PowerMac G4 MDD - Power Supply Unit "fix" Solution

(the issues regarding these powermacs and the power supply failure is prevalent and much discussion has been made regarding replacing the PSU with a new one or modding an ATX power supply)

Hello, I just want to share my experiences and solution with fixing my MDD PowerMac power supply.

First some quick background:

A few years back my Mac would reset its clock back to December 31, 1969.

And at times it would not power-up unless I switched off/on the power switch on the power strip or unplug/plug the power cable.

I replaced the PRAM battery and reset the PMU.

The problem was intermittent and though it was slightly inconvenient I was able to deal with manually updating the date/time settings, etc.

A few weeks ago, I decided to give the inside of my Mac a quick spring cleaning.

When I tried to turn it back on, it was completely unresponsive.

Since I knew the PSU, was prone to catastrophe I decided to open it up and take a peek.

But first a disclaimer:

I am not an expert in electrical engineering so my "solution" may not (or it may) work for your particular case. Additionally I simply stumbled upon this quick solution while I was "poking" around.

When you open up the PSU you should see something that looks like a cathode terminal, a few inches from this is a heat sink. If you happen to accidently touch both pieces at the same time you will receive a nice buzz. That is because the PSU and its capacitors/inductors have electrical charge stored in them.

What I did was simply arc the two pieces of metal using a thick screw driver with a rubber grip. This short circuit will simply discharge the PSU and you will see a spark or two as the electrical current finds the ground. (it also leaves a nice little nick on the shaft of you screw driver)

(That whole last part maybe a bit unneccessary, but it sure is fun to do and it brings back memories of how we use to "hotwire" our HMMWVs)

After checking with a multimeter, I verified that the PSU was getting the correct input voltage.

My next logical step was to check output voltage.

Since I didn't know what to look for in the PSU, I closed it back up.

(I should mention that there were no signs of burnt or exploded circuits, diodes, capacitors etc.. and that I could not locate a standard fuse... it turns out the fuse is a built-in 250 V - 8 amp ceramic fuse and proximal to where the power cord enters the PSU)

The next step was to test the output voltage, so I figured I'll check it at the terminal where it connects to the logic board.

I googled the pin out and then I checked the voltage.

Here's a good resource from forum member JapaMac.

Since I was getting the right voltages I put everything back together and then... VOILA.

My Mac powered on flawlessly, even the date and time settings were preserved.

Hopefully this it will work for others. I am not sure why it worked, but it did after I was unable to power-up my Mac.

With this quick solution I was able to bring my Mac back to life three times using that last trick.

What I simply did was measure the voltage with a multimeter between, if I recall correctly, were Pins 11 and 12.

Try that combination: between Power On and Ground or between the +25 V Standby and Ground.

Thanks for reading. Hopefully this helps out. Let us know if it does.

PowerPC Lives!