IMac Power PC or intel? 20in Power PC at Costco for $1479 w/3 yrs Aplecare!

So, I was planning on getting an iMac in July, and was planning on getting a 17 inch intel iMac, when I noticed iMacs at my local Costco. $1479 for the 20 inch PowerPC version with 3 years of Applecare included! Wow! So, should I get the old one, or the new one? My uses are internet, a little iMovie for home movies, iTunes, and I will be using iWork, for a small business. No serious applications, and not alot at any one time. My Mac mini pushes my limits now, but is still fine for what I use it for mostly. Thanks! I will want to jump on this if the intel will not make that much difference for me.

Depends on how many years you plan on keeping your Mac.
Three years? Go with the G5.
ICD faster? With apps that are Universal, yes. In some cases.
Most software runs under Rosetta and is hit very hard in terms of speed.
I find graphic apps like Bryce run at about G3 speed. WAY slower than G4 and not even CLOSE to G5 speed.
But if your apps aren't heavy graphic-intensive, speed won't be an issue.
Now if you're planning on keeping this computer more than 3 years, go with the ICD. Apple's support will be much stronger in the years to come.
17" iBook Duo Core Mac OS X (10.4.4)

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I can't figure out how to get this to work. Is it possible?

WildBull wrote:
Will it work if I connect via usb instead of firewire?
No. There is no "USB Target Disk Mode".
Just so we're clear, when you start your Mac mini in FireWire Target Disk Mode (TDM - holding T on startup), do you see a FireWire icon on the Mac mini's display? IIRC, it's not required that the Mac mini be connected to a display for TDM, but it would help diagnose whether or not the mini is actually starting in this mode.

Any problem with imac intel-based -power supply?

I just took possesion of an IMAC intel-based with 10.8 os. It is good so far but I'm not sure if alot of on/off power button usage is recommended.Is it better to put it to sleep for 1 hourish, or keep popping the power button. Thanks for reading. Double Thanks for an answer.

Welcome to the Apple Support Communities
You should turn off the iMac only when you aren't going to use it for some days

My IMAC seems to have died after power surge. It shut down and won't start. What shall I do?

my IMAC seems to have died after power surge. It shut down and won't start. What shall I do?

I would recommend you try an SMC reset at this point:
Resetting the System Management Controller (SMC)
Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve
Shut down the computer.
Unplug the computer's power cord.
Wait fifteen seconds.
Attach the computer's power cord.
Wait five seconds, then press the power button to turn on the computer.
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IMac G3 computers still refusing to power up? Here is a solution.

