Anyone know much about Mac Power Supplies?

Inspector Gadget

Hi folks:

This is a long shot, but I thought I'd throw in a post in case.

I was lucky enough a while back there to inherit an old G4 Digital Audio (less RAM, HDD and PSU) from a friend of mine. The more conventional bits I could buy/obtain myself, but luckily for me (considering they're as rare as rocking horse manure) ZENER here on the boards was kind enough to sort me out with a power supply for it (thanks again Ken, much obliged!)

Unfortunately, said PSU (a 338W AcBel model, revision 2 according to itself) has gone west on me - it seems to want to start, but while the white light around the power button comes on as expected when I press it, it goes out again as soon as I let go of said button, and I only get a flicker out of the red SMD LED in the middle of the motherboard. I'm led to believe that Macs (of this generation at least) don't do the PWR_GOOD handshaking that PC PSUs and motherboards do, so I'm inclined to think that it's the PSU that's at fault. That, and the fact that it starts first time with a PSU I was able to borrow for an evening

Anyway, my question(s) are:

1. Has anyone seen this problem before, and/or been able to cure it?
2. Has anyone got access to relevant technical manuals or stuff like that that I could browse? (I have reasonable experience in electronics, including some power stuff, and am reasonably well equipped, so I could have a go at it myself)

Anyway, all help/suggestions appreciated.
Gadget

ZENER

Hi Gadget !

Funny thing about Swords ya know, seems we have an abundance of rocking horses roaming about leaving their little droppings all over the place !!

Have you checked if any of the voltage rails come alive when you press the button ? I assume the switch mode is operating because the 5v stby line is what keeps the logic board ticking over waiting for the on button. Didn't you have to alter the PSU I gave you to get it to work? It was from a Quicksilver case I got it from.

I think I have another one here somewhere I could swap with you if all else fails. I'd like to have a spare - even a faulty one - about the place for emergencies.

ZEN

Inspector Gadget

Hi again!

Never modified the PSU, didn't have to (I'm delighted to add). To be honest, I should have gone digging for a pinout for the PSU - now that you've reminded me that it came from a Quicksilver, I'll do some digging.

I have popped the lid on the thing, but my god it's densely packed in there! I've seen the insides of plenty PSUs in my time, but I wasn't really prepared for this one. On visual inspection, the back-to-back diodes and the two power FETs look fine (unlike a PSU I pulled out of a server at work today, which had burst a fistful of AA_sized electrolytic caps and blown a nice hole in the side of one of the power MOSFETs after the fan seized), I don't see any SMD devices blown open, or any dry joints or evidence of thermal damage (burned resistors, that sort of thing), but I'll have to wait until I get home on the weekend to discharge the lethal caps and get a proper look at it.

Thanks for your kind offer of a swap-out - I don't think it'd be fair to accept another PSU from you, so I'm keen to fix the one I have.

Once I figure out which line is the 5v standby, be sure that it's the first one I'll test...

Thanks (again, and again, and again...)
Gadget

Inspector Gadget

Just to kinda close off this thread, and so that ZENER knows I'm not still looking for one, I've managed to procure another PSU of the same type for my mac. I'm told there's a possibility it may cause the computer to hang some time after it goes to sleep, but that's not exactly a huge deal.

Thanks one and all... now all I have to do is find out what happened to all of my PC133 RAM! (for some reason, my Digital Audio had 3 sticks of PC100 stuck in it, despite the fact that it requires PC133...?)

Gadget

ZENER

my Digital Audio had 3 sticks of PC100 stuck in it, despite the fact that it requires PC133...?

I had 2 sticks of PC100 in my 133 bus mac too along with a 512 PC133 stick and it was grand ! Glad you got sorted with a PSU, that one is still here if you're stuck in the future.

ZEN

Inspector Gadget

Thanks again, for the offer mate - most kind.

The reason I'm doubting the RAM is that the machine was making very un-Mac-like low-frequency tones when I'd switch it on; whack in a stick of PC133, and hey presto! Happy Mac sound!

(For the record, in case anyone thinks I'm being dimwitted and the RAM is banjoed, the same sticks all check out under MemTest+ in a PC)

Cheers,
Gadget