Mac IIfx Power Issue - Requires "on" the be continually pressed

[PaloAlto]

Hi,

I have an IIfx that has fresh batteries in it and weird behavior. Pressing power at the back or on the keyboard generates a split second of power. Pressing the power button on the keyboard AND keeping it pressed boots the machine perfectly. Take your finger off the power button and lose all power.

The motherboard looks pristine to my untrained eye. I know it will need to be recapped, but I do not have the skills for that. Looking into finding a source for that. Is there anything else that might be causing this odd power behavior?  With new access to fresh memory, I am wanting to get this machine running again.

Thanks

 

[hyperneogeo]

The caps will def need to be changed on both the power supply and the computer itself. That could be the problem in itself. 

 

[Jinnai]

It's something in the soft power circuitry that is supposed to have a combination of semiconductors to pass off the power on signal voltage from the 6v PRAM battery to the PSU Power On pin until it's turned off, when the computer sends a "power off" signal to it and cuts the voltage from the PSU to the PSU Power On pin.

 

[PaloAlto]

Thank you both. This II remains on my project list. Hopefully next month I can get back to it.

 

[360alaska]

Look here for more info: https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/

 

[PaloAlto]

Thank you