A Project: iMac G4 with power issues

[indigo_k]

Tinkering with my project iMac yesterday trying to get the power working, I learned that the PSU lacks continuity on the ground line. As in, I couldn't find the ground pin on the power supply connector when I had everything connected and plugged in. I was testing for continuity by holding one probe against the grounding screw attached to the AC Line Filter, and touching the other probe to what should have been Ground connections on the PSU.

Continuity lined up on the bridge cable between the two PSU boards, but there was no continuity on the Power Supply connector. None of the 12 pins had continuity with ground. You're supposed to be able to test here to confirm that 12v power is getting to the logic board, but I was getting no reading at all from any of the pins. I was only able to trace the ground along the underside of the board for a little bit before I accidentally sparked a resistor and lost my nerve out of fear of damaging the board too much to repair.

So, any tips on what I should be testing specifically here? Or is this one of those cases where it's going to be hopeless, and I should just source a replacement to get this computer running again?

Also, I don't know if this is the best Mac forum for troubleshooting this kind of issue, but it seems to be the sort of thing that some folks here might have some previous experience with, so…

 

[Jagmn]

I'd be surprised if you get mains ground continuity on the PSU->Logic board connector. The secondary PSU board is a DC-DC converter, so it should be isolated.

If you have a PSU issue its worth first checking for the 12V supply on the PSU connected to the logic board. From memory the black cables should all be GND and the yellow cables 12V. There's one yellow cable that I think only gets 12V once the machine is booting (for HDD/CDROM).

I recently repaired my iMac G4 700Mhz supply; the main problem was a dead optocoupler (PC1) on the DC-DC board, but several of the DC-DC side caps had unhelpfully high ESRs. Once the optocoupler(s) + caps were replaced, my was happy again.

 

[indigo_k]

the main problem was a dead optocoupler (PC1) on the DC-DC board, but several of the DC-DC side caps had unhelpfully high ESRs.

Thanks for the tips! I'll crack it back open and see what I can find out about the components you mentioned. Any tips for testing the optocoupler with a multimeter? Also, should I be concerned about the resistor I accidentally sparked in my earlier testing?

 

[Mk.558]

You don't have to use the stock power supply if you don't want to. If you supply 12V and GND to two specific pins on the power connector to the motherboard, it'll work fine: all the level converters are on the board. I had 2 iMac G4s and one of them just died one day for reasons I wasn't able to track down, the power supply had failed. The power supplies are kinda "meh" and a modern replacement might be a good option.

 

[indigo_k]

You don't have to use the stock power supply if you don't want to. If you supply 12V and GND to two specific pins on the power connector to the motherboard, it'll work fine: all the level converters are on the board. I had 2 iMac G4s and one of them just died one day for reasons I wasn't able to track down, the power supply had failed. The power supplies are kinda "meh" and a modern replacement might be a good option.

That's an interesting thought - I was considering looking into what it would take to convert the entire iMac to 12v power anyway, so I could run it off a 12v supply. Do you have links to a good 'modern replacement' power supply that would fit inside the iMac G4's unusual shell?

 

[Jagmn]

Thanks for the tips! I'll crack it back open and see what I can find out about the components you mentioned. Any tips for testing the optocoupler with a multimeter? Also, should I be concerned about the resistor I accidentally sparked in my earlier testing?

I tested my optocouplers with a multimeter set to diode mode (so you can get the forward voltage). You can also use a multi-tester or simply desolder and wire up to a test circuit. It was the transistor side of my opto that failed and a cheap multi-tester picked that up whilst it was in-circuit.
With the capacitors; you can just choose to replace them based on their age (or a visual inspection) or get an LC meter / multi-tester that can measure their capacitance and ESR. The ones I replaced had a ESRs of between 2 and 13 Ohms.

 

[indigo_k]

Update! Today I replaced the Power Supply and the iMac booted right up. Still had a couple users on it, and me without the passwords. Found instructions for creating a new Admin user via Single User Mode, and I was in business. Learned that it appears the iMac hasn't been used since 2004, meaning its used lifespan was maybe 2 years? I went ahead and did a clean install of OS 10.2 Jaguar, with a partition for hopeful OS9 install, and played around with it for a bit. I couldn't get any of my OS 9 images to work though, since it looks like the 10.2 install did *not* include a usable Classic environment. So, now I'm updating to 10.4, and I found what I hope will be a good bootable OS 9.2.2 installer which I've burned to a dvd.

Still a lot of upgrades I want to make to the machine, like a new SSD, a massive RAM upgrade, and adding an Airport card so I can get online. Also need to figure out how to tighten up the screen arm, since it's really loose and tips all the way down to the table.

 

[indigo_k]

Today I fixed the droopy screen, following this video's example:

I didn't have the tool he used, but I had some needlenose pliers and those worked just fine. The neck did pop apart a little bit and I had a predictably difficult time getting it back together, but I managed! And now I've got a really decent iMac G4 that stands up nice and straight (as long as it's on level ground)

Spent a little time pricing out the upgrades I want to make - new SDD hard drive, maxed out RAM, and an Airport card. Planning to get it all done at once so I don't have to open it up any more times than I need to.