g4 mdd power supply bad

[newbmac2k12]

I am trying to work on a g4 mdd dual 1.25ghz tower, for someone.  When plugged in no power.  I did take apart the power supply and from what i can tell a ceramic disc capacitor blew.  If I also understand correctly its soldered on top of a diode, the small blk cylinder soldered horizontally.  Anyway I did search and there is a  thread here about a g4 mdd power supply.  The person mentioned that a short happened and it took out the fuse and may even have affected the mainboard.  My question is where in the power supply is the fuse?  I took a few pics of the burnt capacitor and of the underside and nothing else looks burnt other than that capacitor.  What else would i need to check, and do other than replacing the fuse and this capacitor?  I've never worked on power supplies before other than replacing a few swollen caps.  I also did look it up and it is a pain to use an atx power supply and the used power supplies for it are expensive or at least the ones i saw on fleapay.  Finally, this power supply is the acbel brand one, not the samsung one in that thread. I did try to upload the 3 pics i have of the power supply but it wont let me, it shows im not permitted to upload this kind of file?  They are jpgs.

Thanks in advance.

 

[trag]

As wonderful as this forum is, I find that the badcaps forum has beaten us to the punch regarding MDD power supply repair.

Try this thread as a guide to other useful threads:  http://www.badcaps.net/forum/showthread.php?t=25253

That comes up from a google search of "badcaps mdd power supply".

If someone here can answer your questions, that's great, but failing that, try the discussions on badcaps.net.

 

[Macdrone]

Bad caps learned early on is why they have so much info.

They do have great info.

I've been there when I had my first MDD.

 

[Unknown_K]

Working on dead PS is a pain. I just waited for a deal and purchased 2 working MDD supplies (one for a dead unit one as a spare) and put the non working PS on the shelf for when I am really bored enough to try fixing it.

 

[newbmac2k12]

just an update havent had too much time to look into the power supply further.  But one thing, i did stumble across a site called atxg4.  He has adapters to use atx ps in g4's. He has the one i need specifically for the mdd.  Also he has another i could use for a g4 i picked up for myself a few yrs ago for 3$ at a garage sale.  He has the one for the gigabit g4.  Does anyone have experience with these adapters? I understand that the adc would be gone, well that monitor that gets its power from the video port, as well as sleep issues.  These wouldnt be a problem as  i dont have one of those monitors as well as the g4 mdd im wrking on for someone, doesnt have that monitor either.  I am leaning towards these options, as if i try to recap and fix the ps on the mdd, im thinking it might have stressed other components on the ps as well and may lead to other problems down the road.  At least this way that person and i could both just use standard atx ps's on it.

  Finally, on that site the guy can only accepts orders via etsy?  I've never heard of them before, so has anyone ever ordered from him via that site?

Thanks.

 

[uniserver]

yup i sifted through the bull, and found this post, as trag mentioned they have got to it first.

the issue looks most likely caps.   and it looks like they can (with this psu) be dead and not even show.

You'll need 12 caps here. Everyone from the end of the board at the secondary output side:

C7, 8, 10, 13, 16, 29, 21, 60, 59, 41, 35, & 43.

Oh, and invest in a decent 45W soldering iron and a spool of 1/8" (0.100") [blue] solder-wick. Plenty of decent irons can be had for $20 or less.

Toast

 

[trag]

Substituting an ATX supply for the MDD supply is very doable, but there are two issues.

First, physical fit.  The MDD supply is long and low, not boxy.   With some hunting you can probably find an power supply with ATX outputs that will fit in the MDD.  It will require some measureing and hunting, and don't forget that you want the power cord connection to match up properly on the back too.  Some folks remove the MDD optical drive and just install a boxy ATX drive and that works too, but involves sacrifice.

The second issue is electrical.  ATX supplies have no 28V supply.   This is primarily to supply connected ADC displays and absent one of those, you're mostly okay.   However, I have also read that the 28V supply plays some part in the use of the Firewire port.    I'm not sure how accurate this is, as I've never tried it.   Years ago, one fellow posted a  nice little voltage boost circuit to convert a portion of the 5V Standby current to a 28V line.   

Now days, it's simpler to get one of these:

http://r.ebay.com/i96eq2

This is a DC-DC boost circuit.   It supports inputs of 3V - 32V and outputs of 5V - 32V.

It's less than $4 from China.

If I was replacing an MDD power supply with an ATX, I'd get one of these,  and hook the input side of one of these to the 5V standby of the ATX power supply and the output side to the Logic Board 28V input, and adjust the thing to output 28V.   I've read that 24V will work well enough so you could use that setting.  

If I wanted ADC support I'd add one of these:

http://r.ebay.com/p6XlaX

(Slightly less than $5 from China.)

and a couple or four diodes.  Then I'd hook this thing's inputs to the 12V output of the ATX supply (modern ATX's have lots of 12V capacity), and the outputs (through a couple of diodes) to the Logic Board's 28V input and adjust this to output 28V.  Add the remaining two diodes between the previous device's outputs and the logic board.

The diodes are there to prevent current from flowing backwards from one boost circuit to the other, since they're both hooked up to the same pins on the logic board.

Why two boost circuits?

The first one does not have enough capacity to run an ADC monitor.  It just can't output/convert enough power.  Oh, it can probably do a small one, but it's kind of whimpy and the 5V standby can't supply enough power either.   You could overcome the latter issue by connecting the regular 5V supply as well as the 5V standby to the inputs of the boost circuit.   You might need more diodes there, though. I'm not to sure about the prospects of shorting the 5V standby to the regular 5V circuit.   Then again, the power supply itself may do that after power on.  I don't know.

The second boost circuit has plenty of capacity but its input range does not include 5V, so it is not useful for supplying the Stand-by 28V.

"Stand-by" means the voltages/power that is available to the logic board even when the computer is "off".

So, you need the first boost circuit to convert and supply the Stand-by 28V and the second boost circuit to convert and supply the operating 28V.