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Fuel PSU repair

From Higher Intellect Vintage Wiki

Buy one of these:

and one of these:

and one of these:

The wires are easily removed from the molex using two paper staples that has been straightened out. Moved the necessary wires around according to the scheme below. Keep the female side as it were and moved the wires in male molex. The left hand side in the table is thus the female, ATX side and the right is the male, WTX side.

Using ATX pinout and WTX pinout(look at the alternate one) I made this mapping. I also followed the wires back into the WTX PSU since the purpose of the wires were printed on the circuit board end.

1 -- 1    | +3.3V
2 -- 2    | +3.3V
3 -- 4    | GND
4 --      | NC
5 -- 5    | GND
6 -- 6    | +5V
7 -- 8    | GND
8 -- 11   | PWR_OK
9 -- 7    | 5VSB
10 -- 21  | +12V
11 -- 22  | +12V
12 -- 14  | +3.3V
13 -- 13  | +3.3V
14 -- 20  | -12V
15 -- 15  | GND
16 -- 23  | PS_ON
17 -- 16  | GND
18 -- 9   | GND
19 -- 10  | GND
20 --     | NC
21 -- 17  | +5V
22 -- 18  | +5V
23 -- 19  | +5V
24 --     | NC

Three positions on the male connector has get special treatment:

Pin 3: This is +3.3V sense. If I understand sense lines correctly you have to run the sense line as close to the load as possible for proper function. The sense line on the ATX side shares pin 13 with a +3.3V wire, it is usually a brown slightly thinner cable, but you have to open the PSU to be sure. Cut it and extend it to PIN 3 on the WTX side (it is not shared with any other wire)

Pin 12: This is FANC, it should control fan speed. Read more below

Pin 24 This is FANM, it monitors fan speed. Connect to tachyometer pin on fan:

Next you must swap GND and +12V on the P3 connector. Just to be sure, remove wires from pin 4 and 8. Like so:

1 -- 5 | +12V
2 -- 6 | +12V
3 -- 7 | +12V
4 --   | NC
5 -- 1 | GND
6 -- 2 | GND
7 -- 3 | GND
8 --   | NC


  1. The original Fuel PSU has sense lines on +3, +5V and +12V. ATX only on +3V. If accurate +5V and +12V is needed this might be a problem. But according to L1 they are well within margin when the system i idle.
  2. The WTX guidlines state that a 460W PSU should deliver 45A on +3.3V. Not many ATX PSU goes over 30A. Similar for +5V
  3. The FANC on pin 12 doesn't appear to work as expected. I expected that the wire would go directly to the fan header inside the PSU, but it looks like its going into one of the IC's, but I'm not sure. The FANC line stays put at +3.3V even when the system if powered down. This might be to low to start the fan from the PSU. Which might indicate that it is just a signal wire to a fan controller in the PSU.. but I don't know.. perhaps a good reason to get my oscilloscope out.

Additional reference material

ATX Power supply

The ATX specification requires the power supply to produce three main outputs, +3.3 V, +5 V and +12 V. Low-power −12 V and 5 VSB (standby) supplies are also required. A −5 V output was originally required because it was supplied on the ISA bus, but it became obsolete with the removal of the ISA bus in modern PCs and has been removed in later versions of the ATX standard.

Originally the motherboard was powered by one 20-pin connector. An ATX power supply provides a number of peripheral power connectors, and (in modern systems) two connectors for the motherboard: a 4-pin auxiliary connector providing additional power to the CPU, and a main 24-pin power supply connector, an extension of the original 20-pin version.

Four wires have special functions:

  • PS_ON# or "Power On" is a signal from the motherboard to the power supply. When the line is connected to ground (by the motherboard), the power supply turns on. It is internally pulled up to +5 V inside the power supply.
  • PWR_OK or "Power Good" is an output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500 ms) after the PS_ON# signal is pulled low.
  • +5 VSB or "+5 V standby" supplies power even when the rest of the supply lines are off. This can be used to power the circuitry that controls the Power On signal.
  • +3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows for remote sensing of the voltage drop in the power supply wiring.

Generally, supply voltages must be within ±5% of their nominal values at all times. The little-used negative supply voltages, however, have a ±10% tolerance. There is a specification for ripple in a 10 Hz–20 MHz bandwidth:

Supply [V] Tolerance Range (min. to max.) Ripple (p. to p. max.)
+5 VDC ±5% (±0.25 V) +4.75 V to +5.25 V 50 mV
−5 VDC ±10% (±0.50 V) –4.50 V to –5.50 V 50 mV
+12 VDC ±5% (±0.60 V) +11.40 V to +12.60 V 120 mV
−12 VDC ±10% (±1.2 V) –10.8 V to –13.2 V 120 mV
+3.3 VDC ±5% (±0.165 V) +3.135 V to +3.465 V 50 mV
+5 VSB ±5% (±0.25 V) +4.75 V to +5.25 V 50 mV

WTX 12V CPU (P3) Pinout

8 PIN MOLEX 39-29-9082 CONNECTOR at the Motherboard 8 PIN MOLEX 39-01-2080 CONNECTOR at the cable

Pin Name Color Description AWG
1 12Vdig White +12 VDC (Digital) 18
2 12Vdig White +12 VDC (Digital) 18
3 12Vdig White +12 VDC (Digital) 18
4 12Vdigsen Sense +12 VDC (Digital) (Optional?)
5 com Black Ground 18
6 com Black Ground 18
7 com Black Ground 18
8 12Vdigsenrtn Ground Sense +12 VDC (Digital) (Optional?)

See also