This post is the sequel to the discussion "iMac G3 computers still refusing to power up? Here is what I've found out!", accessible under this link http://discussions.apple.com/thread.jspa?threadID=2326420&tstart=0 .
I made it! I have fixed the issue of the iMac DV refusing to power up.
For this, I had to design and build a prototype of a cost-effective nonresident hardware patch, which I am about to describe in this article. I will also give the instructions of use. Subsequently, I will expose the rationale, which led to the background of the design. Finally, I will report the observations after the experiment with the patch and sketch the theory of its actual operation with the PMU firmware.
*TABLE OF CONTENTS*
1. Applicability condition and restriction
2. Description of the PMU hardware patch
3. Interconnections of the PMU microcontroller with the PMU Hardware Patch
4. Instructions to read before mounting and unmounting the PMU Hardware Patch on the iMac
5. Background to the design of the PMU Hardware Patch
5.1. The genesis
5.2. What the PMU Hardware Patch actually does
6. Recommended updates before using the PMU Hardware Patch
7. User's Manual of the PMU microcontroller
8. Datasheets of the 74HC4049 Hex inverting high-to-low level shifter
9. Datasheets of the 74HC193 Presettable synchronous 4-bit binary up/down counter
10. Errata to the discussion start of February 6th, 2010
h1. 1. Applicability condition and restriction
The hardware patch addresses the issue of a slot-loading iMac, which can be powered up only within two seconds after resetting the PMU microcontroller. It is not required otherwise.
Moreover, it does not need to reside in the iMac, but suffices once to permanently cure the power-management firmware.
h1. 2. Description of the PMU hardware patch
As seen on figures 1 and 2, the PMU hardware patch relies on a low part count: two logic ICs (74HC193 and 74HC4049), two 0805-packaged resistors and two 1206-packaged capacitors, which all fit on a 31mm x 45mm single-sided SMD prototyping board. The circuit consumes a current intensity of about 7.5 milliamps for a supply voltage of 3.3 volts.
The PMU hardware patch functions as a frequency divider of the PMU sub-clock generator output, XCOUT. It applies an active low pulse to the PMU NMI_bar non-maskable input, once every sixteen XCOUT rising edge for half a period. It is disabled when the PMU main clock generator output, XOUT, returns to 0 by oscillations, or when the system is being reset with the PMU RESET_bar input asserted to 0. The peak-to-peak 1.5-Volt oscillations of XCOUT are fed through the 1-nanofarad DC-blocking capacitor to the inverter biased in the linear region by a feedback resistor of twice 270 kilo-ohms, for translation to 3.3V-compatible logic levels.
The power-supply bypass capacitor amounts to 100 nanofarads.
*Figure 1: Schematics of the PMU Hardware Patch*
[Figure 1|http://www.flickr.com/photos/aegidius_2/4923600713/in/photostream/|Click to view in a new tab.]
Note the 0.5mm-diameter insulated copper wires in ochre color on the prototype board (figure 2).
*Figure 2: Picture of the PMU Hardware Patch prototype - component side*
[Figure 2|http://www.flickr.com/photos/aegidius_2/4924196064/in/photostream/|Click to view in a new tab.]
The 10cm-long interface wires can be clamped to the board with cylinder blocks made of epoxy glue (Araldite), as seen on figure 3.
*Figure 3: Picture of the PMU Hardware Patch prototype - interface side*
[Figure 3|http://www.flickr.com/photos/aegidius_2/4924196296/in/photostream/|Click to view in a new tab.]
h1. 3. Interconnections of the PMU microcontroller with the PMU Hardware Patch
The PMU Hardware Patch uses the signals RESET_bar, NMI_bar and the clocks XOUT and XCOUT of the PMU microcontroller, U34 on the Logic Board underside. Both are supplied by the voltage available across the tantalum capacitor C131.
Pin 73 of U34 performs the function INT3_bar because it turns out to be permanently configured as an edge-sensitive interrupt input, once the PMU Hardware Patch has been used. This will be justified in section 5.2. When the computer is plugged to the mains, a pressure on any power button can induce a falling edge on pin 73. U34 responds by asserting pin 75 to logic 0, as long as the computer has to remain powered on from the PAV board.
Pin 10 of U34 is the active low reset input, RESET_bar. It toggles from logic 0 to 1, a few hundred milliseconds after U35 has detected the supply voltage across the capacitor C131 has settled above 2.2 volts, either because a good battery has been inserted in the holder, or else because the computer has been plugged to the mains and the trickle power from the PAV board, T5V, is available. It is also asserted to logic 0, as long as the PMU reset button is depressed.
Pin 9 of U34 is the sub-clock output, XCOUT. The PMU microcontroller selects it for its operation in low-power dissipation mode to carry on the timekeeping, normally as soon as the computer is being shut down. The microcontroller shall then draw no more than 40 microamps typically from the battery.
Pin 11 of U34 is the main clock output, XOUT. It is the default clock of the PMU microcontroller after the reset phase. The microcontroler selects it for its operation in high-speed mode, when the computer is powered on.
Pin 15 of U34 is the active low edge-sensitive non-maskable interrupt, NMI_bar. It is normally unused and its voltage remains pulled up to the microcontroller supply through resistor R124. However, it happens to help resolve the power-up issue when it is controlled by the PMU Hardware Patch. The justifications are exposed in section 5.2.
Figure 4 shows an easier access to the clock and control signal pins of U34, to be connected to the PMU Hardware Patch, from the top of the Logic Board. You will have to solder the PMU Hardware Patch interface wires at the locations pointed to.
*Figure 4: View of the access to the PMU U34 chip signals for the PMU Hardware Patch*
[Figure 4|http://www.flickr.com/photos/aegidius_2/4929783304/in/photostream/|Click to view in a new tab.]
h1. 4. Instructions to read before mounting and unmounting the PMU Hardware Patch on the iMac
All possible updates accessible from the links in section 6 shall be installed. Then remove the battery. All this ensures your iMac will be prepared to interact with the PMU Hardware Patch under the same initial conditions as in the experiment I have carried out.
Afterwards, remove the bottom housing, the metal shield, the SDRAM, the IDE cable and disconnect the hard-drive power cable. Take off the Logic Board together with the Down Converter board and the modem and place it on your workbench. Now carefully solder the PMU Hardware Patch interface wires at the locations pointed to in figure 4. Figure 5 illustrates the result.
Install the boards back in the iMac, reconnect the hard-drive power cable and the IDE cable, and re-insert the SDRAM in its original slot. Put the computer on a large flat and safe area near a power socket. Stand up the PMU Hardware Patch on a plastic pouch, to insulate it for the parts of the Logic Board, as seen in figure 6. Connect the keyboard and the mouse.
Once you have plugged the computer back to the mains, don't take the risk to touch the computer except the keyboard and the mouse, because quite a few visible metal parts are live and lethal by body contact!
Wait for twenty seconds and then unplug the computer from the mains. Wait for a minute. Again, supply the mains to the computer and wait for twenty seconds, before you press the power button on the keyboard. Then let the operating system load until you can see the menu bar. Maybe a window will pop up to warn you that the system date is too ancient, but you can skip it. Now shut down the computer by selecting the command in the Special menu. I know it is awkward to move the mouse pointer when the picture is upside down. But take it easy!
Unplug the computer from the mains and unmount the PMU Hardware Patch. Once you have re-assembled your iMac, you can power it up and use it without the constraint to reset the PMU each time before and then to press on a power button in a hurry within two seconds.
A good tip: get all this task cleanly done by a professional, like a repairman or an electronics technician, if you feel you don't have enough skill in soldering!
*Figure 5: View of the Patch-PMU interconnections*
[Figure 5|http://www.flickr.com/photos/aegidius_2/4929190683/in/photostream/|Click to view in a new tab.]
*Figure 6: View of the mounted PMU Hardware Patch*
[Figure 6|http://www.flickr.com/photos/aegidius_2/4929190963/in/photostream/|Click to view in a new tab.]
h1. 5. Background to the design of the PMU Hardware Patch
h2. 5.1. The genesis
My idea to the design of the PMU Hardware Patch started with the study of the user's manual of the PMU microcontroller and with my observations on the clocks XOUT and XCOUT.
The symptom of the power-up default was accompanied by the disappearance of the clocks XOUT and XCOUT, if no power button was depressed within two seconds after pushing on the PMU reset button. I concluded, it was the reason why the PMU microcontroller could no longer react to a power button signal on pin 73 (P15/D13/INT3) and perform any timekeeping until the next PMU reset.
By looking at the block diagram Figure 1.10.3 (Clock generating circuit) of the microcontroller user's manual, I noticed that any clock oscillator, XOUT and XOUT, can be re-enabled after the CM10 bit has pulsed to logic 1, provided that the SR latch is reset by the NMI_bar non-maskable interrupt line or by the RESET_bar input. If bit CM04 was set to logic 1, then XCOUT resumes the clock oscillations, and likewise for XOUT if bit CM05 was set to logic 1.
I assumed the situation CM04 = 1 and CM05 = 0, as the PMU microcontroller enters the STOP mode. My idea was then to let it avoid the STOP mode and continue operating in the low-power dissipation mode, by periodically refreshing the SR latch reset through a low pulse on the NMI_bar input. Since the 32.768-kHz oscillations on XCOUT fade within a few hundred microseconds, repeating the pulse on NMI_bar at a quicker rate, once every 488 microseconds (i.e. once every sixteen XCOUT clock period) is acceptable.
However, I could not foresee how the non-maskable interrupt routine would interfere with the PMU firmware, whenever the NMI_bar pulse would be acknowledged to let the microcontroller program counter branch to it. I know that events on edge-sensitive interrupt lines are normally latched, which allows the microcontroller state machine to detect them even if they are too short compared to the clock period. And yet, I assumed it differently, after reading this excerpt in the microcontroller user's manual in the section "Precautions for Interrupts": "Signals input to the NMI pin require an “L” level of 1 clock or more, from the operation clock of the CPU". I decided to design and build a PMU Hardware Patch, that would generate a pulse on NMI_bar for a duration of less one XCOUT clock period, actually for half of it. If it is synchronised with the appropriate edge of XCOUT, then maybe the pulse would not cause the branch to the NMI routine. Moreover, when both XOUT and XCOUT are running, the duration of the pulse from the PMU Hardware Patch would exceed the period of the XOUT clock. To avoid the risk that the pulse occurs for more than one period of XOUT, as the operation clock of the microcontroller, I designed the PMU Hardware Patch, so that it remains inactive as long as XOUT is not steadily asserted to logic 1. The PMU Hardware Patch will not either generate any low pulse during the reset phase, as long as RESET_bar is asserted to 0, and as required in the microcontroller user's manual.
Once the PMU Hardware Patch assembled, I took the risk to use it in the iMac DV. What a relief it was, whenever I turned on the computer again, without the need to press the PMU reset button each time. The PMU Hardware Patch works!
After a power cycling of one minute, without unplugging the iMac from the mains, I looked at the time and date I had initially set. The time had not advanced correctly, while the computer was turned off. Instead it was lagging: it recovered with the value I had initially entered, plus about one minute that the operating system takes to load. That means that no clock was running again, while the computer was turned off. Nonetheless I unmounted the PMU Hardware Patch and tried power cycling the computer at any time interval. Powering it on always succeeded without the need anymore to reset the PMU microcontroller before each attempt.
h2. 5.2. What the PMU Hardware Patch actually does
This means the PMU Hardware Patch had definitely interfered with the PMU firmware and cured it with respect to the power-up issue once and for all. I have figured out it interrupts the firmware through the NMI routine, which restores a few data in the non-volatile FLASH memory. These data are used to program the interrupt control register INT3IC, so that pin 73 behaves as the edge sensitive interrupt INT3_bar. This eliminates the need to have any clock running to register a request on pin 73 to power up the computer. Furthermore, when INT3_bar senses a low pulse because a power button is being pushed on, it urges the internal signal "Interrupt request level judgment" to reset the SR latch register, which re-enables any clock oscillator, as seen in figures 1.11.9 (Maskable interrupts priorities (peripheral I/O interrupts)) and 1.10.3 (Clock generating circuit). The PMU microcontroller can resume its operations and look for the source of the interruption. It finds out it has to power up the iMac, because INT3_bar had been latched in as active.
h1. 6. Recommended updates before using the PMU Hardware Patch
+Mac OS 9: Available Updates+
http://support.apple.com/kb/HT1387
+Power PC-based iMac: When to install available updates+
http://support.apple.com/kb/HT2560?viewlocale=enUS&locale=enUS
+iMac: How to Install an iMac Firmware Update+
http://support.apple.com/kb/HT2561
h1. 7. User's Manual of the PMU microcontroller
http://documentation.renesas.com/eng/products/mpumcu/62aeum.pdf
h1. 8. Datasheets of the 74HC4049 Hex inverting high-to-low level shifter
http://www.nxp.com/documents/datasheet/74HC4049CNV.pdf
http://www.st.com/stonline/products/literature/ds/1965/m74hc4049.pdf
http://www.ti.com/lit/gpn/cd74hc4049
http://www.fairchildsemi.com/ds/MM/MM74HC4049.pdf
h1. 9. Datasheets of the 74HC193 Presettable synchronous 4-bit binary up/down counter
http://www.nxp.com/documents/datasheet/74HCHCT193.pdf
http://www.ti.com/lit/gpn/cd74hc193
h1. 10. Errata to the discussion start of February 6th, 2010
1. "PRAM battery": Wrong naming! The Logic-Board battery only serves to support the timekeeping operation when the mains is removed from the computer, whereas the PRAM data are stored in the non-volatile FLASH memory of the PMU microcontroller (in slot-loading iMacs).
2. "(...) sets bit 1 of the CM1 register (system clock control register 1) to 1 (...)". This fragment shall rather read: "(...) sets bit 0 of the CM1 register (system clock control register 1) to 1 (...)".
Aegidius_2
Keep perseverance and reach your goal!

