iMac G5 17" ALS - Troubleshoot - bad PSU?

[Bandolino]

Hello all,

This is my first post in this forum. First of all, a big thank you to the folks on this forum! I am feeling lost with this problem, and I'm grateful that I can ask for help from people who know a lot more than I do. I don't have any experience diagnosing electronic components other than by replacing with a known good component. My first attempt at soldering was about a month ago with a Wifi antenna. However, I am open to learning more and possibly trying out a recap of this iMac.

I am hoping to get guidance troubleshooting an iMac G5 17" ALS (2005 model). I have read extensively online about the problems that this and similar models of iMac have had, and from the anecdotal evidence, it seems likely that the problem may be bad caps on the motherboard or the power supply. I have visually inspected the caps on both boards, and I don't see any obvious failed caps. I am attaching a couple photos of the power supply with this post in hopes that readers might see something. I think this power supply is an AcBel API4PC47. Almost all the caps are Ltec, except the largest one, which is Nippon Chemi-Com.


Here are the symptoms:

1. At power on, computer gives start-up chime, fans start, but no video. Display is black, with no backlight visible. I have not tried using an external monitor, because I don't have the required connector.

2. Apparently at random, power on completes correctly, video works fine, boot to OS works fine, CPU temperature is ok (I think), computer will run for weeks with no problems, as long as it is not turned off.

I have found no pattern to when the boot works correctly. For example, sometimes it boots when it has been running for a few hours, and is warm, sometimes it boots after being powered off for a few minutes or a few days. Sometimes it boots after an SMU reset (system management unit - controls power regulation, fans, temperature monitoring), and sometimes SMU reset doesn't help.

3. On several occasions, the computer has been unable to wake from sleep properly. The display flashes briefly upon waking, then goes black. The computer is still on, but no display. Other times, it wakes from sleep fine.

4. One time, the CPU was overheating, and the computer went into sleep automatically to protect the CPU. I replaced the thermal compound on the CPU and the U3L IC (graphics bus chip on bottom side of motherboard). The computer appeared to run fine and at normal temperatures after that. I have monitored the CPU temp, and it usually runs between 50-75ºC. The temperature utility I use places the upper limit at 85ºC.

5. I have been unable to find a software utility to monitor the voltages of CPU, RAM, and such.

6. I have run Apple's hardware test boot CD a few times, with no problems detected. I have clean installed the OS, with no change in symptoms.

7. A few messages from the console indicate possible power supply or SMU problems. I'm including a sample of boot-time console messages to illustrate:

AppleSMU -- shutdown cause = -110
3/25/10 8:13:03 AM kernel Got boot device = IOService:/MacRISC4PE/ht@0,f2000000/AppleMacRiscHT/pci@3/IOPCI2PCIBridge/k2-sata-root@C/AppleK2SATARoot/k2-sata@0/AppleK2SATA/ATADeviceNub@0/AppleATADiskDriver/IOATABlockStorageDevice/IOBlockStorageDriver/ST3160023AS ST3160023AS/IOApplePartitionScheme/Untitled@3
3/25/10 8:13:03 AM kernel BSD root: disk0s3, major 14, minor 2
3/25/10 8:13:03 AM kernel [Bluetooth::CSRHIDTransition] switchToHCIMode (legacy)
3/25/10 8:13:03 AM kernel AppleSMU::PMU vers = 0x000d006b, SPU vers = 0x2b, SDB vers = 0x01,
3/25/10 8:13:03 AM kernel [Bluetooth::CSRHIDTransition] transition complete.
3/25/10 8:13:03 AM kernel [HCIController][configurePM] power parent ready after 1 tries
3/25/10 8:13:03 AM kernel Jettisoning kernel linker.
3/25/10 8:13:03 AM kernel Resetting IOCatalogue.
3/25/10 8:13:03 AM kernel Matching service count = 0
3/25/10 8:13:03 AM kernel Matching service count = 1
3/25/10 8:13:03 AM kernel Matching service count = 1
3/25/10 8:13:03 AM kernel Matching service count = 1
3/25/10 8:13:03 AM kernel Matching service count = 1
3/25/10 8:13:03 AM kernel Matching service count = 1
3/25/10 8:13:03 AM kernel Matching service count = 2
3/25/10 8:13:03 AM kernel PowerMac8,2: stalling for module
3/25/10 8:13:03 AM kernel Matching service count = 1
3/25/10 8:13:05 AM kernel PowerMac8_1_SystemFansCtrlLoop::calculateNewTarget() linkedControl not ready
3/25/10 8:13:05 AM bootlog[38] BOOT_TIME: 1269519176 0 3/25/10 8:13:05 AM

In some online sources, "AppleSMU -- shutdown cause = -110" has been linked to a bad power supply. The "kernel [HCIController][configurePM] power parent ready after 1 tries" message occurs just before an extended pause (15+ minutes) in boot sequence, at times.
_________________________________________________________

Well, hopefully that is enough information to start with. Here are the photos:

[deckart]

Re: iMac G5 17" ALS - Troubleshoot - bad PSU?

Google istat pro or Hardware monitor,just make sure you get the right one for whatever you have ie,Tiger etc.

[Bandolino]

Re: iMac G5 17" ALS - Troubleshoot - bad PSU?

Thanks for the suggestion, deckart. I'll look for one of those.

Would it be appropriate at this time to try testing the voltages from the power supply with a multimeter? As I'm a beginner at this, I'm not sure how I would do so. Is there a decent tutorial or book that shows how? I don't want to electrocute myself!

Along the same lines, is there a safe way to test individual caps while they are still on a board?

Can anyone tell from the photos, the symptoms, or the basic history of this model if it makes sense to continue with this as a caps problem?

I know I'm a noob at this. Am I treading in areas I shouldn't go without more experience or training?

Thanks again!

[Toasty]

Re: iMac G5 17" ALS - Troubleshoot - bad PSU?

Quoting myself from last year:

I suggest using the "advanced search" function on the forums. There are at least 20 topics on iMacs. Interesting reading and a ton of info on the PSU's and Midplane (Apple's name for the motherboard).

Search for: imac
Select from the drop-down box: Search Titles Only
At the bottom left- 'Show Results as:' click on 'Threads' button.

See Post # 10 in this thread for LED indications-

https://www.badcaps.net/forum/showthread.php?t=7124

See Post # 37 for the connector pinout-

https://www.badcaps.net/forum/showpost.php?p=78821

Toast

[Toasty]

Re: iMac G5 17" ALS - Troubleshoot - bad PSU?

>>Along the same lines, is there a safe way to test individual caps while they are still on a board?<<

ESR Meter will test the PSU ones in circuit. But not the mobo ones because they are "banks" of caps in parallel. Impossible to pick bad one out of 20.

There are several brands of ESR meters from kits to complete units. Just search on that in here too. I use the Blue ESR meter from Bob Parker / AnaTek:

http://www.anatekcorp.com/blueesr.htm

Toast

[Toasty]

Re: iMac G5 17" ALS - Troubleshoot - bad PSU?

Any Progress here ? Just dropped dead....



Toast

[koektrommel]

Re: iMac G5 17" ALS - Troubleshoot - bad PSU?

Well, i can use some help.

Im looking for a certain diode on the PSU, labeled as ZD6.
I have one on my failed PSU, where transistor Q1 was blown and i think it took diode ZD1 with it. This diode gives weird readings: unsoldered it measured 045 und 045. On another, well working psu, the diode on the same place gave a totally different reading. It acted as a normal diode, showing reading nearby 7-800 and 1.

Can anyone tell me what kind of diode this is, and how and where to order one?