing wires from a pre-exiting Molex connector is difficult and it is likely that the barbs at the end of the connector will be crushed. This prevents the wire from being re-used because it will come loose from the socket. If possible, it is best to purchase a new 24pin female receptacle and male plug as well the appropriate male plugs for the motherboard if you cannot remove the wires cleanly.

There are two alternatives to replacing the pre-existing power supply connection

1. Cut the require wires from the ATX extender cable and original PSU cable and soldering them together.

2. Creating a new adapter by using either crimping new Molex wires or using pre-crimped ones. This option is good if the connector on the motherboard is a Molex connector.,

The connectors themselves will have the pin numbers written on the plug to make it easier to determine which one is which

The connector pin numbering should be as follows. With the latch facing up it goes

---

|--------|----|-----|

12 11 10 9 8 7

6 5 4 3 2 1

|------------------|

The ATX PWK_OK line should be used for when the motherboard need a "Power Sense" line

The ATX PSU_ON line should be connected to ground. I joined a spare ground(COM) line to the PS_ON line.

Some people will normally wire the PSU_ON and its associated ground through a switch so that they can turn the PSU on and off.

Some PSUs have their own power switch on the PSU, which eliminates the need for that other switch.

Consult the schematics and service manuals for the machine in question for power pin layouts.

Example: PS for sun4m lunchboxes

Example: PS for sun4c lunchboxes

Example: PS for sun4c lunchboxes

Example: PS for sun4c pizzaboxes

Example: PS for SGI O2s

Example PS for the IBM RS/6000 Model 43P-150