This post is the sequel to the discussion "iMac G3 computers still refusing to power up? Here is what I've found out!", accessible under this link http://discussions.apple.com/thread.jspa?threadID=2326420&tstart=0 .
I made it! I have fixed the issue of the iMac DV refusing to power up.
For this, I had to design and build a prototype of a cost-effective nonresident hardware patch, which I am about to describe in this article. I will also give the instructions of use. Subsequently, I will expose the rationale, which led to the background of the design. Finally, I will report the observations after the experiment with the patch and sketch the theory of its actual operation with the PMU firmware.
*TABLE OF CONTENTS*
1. Applicability condition and restriction
2. Description of the PMU hardware patch
3. Interconnections of the PMU microcontroller with the PMU Hardware Patch
4. Instructions to read before mounting and unmounting the PMU Hardware Patch on the iMac
5. Background to the design of the PMU Hardware Patch
5.1. The genesis
5.2. What the PMU Hardware Patch actually does
6. Recommended updates before using the PMU Hardware Patch
7. User's Manual of the PMU microcontroller
8. Datasheets of the 74HC4049 Hex inverting high-to-low level shifter
9. Datasheets of the 74HC193 Presettable synchronous 4-bit binary up/down counter
10. Errata to the discussion start of February 6th, 2010
h1. 1. Applicability condition and restriction
The hardware patch addresses the issue of a slot-loading iMac, which can be powered up only within two seconds after resetting the PMU microcontroller. It is not required otherwise.
Moreover, it does not need to reside in the iMac, but suffices once to permanently cure the power-management firmware.
h1. 2. Description of the PMU hardware patch
As seen on figures 1 and 2, the PMU hardware patch relies on a low part count: two logic ICs (74HC193 and 74HC4049), two 0805-packaged resistors and two 1206-packaged capacitors, which all fit on a 31mm x 45mm single-sided SMD prototyping board. The circuit consumes a current intensity of about 7.5 milliamps for a supply voltage of 3.3 volts.
The PMU hardware patch functions as a frequency divider of the PMU sub-clock generator output, XCOUT. It applies an active low pulse to the PMU NMI_bar non-maskable input, once every sixteen XCOUT rising edge for half a period. It is disabled when the PMU main clock generator output, XOUT, returns to 0 by oscillations, or when the system is being reset with the PMU RESET_bar input asserted to 0. The peak-to-peak 1.5-Volt oscillations of XCOUT are fed through the 1-nanofarad DC-blocking capacitor to the inverter biased in the linear region by a feedback resistor of twice 270 kilo-ohms, for translation to 3.3V-compatible logic levels.
The power-supply bypass capacitor amounts to 100 nanofarads.
*Figure 1: Schematics of the PMU Hardware Patch*
[Figure 1|http://www.flickr.com/photos/aegidius_2/4923600713/in/photostream/|Click to view in a new tab.]
Note the 0.5mm-diameter insulated copper wires in ochre color on the prototype board (figure 2).
*Figure 2: Picture of the PMU Hardware Patch prototype - component side*
[Figure 2|http://www.flickr.com/photos/aegidius_2/4924196064/in/photostream/|Click to view in a new tab.]
The 10cm-long interface wires can be clamped to the board with cylinder blocks made of epoxy glue (Araldite), as seen on figure 3.
*Figure 3: Picture of the PMU Hardware Patch prototype - interface side*
[Figure 3|http://www.flickr.com/photos/aegidius_2/4924196296/in/photostream/|Click to view in a new tab.]
h1. 3. Interconnections of the PMU microcontroller with the PMU Hardware Patch
The PMU Hardware Patch uses the signals RESET_bar, NMI_bar and the clocks XOUT and XCOUT of the PMU microcontroller, U34 on the Logic Board underside. Both are supplied by the voltage available across the tantalum capacitor C131.
Pin 73 of U34 performs the function INT3_bar because it turns out to be permanently configured as an edge-sensitive interrupt input, once the PMU Hardware Patch has been used. This will be justified in section 5.2. When the computer is plugged to the mains, a pressure on any power button can induce a falling edge on pin 73. U34 responds by asserting pin 75 to logic 0, as long as the computer has to remain powered on from the PAV board.
Pin 10 of U34 is the active low reset input, RESET_bar. It toggles from logic 0 to 1, a few hundred milliseconds after U35 has detected the supply voltage across the capacitor C131 has settled above 2.2 volts, either because a good battery has been inserted in the holder, or else because the computer has been plugged to the mains and the trickle power from the PAV board, T5V, is available. It is also asserted to logic 0, as long as the PMU reset button is depressed.
Pin 9 of U34 is the sub-clock output, XCOUT. The PMU microcontroller selects it for its operation in low-power dissipation mode to carry on the timekeeping, normally as soon as the computer is being shut down. The microcontroller shall then draw no more than 40 microamps typically from the battery.
Pin 11 of U34 is the main clock output, XOUT. It is the default clock of the PMU microcontroller after the reset phase. The microcontroler selects it for its operation in high-speed mode, when the computer is powered on.
Pin 15 of U34 is the active low edge-sensitive non-maskable interrupt, NMI_bar. It is normally unused and its voltage remains pulled up to the microcontroller supply through resistor R124. However, it happens to help resolve the power-up issue when it is controlled by the PMU Hardware Patch. The justifications are exposed in section 5.2.
Figure 4 shows an easier access to the clock and control signal pins of U34, to be connected to the PMU Hardware Patch, from the top of the Logic Board. You will have to solder the PMU Hardware Patch interface wires at the locations pointed to.
*Figure 4: View of the access to the PMU U34 chip signals for the PMU Hardware Patch*
[Figure 4|http://www.flickr.com/photos/aegidius_2/4929783304/in/photostream/|Click to view in a new tab.]
h1. 4. Instructions to read before mounting and unmounting the PMU Hardware Patch on the iMac
All possible updates accessible from the links in section 6 shall be installed. Then remove the battery. All this ensures your iMac will be prepared to interact with the PMU Hardware Patch under the same initial conditions as in the experiment I have carried out.
Afterwards, remove the bottom housing, the metal shield, the SDRAM, the IDE cable and disconnect the hard-drive power cable. Take off the Logic Board together with the Down Converter board and the modem and place it on your workbench. Now carefully solder the PMU Hardware Patch interface wires at the locations pointed to in figure 4. Figure 5 illustrates the result.
Install the boards back in the iMac, reconnect the hard-drive power cable and the IDE cable, and re-insert the SDRAM in its original slot. Put the computer on a large flat and safe area near a power socket. Stand up the PMU Hardware Patch on a plastic pouch, to insulate it for the parts of the Logic Board, as seen in figure 6. Connect the keyboard and the mouse.
Once you have plugged the computer back to the mains, don't take the risk to touch the computer except the keyboard and the mouse, because quite a few visible metal parts are live and lethal by body contact!
