Re: All replacement capacitors in iMac G5 PSU burned up

Confirmed, L6 is a saturable and there's a mag-amp circuit; Q14 controls L6's saturation through D24. Ultimately, regulation is performed by TL431 part M9. I haven't traced the whole circuit out yet, the layout of the board is horrible for tracing out and I'm too busy doing other stuff.

If the circuit fails such that L6 is almost always saturated, the +3.3V output will rise to +5V. Also, I don't know if this magamp controller circuit is sensitive to the amount of output capacitance, output capacitor ESR or anything...

Re: All replacement capacitors in iMac G5 PSU burned up

I just had a chat with a SMPS guy here at work, who had some insight. L6 is probably a saturable inductor, and is used as a mag-amp circuit. There's a bit of circuitry somewhere here which tags on between L6 and D33 and varies the saturation of L6 to regulate the +3.3V output.

Searching a bit more on Google, check out this book (click 'preview') pages 800-804. From the schematic on 804, it looks like this circuit could be hiding amongst the 'jellybean' SMT bits on the supply that I haven't considered yet....

Re: All replacement capacitors in iMac G5 PSU burned up


I've designed and worked on several UC384x based supplies. Behind the TL494, it's probably the most common, well understood SMPS controller on the planet. It's definitely not crap if it's implemented correctly.

Toasty: I'll look into that when I get home later today. Do these supplies have a habit of burning vias between top/bottom?...

Re: All replacement capacitors in iMac G5 PSU burned up

It's a 614-0293. I replaced all electrolytics on the secondary side of the supply, 9 in total. There are four other electrolytics on the primary side, plus the two mains caps, I didn't replace these. I used a mixture of Nichicon HE and Panasonic FM for the recap.

Eyeballing the layout, it looks like the TOP244Y is used for +5.1VSB - this is a different circuit from L8's, so I'm not concerned about that.

The main rails are run off the UC3845 switcher. I haven't traced out how the primary side works....

Re: All replacement capacitors in iMac G5 PSU burned up

Hey folks,

I've got an iMac G5 17" power supply which also has a burnt L8. I recapped this supply a couple of weeks ago, L8 looked good but the power supply quit working and it's back in my hands again.

Before I pull this supply apart, trace out a schematic and test every active component in the supply... has anyone figured out yet what destroys this coil?