G4 Tower: Doesn't power up!

Question

I have a blue and grey G4 tower. It has sat at work for about 4 years and only been used half dozen times. So I took it home and have been happily using it for about 3 months. I got a Virgin Media broadband account and all was fine till the other day when (I believe) I shut down the mac ok but I lost the phone connection at the same time. I am getting the phone connection restored but it is possible a mains spike caused the problem as I remember shutting down the mac, switching the monitor off and then walking away. Just before i left my flat I noticed the mac was still on (it shouldn't have been) so I powered the monitor on and then shut the Mac down. now it won't start, either using the large 'power' button or the small 'reset' button. The mac IS 4 years old but has hardly been used. SOmebody suggested I clean the PSU unit but I looked and even the fans are still pretty clean. the inside of the Mac is spotless. SOmebody else suggested maybe there is a battery that has run out. I remember old Macs used to have a battery for storing time but surely this wouldn't stop it powering up?
The only other thing I noticed is that there is a small silver, braided wire with a funny 'nipple' connector on the end (in brass I think) which looks like it is disconnected from a point near the power button, but then again it could be just meant to be disconnected (ie ready for secondary media like DVD which this Mac doesn't have etc). I couldnt' see anywhere this could be connected.
Can anybody help? I have had macs for years and I have never had one just refuse to start before, especially one so little used - they usually soldier on for years. I do not have access to a manual or diagrams of what should be inside.
Oh I forgot to mention, the Monitor gets its power from the Mac and its light is on, which shows that power IS getting to the Mac)

Answer 1

Hi Steve!
I just bumped into your thread and I'm hoping that for now, at least, you still haven't managed todisconnect your PSU harness at the logic board. Unless you're bench testing the PSU with a resistance connected to the wiring, it's bad practice to energize it with the harness disconnected. Doing so can burn out the PSU in a very short amount of time.
Test the PSU using the link that Don kindly provided, but do so with the PSU installed and connected to the logic board. Insert the voltmeter probes into the wire side of the connector at the logic board, inserting them into the appropriate wire sockets right alongside the wires. Toward the end of each wire is a metal pin connector which you'll want to insure to make contact with. (Please note that after each test in the linked testing procedure, Apple advises to retest if you don't get a reading the first time. I take this as a double check to insure that probe to pin contact was made.)
I'd start by using the voltmeter set to AC voltage and first test for normal line voltage at the wall receptacle. If normal, connect your power cord to it and test for line voltage at the female end of the cord. If normal, connect the power cord into the Mac, switch the meter to DC voltage, open the side door, and insert the positive (red) probe into pin socket #22, which should be the white wire at the end of the connector for your Mac model. Insert the negative (black) probe alongside pin socket #11, which should be the black wire at the same end of the connector, just across from #22. This should produce a reading of +28 volts, which is the required trickle charge output for an energized power supply on a Gigabit Ethernet or Digital Audio Mac. If the reading is zero, retest to insure your probes indeed made good contact. If your probes are thick or you're unsure that you made metal to metal contact, you might use two straightened paper clips (much thinner) to insert alongside the wires, being careful not to touch them to each other or to touch both of them at the same time with a single probe, creating a short circuit. Then probe the protruding paper clips. If the reading is then still zero, the PSU is defective.
Regarding removing the connector... There's a plastic protrusion at the middle of the backside of the connector which you must press in near the top of the connector to release it from where it latches on a mating part just above the logic board. The harness connector should then pull straight up and off. If it's stubbornly stuck, then I'd first completely de-energize the Mac by unplugging the power cable, removing the internal battery, depressing the front power button for a few seconds, and then allowing the tower to sit for thirty minutes or so to insure that the logic board capacitors are discharged. Then try using a pair of pliers to grasp and carefully pull upward on the connector while depressing the top of the latch I spoke about. That should help you free it. If it still won't loosen, you might try spraying an aerosol dielectric electronics cleaner into the connector, wait a minute or so, then try the pliers again. If, instead, the problem is that the latch doesn't release at the bottom, try inserting a small tool between the connector body and the latch and gently pry the latch outward at the bottom just a tad. It only needs to move less than an eighth of an inch to release it. Then pull the connector straight upward.
Gary