Wait for twenty seconds and then unplug the computer from the mains. Wait for a minute. Again, supply the mains to the computer and wait for twenty seconds, before you press the power button on the keyboard. Then let the operating system load until you can see the menu bar. Maybe a window will pop up to warn you that the system date is too ancient, but you can skip it. Now shut down the computer by selecting the command in the Special menu. I know it is awkward to move the mouse pointer when the picture is upside down. But take it easy!
Unplug the computer from the mains and unmount the PMU Hardware Patch. Once you have re-assembled your iMac, you can power it up and use it without the constraint to reset the PMU each time before and then to press on a power button in a hurry within two seconds.
A good tip: get all this task cleanly done by a professional, like a repairman or an electronics technician, if you feel you don't have enough skill in soldering!
*Figure 5: View of the Patch-PMU interconnections*
[Figure 5|http://www.flickr.com/photos/aegidius_2/4929190683/in/photostream/|Click to view in a new tab.]
*Figure 6: View of the mounted PMU Hardware Patch*
[Figure 6|http://www.flickr.com/photos/aegidius_2/4929190963/in/photostream/|Click to view in a new tab.]
h1. 5. Background to the design of the PMU Hardware Patch
h2. 5.1. The genesis
My idea to the design of the PMU Hardware Patch started with the study of the user's manual of the PMU microcontroller and with my observations on the clocks XOUT and XCOUT.
The symptom of the power-up default was accompanied by the disappearance of the clocks XOUT and XCOUT, if no power button was depressed within two seconds after pushing on the PMU reset button. I concluded, it was the reason why the PMU microcontroller could no longer react to a power button signal on pin 73 (P15/D13/INT3) and perform any timekeeping until the next PMU reset.
By looking at the block diagram Figure 1.10.3 (Clock generating circuit) of the microcontroller user's manual, I noticed that any clock oscillator, XOUT and XOUT, can be re-enabled after the CM10 bit has pulsed to logic 1, provided that the SR latch is reset by the NMI_bar non-maskable interrupt line or by the RESET_bar input. If bit CM04 was set to logic 1, then XCOUT resumes the clock oscillations, and likewise for XOUT if bit CM05 was set to logic 1.
I assumed the situation CM04 = 1 and CM05 = 0, as the PMU microcontroller enters the STOP mode. My idea was then to let it avoid the STOP mode and continue operating in the low-power dissipation mode, by periodically refreshing the SR latch reset through a low pulse on the NMI_bar input. Since the 32.768-kHz oscillations on XCOUT fade within a few hundred microseconds, repeating the pulse on NMI_bar at a quicker rate, once every 488 microseconds (i.e. once every sixteen XCOUT clock period) is acceptable.
However, I could not foresee how the non-maskable interrupt routine would interfere with the PMU firmware, whenever the NMI_bar pulse would be acknowledged to let the microcontroller program counter branch to it. I know that events on edge-sensitive interrupt lines are normally latched, which allows the microcontroller state machine to detect them even if they are too short compared to the clock period. And yet, I assumed it differently, after reading this excerpt in the microcontroller user's manual in the section "Precautions for Interrupts": "Signals input to the NMI pin require an “L” level of 1 clock or more, from the operation clock of the CPU". I decided to design and build a PMU Hardware Patch, that would generate a pulse on NMI_bar for a duration of less one XCOUT clock period, actually for half of it. If it is synchronised with the appropriate edge of XCOUT, then maybe the pulse would not cause the branch to the NMI routine. Moreover, when both XOUT and XCOUT are running, the duration of the pulse from the PMU Hardware Patch would exceed the period of the XOUT clock. To avoid the risk that the pulse occurs for more than one period of XOUT, as the operation clock of the microcontroller, I designed the PMU Hardware Patch, so that it remains inactive as long as XOUT is not steadily asserted to logic 1. The PMU Hardware Patch will not either generate any low pulse during the reset phase, as long as RESET_bar is asserted to 0, and as required in the microcontroller user's manual.
Once the PMU Hardware Patch assembled, I took the risk to use it in the iMac DV. What a relief it was, whenever I turned on the computer again, without the need to press the PMU reset button each time. The PMU Hardware Patch works!
After a power cycling of one minute, without unplugging the iMac from the mains, I looked at the time and date I had initially set. The time had not advanced correctly, while the computer was turned off. Instead it was lagging: it recovered with the value I had initially entered, plus about one minute that the operating system takes to load. That means that no clock was running again, while the computer was turned off. Nonetheless I unmounted the PMU Hardware Patch and tried power cycling the computer at any time interval. Powering it on always succeeded without the need anymore to reset the PMU microcontroller before each attempt.
h2. 5.2. What the PMU Hardware Patch actually does
This means the PMU Hardware Patch had definitely interfered with the PMU firmware and cured it with respect to the power-up issue once and for all. I have figured out it interrupts the firmware through the NMI routine, which restores a few data in the non-volatile FLASH memory. These data are used to program the interrupt control register INT3IC, so that pin 73 behaves as the edge sensitive interrupt INT3_bar. This eliminates the need to have any clock running to register a request on pin 73 to power up the computer. Furthermore, when INT3_bar senses a low pulse because a power button is being pushed on, it urges the internal signal "Interrupt request level judgment" to reset the SR latch register, which re-enables any clock oscillator, as seen in figures 1.11.9 (Maskable interrupts priorities (peripheral I/O interrupts)) and 1.10.3 (Clock generating circuit). The PMU microcontroller can resume its operations and look for the source of the interruption. It finds out it has to power up the iMac, because INT3_bar had been latched in as active.
h1. 6. Recommended updates before using the PMU Hardware Patch
+Mac OS 9: Available Updates+
http://support.apple.com/kb/HT1387
+Power PC-based iMac: When to install available updates+
http://support.apple.com/kb/HT2560?viewlocale=enUS&locale=enUS
+iMac: How to Install an iMac Firmware Update+
http://support.apple.com/kb/HT2561
h1. 7. User's Manual of the PMU microcontroller
http://documentation.renesas.com/eng/products/mpumcu/62aeum.pdf
h1. 8. Datasheets of the 74HC4049 Hex inverting high-to-low level shifter
http://www.nxp.com/documents/datasheet/74HC4049CNV.pdf
http://www.st.com/stonline/products/literature/ds/1965/m74hc4049.pdf
http://www.ti.com/lit/gpn/cd74hc4049
http://www.fairchildsemi.com/ds/MM/MM74HC4049.pdf
h1. 9. Datasheets of the 74HC193 Presettable synchronous 4-bit binary up/down counter
http://www.nxp.com/documents/datasheet/74HCHCT193.pdf
http://www.ti.com/lit/gpn/cd74hc193
h1. 10. Errata to the discussion start of February 6th, 2010
1. "PRAM battery": Wrong naming! The Logic-Board battery only serves to support the timekeeping operation when the mains is removed from the computer, whereas the PRAM data are stored in the non-volatile FLASH memory of the PMU microcontroller (in slot-loading iMacs).
2. "(...) sets bit 1 of the CM1 register (system clock control register 1) to 1 (...)". This fragment shall rather read: "(...) sets bit 0 of the CM1 register (system clock control register 1) to 1 (...)".
Aegidius_2
Keep perseverance and reach your goal!

