(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.
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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.
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.
Car rental landing page free download windows 10. 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.
MDD Dual 1.42 GHz PowerPC G4, Mac OS X (10.5.8)
Posted on Jun 17, 2013 12:24 PM