Macintosh IIfx tries to start but won't catch

[plat]

I'm guessing this is a common problem as they age but I have had trouble describing the issue and finding information. Basically when you hit the power switch there is at least a click and sometimes it begins to power up to the point of starting the chime and then quits. It was working perfectly when last run probably 10 years ago. I have already replaced the pram batteries and re-seated everything including the memory chips and power supply connection to no effect.

I suspect bad capacitors at this point? If so, more likely on the motherboard or in the power supply? I just want to be cautious with it and not take the risk in doing anything unnecessary if I can avoid it.

I appreciate any thoughts on this.

 

[David Cook]

There are a couple of different versions with different quantities of electrolytic capacitors of the motherboard. Can you take pictures of the motherboard? (Particularly the right side --that's where the power on/off logic is located)

 

[plat]

Thanks for looking at it. I noticed there are a couple of spots that look like some kind of residue on the surface of the motherboard on the right side, wondering if that's from the nearby capacitor to each one leaking? I'm not aware of any foreign substance getting in.

 

[joshc]

A common problem because of the couple of electrolytic capacitors fitted to some IIfx boards (some were tantalums only as David is alluding to).

The only capacitors you need to be concerned about are C9 and C24 as they have leaked, notice the extra dirt around those. Replace these. Clean the area thorougly with isopropyl alcohol. On these boards Apple provided two sets of pads, one for tantalums and one for SMD electrolytics, you can use either depending on what size package the new capacitors are.

If this doesn't solve the problem, further diagnosis of the startup circuit will be required, explained extremely well here: https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/

 

[plat]

Thanks, this is great information and I will see to it. And to confirm, I can replace with any type of capacitor of equivalent value and suitable voltage rating?

 

[finkmac]

stick to reputable brands, replace with the same mounting type. i also advise using aluminum polymer SMD capacitors instead of electrolytic/tantalum

 

[Juror22]

Yes, Polymer and Tantalums are the popular choices - for the capacitors indicated, replace them with a capacitance of 47uF and a voltage of 16V or higher. On the board there are pads for both tantalums and the SMD caps.
Cleaning up after removing the old capacitors is key - the cap goo can be under nearby components, so clean up the board thoroughly. There are plenty of threads on the forum with specifics on the various methods of cap removal, cleanup and replacement.