My iMac will not load since a power flux

Today there was a power flux as i was using my iMac. I did not try to turn it on for 2-3hrs after the iMac screen went black as it turned off due to lack of power. Later i did try to turn the computer on twice, after unplugging it in. Each time I did turn it on the screen lights up with a white background, grey Apple logo, grey pinwheel turning, then switches to a white screen with nothing and the only response to a left mouse click is the rainbow pinwheel for a time, then nothing but a mouse arrow with the whte screen. The only way I can turn it off is by unplugging it since the power button does not respond.
Help, Please!

First work thru the steps at > Mac OS X: Gray screen appears during startup
Then if you get no where with that, go to > OS X: About OS X Recovery

My 24" iMac just died and will not power-up. What can I do?

My 24" iMac just died and will not power-up. What can I do?

Bring it into an Apple Store for a free diagnosis and quote for cost of repair/replacement of parts.
If the costs are $500 or over, consider using the money toward a new or "newer" (used) Mac.

I would like to leave my iMac on Snow Leapord to use Power PC software, and run Mountain Lion on my Macbook pro. I am getting an iPhone 5. Is there a way to use mail on all of them?

I would like to leave my iMac on Snow Leapord to use Power PC software, and run Mountain Lion on my Macbook pro. I am getting an iPhone 5. Is there a way to use mail on all of them with the same accounts?

Any IMAP e-mail account can be accessed from all devices just fine. You don't need to upgrade from Snow Leopard to be able to access an IMAP e-mail account. However, Snow Leopard won't have built-in support for iCloud. As mende1 points out, you can still read your iCloud e-mail from Snow Leopard, but you have to set it up manually. You won't be able to do things like sync your iCloud calendar or addresses with iCal or Address Book on the iMac, though.

Wait for Intel G5 or buy power mac pro intel now????

This is for video and multimedia processing and i know Qaud g5 is best on the market right now but i need somnething within a few months with fast processing power with intel because that will be the newest tech. that wont go out of date anytime soon. So what do I do Wait for power mac G5 Intel which i predict in year or just buy the mac book pro but scared it won't process as fast as i think it will. I just absolutly cant handle when using multiple layers and when I edit them waiting for the process time to make each of the changes. WHAT SHOULD I DO!!!!!!

