Bad power supply on a molar mac G3

[Unknown_K]

I have an Apple G3 AIO (molar mac) with a bad power supply (12V is under 2V, 5V is good).

Anybody have schematics for the power supply (661-2042)? These are kind of rare since the supply was only used with the AIO and provides power for the CRT as well as the motherboard.

Had a hard time taking the supply out since the plastics are so brittle.

 

[GiGaBiTe]

Not much information exists about that era of Mac. Sounds like you have a short to ground though if the 12v rail is being pulled down that low.

 

[Unknown_K]

So look for a shorted capacitor?

 

[GiGaBiTe]

It could be several things, but a recap is a good place to start. There are very few Macs today that are untouched by shitty SMD electrolytics, capacitor plague era caps or bad radials in power supplies.

Since you have an AIO unit, you'll also want to probably start reflowing all of the solder joints due to potential cracks, which are common on most AIO macs from the heat. Other passive components like diodes and resistors are also problematic and should be checked as well.

 

[Unknown_K]

I looked around at the solder joints and didn't see anything that looked bad. I did reflow the connector that goes to the motherboard anyway. I found some resisters that looked like they were shorted but reading the color codes they are .22ohm and good.

 

[GiGaBiTe]

I would do a complete recap if you haven't already just to rule that out as a problem and then go back to the power circuitry.

If you don't have a powerful magnifying glass or a microscope, I'd still recommend redoing all of the solder joints. Some cracks can be so small you can't see them with the eye. The solder can also let go of the component leg its supposed to hold and not show any signs of failure at all. I've worked on many power supplies, including Macintosh power supplies, that had such failures. I'd be pulling my hair out trying to figure out why it wasn't working when everything was good, and it turned out to be a failed solder joint.

If you take care of those two issues and the +12v rail is still low, need to start looking for shorted components. If you can figure out a way to have the power supply turn on without the logic board connected, and the power rail comes back up, it'd tell you something was wrong on the logic board. If it doesn't, something is still wrong with the supply.