C900 and ATX Power Supplies...

From: Bo Zimmerman (bo_at_zimmers.net)
Date: 2005-08-03 10:01:59

I've been trying to figure out how to use an ATX power supply to power my
C900, whose internal power supply is quite unable to react to my US
frequency 220V transformer.

The C900 has the same motherboard connector and colored wires as the
CBM-Iis, so I used them as a reference to discover the voltages to supply to
the motherboard, those being 5V, 12V, GND, -12V, and 2.5V.  The ATX supply
does not supply 2.5 normally, but a few resisters on the ATX 3.3V supply
fixed that well enough.

My problem is that, for some unknown reason, whether under load or not, the
voltages are unstable.  The 5V, according to my little multimeter, drops to
3.9V occasionally, going up to 4.9V. Likewise, the other voltages go up and
down similarly.  While I don't have a oscilliscope, I suspect this is
happening more drastically and more often than my multimeter is catching,
as, during my one attempt with the supply hooked to the C900 MB, the power
light on the C900 blinked about 2 times a second, and the floppy drive motor
did a spin-and-stop on the same interval.

The only thing I'm doing strangely when the ps is not under load is that I
cross the always-on 5V line on the ATX with the Power-On line on the ATX to
force the power supply to come on.  I suspect, perhaps, that this might have
been the wrong thing to do.

Any suggestions are welcome.  Without an answer, I'll have to go try and dig
up a really small non-atx supply I suppose.. One that doesn't require any
"tricks" to turn on.

- Bo

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