You're not even comparing apples to apples here. The MBP is faster than the G4 Powerbooks, but it will in no way compete with a Quad G5! Not to mention, the MBP maxxes out at 2GB memory, which although good, is not nearly enough for serious mutimedia processing like you mentioned. If I didn't already have a D2.5, I'd buy a Quad now - why wait.

My iMac won't turn on after power flickered on and off.Is there a virus affecting it or is something ele wrong?

My iMac was on and we had our power flicker because of weather conditions. I know the computer is hooked up right because it was on before the power went out.So my question is, is there a virus that had been affecting it before it went off, or is the power button broken? And any help would be appreciated because we have just taken in other computers for various reasons and we really don't want to take it in again.Oh, and i don't really know which model it is. Thanks

Try an SMC reset - sometimes works:
SMC Reset
Shut down the computer.
Unplug the computer’s power cord and disconnect peripherals (printers/external hard disks, etc. but not keyboard or mouse).
Press and hold the power button for 5 seconds.
Release the power button.
Attach the computers power cable.
Press the power button to turn on the computer.

My 2002 iMac G4 went down during a power out. won't restart. tried PMU button. still won't start. fan tries for 2 sec, but nothing

my 2002 iMac G4 went down during a power out. won't restart. tried PMU button. still won't start. fan tries for 2 sec, but nothing

Sometimes, a replacement 1/2AA 3.6v Lithium PRAM battery is needed
to get these back into running form again, because these are one-way
and without a good battery, settings are lost or easily corrupted. And this
can contribute to a failure on startup. But that is a troubleshooting measure.
There could be another issue, although these batteries are only good for a
few years at best. A power outage can revoke any remainder of life out of it.
The take-apart is tedious and detail oriented; to include new thermal paste
and some special tools, good instructions, and some skills also help. An
ifixit.com repair guide may help; as would an original Apple Service guide.
Not sure if other good online guides or youtube video are available.
If the power supply is bad, the test ports near the PMU button are where
to check; with multi-meter. Also power supplies may develop issues with
bad capacitors. They have been said to be repairable.
Here's an older site with a take-apart from some time ago.
http://www.xlr8yourmac.com/systems/imac_g4/imacg4_takeapart.html
This also would be a good time to consider, if battery is all that's failed, to
upgrade the internal RAM chip, or upgrade the hard disk drive, etc.
Good luck & happy computing!
edited

How can I run Power processor applicationson Intel systems?

I have Photshop CS which ran perfectly well on my old Power desktop and Intel Mac book Pro. Since upgrading to OS10.8.5, I cannot use it on either. Is there a fix or a translator that will let me run CS on this updated OS?

Although some posters on this forum like to pretend that a solution without upgrading doesn't exist (excuse the double negative):
If you do not want to upgrade you could always run Photoshop CS in Snow Leopard Server ($20) installed into Parallels ($79 or less) for use in Lion/Mt. Lion/Mavericks:
[click on image to enlarge]
More information here:
http://forums.macrumors.com/showthread.php?t=1365439

After reading and resetting everything with Keyboard I still get blank white screen. The only way I can boot to Mavericks is unplug power cord, push and hold power button while plugging power cord in. Fans at full speed.

After reading and resetting everything with Keyboard I still get blank white screen on 2nd? page of boot. The only way I can boot to Mavericks is unplug power cord, push and hold power button while plugging power cord in. Fans run at full speed, machine boots then runs normal except the dvdrw will not . The mid 2011 IMAC had the same problem with LION. I changed hard drives, formatted, and installed a clean install of latest os x mavericks. Any help would be greatly appreciated.
EtreCheck version: 1.9.15 (52)
Report generated August 30, 2014 at 6:56:41 PM EDT
Hardware Information: ?
    iMac (21.5-inch, Mid 2011) (Verified)
    iMac - model: iMac12,1
    1 2.5 GHz Intel Core i5 CPU: 4 cores
    4 GB RAM
Video Information: ?
    AMD Radeon HD 6750M - VRAM: 512 MB
        iMac 1920 x 1080
System Software: ?
    OS X 10.9.4 (13E28) - Uptime: 0 days 0:16:53
Disk Information: ?
    ST3120026AS disk0 : (120.03 GB)
    S.M.A.R.T. Status: Verified
        EFI (disk0s1) <not mounted>: 209.7 MB
        Untitled (disk0s2) / [Startup]: 119.17 GB (87.12 GB free)
        Recovery HD (disk0s3) <not mounted>: 650 MB
    HL-DT-STDVDRW GA32N
USB Information: ?
    Apple Inc. FaceTime HD Camera (Built-in)
    CHICONY USB NetVista Full Width Keyboard
    Apple Inc. BRCM2046 Hub
        Apple Inc. Bluetooth USB Host Controller
    Apple Computer, Inc. IR Receiver
    Apple Internal Memory Card Reader
Thunderbolt Information: ?
    Apple Inc. thunderbolt_bus
Gatekeeper: ?
    Anywhere
Kernel Extensions: ?
    [loaded]    com.nvidia.CUDA (1.1.0) Support
    [loaded]    com.sophos.kext.sav (9.0.61 - SDK 10.7) Support
    [loaded]    com.sophos.nke.swi (9.0.53 - SDK 10.8) Support
Startup Items: ?
    CUDA: Path: /System/Library/StartupItems/CUDA
    FanControlDaemon: Path: /Library/StartupItems/FanControlDaemon
Launch Daemons: ?
    [loaded]    com.adobe.fpsaud.plist Support
    [running]    com.arcsoft.eservutil.plist Support
    [running]    com.bjango.istatmenusdaemon.plist Support
    [loaded]    com.oracle.java.Helper-Tool.plist Support
    [running]    com.sophos.autoupdate.plist Support
    [running]    com.sophos.configuration.plist Support
    [running]    com.sophos.intercheck.plist Support
    [running]    com.sophos.notification.plist Support
    [running]    com.sophos.scan.plist Support
    [running]    com.sophos.sxld.plist Support
    [running]    com.sophos.webd.plist Support
Launch Agents: ?
    [running]    com.arcsoft.esinter.plist Support
    [running]    com.bjango.istatmenusagent.plist Support
    [loaded]    com.nvidia.CUDASoftwareUpdate.plist Support
    [loaded]    com.oracle.java.Java-Updater.plist Support
    [running]    com.sophos.uiserver.plist Support
User Login Items: ?
    Macs Fan Control
    Firefox
Internet Plug-ins: ?
    FlashPlayer-10.6: Version: 14.0.0.176 - SDK 10.6 Support
    Flash Player: Version: 14.0.0.176 - SDK 10.6 Support
    QuickTime Plugin: Version: 7.7.3
    JavaAppletPlugin: Version: Java 7 Update 67 Check version
    Default Browser: Version: 537 - SDK 10.9
Audio Plug-ins: ?
    BluetoothAudioPlugIn: Version: 1.0 - SDK 10.9
    AirPlay: Version: 2.0 - SDK 10.9
    AppleAVBAudio: Version: 203.2 - SDK 10.9
    iSightAudio: Version: 7.7.3 - SDK 10.9
iTunes Plug-ins: ?
    Quartz Composer Visualizer: Version: 1.4 - SDK 10.9
3rd Party Preference Panes: ?
    CUDA Preferences Support
    Fan Control Support
    Flash Player Support
    Java Support
Time Machine: ?
    Time Machine not configured!
Top Processes by CPU: ?
         1%    WindowServer
         1%    fontd
         0%    firefox
         0%    SystemUIServer
         0%    SophosWebIntelligence
Top Processes by Memory: ?
    229 MB    firefox
    156 MB    SophosScanD
    152 MB    InterCheck
    131 MB    com.apple.IconServicesAgent
    115 MB    SophosAntiVirus
Virtual Memory Information: ?
    424 MB    Free RAM
    1.53 GB    Active RAM
    1.37 GB    Inactive RAM
    699 MB    Wired RAM
    1.26 GB    Page-ins
    0 B    Page-outs

