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- James Midolo
February 15, 2007, 10:23 am
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I have recently tried to get my very old Unisys 286 computer with a
Mitsubishi-made motherboard working. I switched on the power supply
and nothing happened, except for a squealing noise from the power
supply. The fan would normally switch on when the power supply was
turned on, so this indicated a possible problem with the power supply.
I pulled the power supply plugs out of the motherboard and tested them
with a multimeter and barely any voltage (less than a volt) was
registered between any of the wires and ground. I then opened up the
power supply and applied a voltage across the fan and it worked. I
have decided to replace the faulty power supply, but I'm having
trouble with the colours of the wires and what voltages they are meant
to be at.
The power supply is a Mitsubishi Electric PC15WB (REV. D). There are
two plugs going to the motherboard. One plug is a 3x2 with three reds
going down one "column" and three blacks going down the other. The
other plug has four wires: brown, blue, yellow and white. You can see
this plug at http://www26.brinkster.com/darthvader920/jamesmidolo /
The output voltages according to the power supply label are +12V, +5V,
-5V and -12V. I am wondering if anyone knows or can guess which
voltages are related to which wire, especially for the brown, blue and
white wires. I am not worried if I do apply the wrong voltage to the
wrong wire, because it is an old and useless computer, but it would be
nice to see it work again.
If anyone wishes to know anything further or for me to take more
pictures, please let me know. Thanks.
Re: Colours of wires from old Mitsubishi power supply
The red is almost certain to be 5V, the most loaded rail on
the system and thus, 3 wires (not to mention, red is usually
5V anyway). Black, ground.
As for the rest, I cannot guarantee the following, a
proprietary supply could be wired any way they wanted to do
it, but this what the standard AT supplies used and what I'd
Blue : -12V
White : -5V
Brown is the puzzling one. The natural choice seems to be
Power Good, but it was usually an orange wire. Brown on AT
was most commonly the high voltage AC input. I'm not
thinking they took a high voltage AC to the motherboard in
any situation, so Power Good seems to be what's left.
You might see some silkscreening on the PCB inside the PSU
that indicates for certain what these colors correspond to,
or you might be able to trace back the circuit and surmise
it, or if you took very good pictures and posted a link to
them here, someone else might be able to trace the circuit,
though personally I don't recall ever seeing a Mitsubishi
supply like that before, can only imagine it might be fairly
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