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Quadra 950 Power Supply Startup Issue & Weird ON/OFF Workaround


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I have a monstrous Attila the Mac 950 Quadra tower that is a fantastic machine. Works great with one strange startup quirk: Before powering on, I have to turn off the power to the machine (using a power strip on/off button), then on again immediately prior to powering up the 950. If I do this it boots 100% of the time, very reliably. 

 

If I DON'T do this power strip on/off step, the machine fails to boot properly and hangs with a grey screen at startup. Very reliably. 

 

BTW I didn't just stumble up on this startup workaround, I found a hint during my hours of googling vintage mac startup problems and going through my old mac reference books. But alas I have been unable to find the source of this tip, which might provide hints as to what's going on and what I can do about it. Replacing the PSU is one obvious option to try ... but perhaps it's the board that's faulty? 

 

Anybody here have experience or knowledge of this issue? Any suggestions appreciated.

 

Thanks,

Craig

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By chance I found the 'hint' I mentioned about the workaround of using a power strip OFF/ON before powering on the Q950. This describes a known issue with the IIci and IIcx not the Q950, but I'm successfully using this trick on the Q950.

 

http://www.shobaffum.com/iici/faq.html#11

 

And here's the text with a proposed explanation of why this works:

 

"There is a known issue with IIci power supplies. This involves the +5 V trickle that is present even when the computer is off (but plugged in). Over time, the charge dwindles down to 0 Volts, making it impossible to start up.

The usual symptoms for this problem are "clicking" sounds from the power supply, or a startup chime followed by nothing or shutdown."

...
 
"Before you buy a new power supply, try this: Get ahold of a cheap "power strip" with an on/off switch. Plug the IIcx into the power strip. When the IIcx is off, leave the power strip switch in the OFF position.
When you're ready to boot up, reach down and flip the power strip on. Wait a few seconds, and press the keyboard power-on button. When it's time to shut down the computer, go through the normal shutdown procedure.
After the IIcx [or IIci] "turns itself off", reach down and flip the power strip to OFF. Try this - you may be VERY pleasantly surprised with the results..."
 
"The theory is that the +5 V charge is still there briefly when you send current to the power supply. If you can start up the Mac quickly enough, you won't need the trickle charge while it's running."
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