I'd start by getting rid of the following software responsible for these extensions.
Kernel Extensions: ?
    [loaded]    com.nvidia.CUDA (1.1.0) Support
    [loaded]    com.sophos.kext.sav (9.0.61 - SDK 10.7) Support
    [loaded]    com.sophos.nke.swi (9.0.53 - SDK 10.8) Support
Startup Items: ?
    CUDA: Path: /System/Library/StartupItems/CUDA
    FanControlDaemon: Path: /Library/StartupItems/FanControlDaemon
Use the uninstaller provided with the Sophos software. You can uninstall CUDA via the preference pane. Be sure you remove the com.nvidia.CUDA extension which is located in the /System/Library/Extensions/ folder. Not sure if Fan Control has an uninstaller so you will have to do it manually:
Uninstalling Software: The Basics
Most OS X applications are completely self-contained "packages" that can be uninstalled by simply dragging the application to the Trash. Applications may create preference files that are stored in the /Home/Library/Preferences/ folder. Although they do nothing once you delete the associated application, they do take up some disk space. If you want you can look for them in the above location and delete them, too.
Some applications may install an uninstaller program that can be used to remove the application. In some cases the uninstaller may be part of the application's installer, and is invoked by clicking on a Customize button that will appear during the install process.
Some applications may install components in the /Home/Library/Applications Support/ folder. You can also check there to see if the application has created a folder. You can also delete the folder that's in the Applications Support folder. Again, they don't do anything but take up disk space once the application is trashed.
Some applications may install a startupitem or a Log In item. Startupitems are usually installed in the /Library/StartupItems/ folder and less often in the /Home/Library/StartupItems/ folder. Log In Items are set in the Accounts preferences. Open System Preferences, click on the Accounts icon, then click on the LogIn Items tab. Locate the item in the list for the application you want to remove and click on the "-" button to delete it from the list.
Some software use startup daemons or agents that are a new feature of the OS. Look for them in /Library/LaunchAgents/ and /Library/LaunchDaemons/ or in /Home/Library/LaunchAgents/.
If an application installs any other files the best way to track them down is to do a Finder search using the application name or the developer name as the search term. Unfortunately Spotlight will not look in certain folders by default. You can modify Spotlight's behavior or use a third-party search utility, EasyFind, instead.
Some applications install a receipt in the /Library/Receipts/ folder. Usually with the same name as the program or the developer. The item generally has a ".pkg" extension. Be sure you also delete this item as some programs use it to determine if it's already installed.
There are many utilities that can uninstall applications. Here is a selection:
    1. AppZapper
    2. AppDelete
    3. Automaton
    4. Hazel
    5. AppCleaner
    6. CleanApp
    7. iTrash
    8. Amnesia
    9. Uninstaller
10. Spring Cleaning
For more information visit The XLab FAQs and read the FAQ on removing software.
Be sure to remove your two Login Items. Finally do this:
Reinstall Lion, Mountain Lion, or Mavericks without erasing drive
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu.
Reinstall Lion, Mountain Lion, or Mavericks
OS X Mavericks- Reinstall OS X
OS X Mountain Lion- Reinstall OS X
OS X Lion- Reinstall Mac OS X
     Note: You will need an active Internet connection. I suggest using Ethernet
                 if possible because it is three times faster than wireless.

Late 2006, 24" iMac, 2.16 GHz Intel Core 2 Duo, Screen Tearing, nVidia 7300

Late 2006, 24" iMac, 2.16 GHz Intel Core 2 Duo, Screen Tearing, nVidia GeForce 7300 GT
I've been suffering with screen tearing, strange screen artifacts, etc. followed by totally unresponsive freezing machine. This problem has been happening to varying degrees for about 9 months.
Initially, I was able to mitigate the problem with smcFanControl. Adjusting the fan speeds up helped keep the machine cool and prevent most of the problems. Recently, we had a spell of hot weather and the fans just couldn't keep up. If you're using smcFanControl, the only fan speed that will really make a difference is the CPU. The HDD and ODD fans are no where near where you need to be.
Also, I noticed the machine was LOADED with dust when I took it apart, including the heat sink fins for the graphics card. If you have time you may want to try a thorough cleaning with compressed air before replacing the card. You just need to get through step 11 below to access what needs to be cleaned.
Based on reading here and elsewhere, I decided to replace the graphics card. To give back to the community, I wanted to layout the procedure here.
Where to buy the card.
I had a tough time finding the right card. I did exhaustive web searches using every combination of Nvidia 7300 GT, etc. You really need to search by the Apple part number. The Apple part number for the 7300 is 661-4179, for the 7600 is 661-4180. I found it at We Love Macs and at dv warehouse. I paid $248 for the 7600 card. BE CAREFUL buying a "refurbished" card. At We Love Macs, "refurbished" means "pulled from a working machine." Could be a machine with as much mileage as yours and prone to the same failures as yours.
The card is an MXM style card (generally used in laptops), with a giant heat sink and heat pipe assembly. The card can be separated from the heat assembly. If I had time, I would have tried a generic, cheaper, MXM style card.
Dispelling some myths.
People here and elsewhere have said that no Intel iMac graphics card can be replaced, that they're wedded to the logic board. The one on this model can be replaced, I've done it. Also, some have said you can't put a 256MB, 7600 GT in a machine that originally had a 128MB, 7300 GT. Also, not true. That's what I did to my machine and it is working fine.
So here's the step-by-step on my take apart and replacement. This is just one user helping out others, PROCEED AT YOUR OWN RISK. This takes about 1 1/2 hours to take apart and 2 to put back together.
1) Install the iStat widget and monitor the temperatures for a few days and record the temperatures (to be able to verify that you've completed the repair correctly later)
2) Make a complete backup of any internal drives and make sure the backup will be bootable (use Carbon Copy Cloner)
3) Use static safeguards, like an anti-static wrist strap.
4) Get a pen a stack of paper envelopes to separate and label the pieces you remove.
5) You'll need a #2 Phillips and #6, 8, 10 Torx. Most of the screws at T-8. There are 8 T-8s holding in the LCD. They are about 1 1/4" down. You'll need a long, skinny T-8 to get to them.
6) Unplug the machine and place it on it's back on a large white sheet of paper.
7) Remove RAM door, #2 Phillips x 2.
8) Remove RAM.
9) Remove 4 T-8s along bottom.
10) Remove large rectangular plastic bezel surrounding the whole machine, tilt up being careful not to stress iSight cables. You'll need to stretch it around the RAM ejection tabs.
11) Detach two iSight cables.
12) Peel up black stickers at lower corners of speakers.
13) Remove LCD screws, T-8 x 8. You'll probably drop a few, they're pretty easy to retrieve.
14) Remove two retaining screws on right hand LCD cable. T-6 x2 Unplug cable.
15) Remove LCD inverter cable, left.
Start unplugging everything connected to the logic board.
16) Unplug cable coming from right speaker. Note cable routing, it is the only one that's a little tricky. Also, note that the left one plugs in on the right and the right one plugs in on the left.
17) Remove right speaker. T-10 Note: left and right speaker screws are different lengths.
18) Unplug ODD blower cable.
19) Unplug ODD temp cable and lift up tape.
20) Unplug HDD blower cable.
21) Unplug the HDD temp cable.
22) Unplug the camera & mic cable.
23) Unscrew the clamp securing the airport card (has two antenna cables going to it, labeled PCIE), T-6 x 2
24) Unplug SATA cable.
25) Unplug Power cable, thick black plastic harness, socket.
26) Unplug ambient temp cable.
27) Unscrew the bluetooth card, T-6 x2.
28) Unplug CPU blower cable.
29) Unplug power button cable, 2 conductor, tiny cable, bottom left.
30) Unscrew left speaker, T-10, unplug from logic board, but just leave dangling, no need to remove cable completely.
31) Remove logic board screws. Be careful to segregate them, they have different lengths and different thread pitches and thread types. Some are machine threads going into metal, some are coarse threads going into plastic.
31a) Heat sink frame to case T-8 x 2
31b) Logic board to case T-8 x 3
31c) Screws at heat sink from arrows T-8 x 3
31d) Logic board at bottom near RAM slot T-8 x 2
32) Unplug ODD ribbon cable, be extremely careful. Release plastic tabs on each side to remove. Be sure it is fully and evenly seated when you put it back later.
33) Remove logic board with attached graphics card.
34) Unplug graphics card heat sink sensor cable at logic board end.
35) Remove graphics card screws 2 x T-6
36) Remove single screw from small L-bracket on heat sink 1 x T-6
Replace graphics card.
Reverse steps, put everything back together. Be careful not to pinch any cables when you put the logic board back in. When you put the LCD back in, if you can't get the screws to stick to the driver, use some glue stick from your kid's art supplies.
Test everything. You've disconnected every cable in the system.
I torture tested the machine with a space heater on it, 5 different videos of different formats playing at the same time. No problems.
I took a lot of pictures inside the machine. As soon as I figure out a good place to post them, I'll try to add a link here.
Good luck

Greetings!
I know this is in response to your post from over 2 years ago, but I was searching on Apple's site for the max RAM I can upgrade our refurbed iMac too and it brought up this post.
Oddly enough, I've been experiencing the same screen issues and locking up on our iMac as well. I was concerned it was a bad memory module, but now I'm thinking it's the same issue you found.
Because I bought this some 2+ years ago refurbed from Apple, I'm not surprised that could be the actual problem.
I've pulled this thing apart once to replace the hard-drive which crashed about a year ago, so I'm not totally new at opening it up, but this process you mention sounds pretty intense...just wanted to get your second take on it based on what I've done thus far.
Thanks again for posting this...it was an eye-opener as to what might actually be causing our issues with our trusted Mac!
Joshua

24" iMac 2.8 GHz Intel Core 2 Duo OSX v10.5.5 shutdown anomaly

24” iMac 2.8 GHz Intel Core 2 Duo OSX v10.5.5 shutdown anomaly.
I have noticed that when my iMac shuts down, that initially the One Touch Maxtor 500G external drive attached to the Firewire 400 port also does a normal shut down and the light goes out. Within a few minutes I noticed that the light on the Maxtor is back on. This does not happen if I use the USB 2.0 port to attach the hard drive and iMac, the light remains off. It appears the FW400 port is active on the iMac even when in the shutdown mode. Is this normal for the iMac?
I have a 13”MacBook 2.4Ghz that does not activate the same Maxtor when it is shut down. I attach it via the MacBook FW400 port.
I hooked up an ammeter to the iMac power cord and observed the following readings:
iMac Off: From 50ma to 180ma cycling at a rate of 23 cycles/minute. Using the Watt reading of the instrument, that amounted to cycling from 0 to 5 Watts every 4 seconds.
iMac On: 1.16A 133Watts.
This iMac is new, 1 week old, does it need repair or is this normal? I like to get a FW800 external HD, but I don’t want to have to pull the plug on the HD every time I want to shut down.

Since my Maxtor doesn’t have an on/off switch, the best solution I have found so far is to use my APC 750 UPS to turn off the Maxtor. This APC has a master plug outlet that senses when the computer is turned on and activates 3 other outlets to on when the computer reaches a certain power threshold. Fortunately the threshold is user adjustable and I was able to set it high enough to turn off the Maxtor outlet when the iMac is off.
Is anyone else able to confirm that their iMac cycles like mine does, or is this peculiar to my iMac? It is new and if it needs service I would prefer to do it while it is covered under warranty.

IMac Power PC or intel? 20in Power PC at Costco for $1479 w/3 yrs Aplecare!

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