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Measuring iMac G5 Power Supply Voltages at Connector Plug Pinouts

Exactly how how do you test the PSUs out on the Apple iMacs?
How do you turn on the iMac G5 power supply when it is out of the computer?

Apple iMac G5 PSU With Ambient Light Sensor Shown

iMac G5 Power Supply Connector – P-1

1. +3.3 – BlackA3B 12. +3.3 – BlackA3B
2. +3.3 – BlackA3B 13. +12v – BrownA3B
3. GND – BlackB4B 14. GND – BlackC4B
4. +5v Gray/PurpleA4B 15. On/Off – Gray
5. GND – BlackB4B 16. GND – BlackC4B
6. +5v Gray/PurpleA4B 17. GND – BlackC4B
7. GND – BlackB4B 18. GND – BlackC4B
8. PG – Blue 19. +12v – BrownA3B
9. +5.1Vsb – Purple 20. +5v Gray/PurpleA4B
10. +12v – BrownA3B 21. +5v Gray/PurpleA4B
11. GND 22. +20v to +24v – Brown

On the 661-3350 power supply and other similar PSUs, jumper pins 15 (gray) and 16 (blk/gnd) to turn the PSU on. With the power supply on, you should be able to read all the voltages listed above with a DC voltage meter. Between pins 22 and any GND pin, you will see approx. 24vdc, which is needed for the backlight. Note that the plug is numbered on the wired side of the connector plug. You will need some good lighting to see the numbers. A small flashlight comes in handy.

When reading the voltages, it is always a good idea to check both for proper DC voltage and AC ripple voltage. Note that AC ripple voltage should be relatively low, in the low millivolts range.

From what I have been able to tell, the other power supplies in both the 17 inch and 20 inch iMac G5 computers all used the same DC voltage pinout arrangements. I believe the voltage pinout arrangements are different on the Intel iMacs and some of the iSight models.

Please read the detailed information for repairing Apple iMac G5 power supplies.

Note: (see below) PG acronym represents the Power Good signal and VSB represents Voltage Stand By.

If anyone has more information for any of the power supplies (especially the pinout arrangement) used on the various Mac products, including the various PPC models, Intel Models, and Power Macs, please send it to me.

Thank you.



Voltage pinouts listed above cover the following Apple PSUs: 661-3350, 661-3289, 614-0353, 614-0296, 614-0297, 661-3351 614-0923 614-0352 614-0294, AP14P46, 614-0398, 614-0326, 661-3625, 661-3289, AP13PC97, 614-0279, DPS-180SB, A 614-0334, 614-0366, 614-0326, 614-0398, 614-0353, 614-0328, 661-3627, 614-0327, 614-0325, 614-0365, 614-0329, 661-3290, Manufacturer Part Numbers: DPS-180QB-1A Rev 01, API4PC47, DPS-180QB ) along with a number of different manufacturers including: (ACBel API3PC96 – Celetronix Q45B – NPFC), and additional manufacturers’ part numbers than what is listed here.

PG acronym represents the Power Good signal. I found the following reference in the Wikipedia when it comes to talking about power supplies in general for PCs.

In addition to the voltages and currents that a computer needs to operate, power supplies also provide a signal called the Power-Good signal, sometimes written as Power_OK or Power_Good or you can distinguish it by its gray color. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present at startup, the CPU is held in reset state. If a Power-Good signal goes down during operation the CPU will shutdown. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself.

The ATX specification defines the Power-Good signal as a +5 volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard, where it is received by the processor timer chip that controls the reset line to the processor.

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Category: Apple, G5, iMac, iMac G5, PSU
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  • Mike Purdy says:

    Hi Jim, I have a first generation G5 that fell in to the Apple extension program and they fixed it some time ago I believe by replacing the mother board. 2-3 years on and I have been having video problems and now the unit is dead, well not quite dead as the first two test LED’s come on but not the 3rd nor do the fans work, the disk does power up but no video. I have been following your excellent article and have now replaced all the caps on the mother board but unfortunately that didn’t fix it. I have read on the net that if the 2nd led is on then the power supply is OK. So before I have a go at replacing caps on the PS have you any tips regarding the the symptoms. Incidentally when I tested the Caps I removed from the MB they all tested ok and all within there tolerances. Thanks again for your well documented articles on the G5 I will now have a go and test the voltages from the Power supply. One more thing, do you ship the Cap kits to UK? I have had great difficulty in sourcing the correct sizes etc.


    March 27, 2010 at 11:06 AM
  • Jim Warholic says:

    One of the aspects about a power supply being arbitrarily labeled good with just indicator LEDs coming on is that folks can be easily fooled into a type of go no go indicator.

    First off, there are numerous voltages that could be partially faulty, through excessive AC ripple, because of bad caps, that can directly affect various areas of the iMac, and still allow it to somewhat boot up.

    I would personally do a physical inspection of all the caps inside the power supply itself, and also do a very close inspection of the logic board capacitors. Look for any bad caps with even a slight amount of bulging. Sometimes it is very subtle on the bad caps themselves. There might only be a slight bulging top on a bad cap. Additionally, folks are starting to find that many of the bad caps are not even showing physical signs of being bad. Some folks have replaced the caps on the MOB and low and behold, their iMac now works. If you have tried everything else, then this really is about the only option left prior to just giving it away, or selling it on Craigslist or eBay for iMac G5 parts.

    As far as measuring the caps are concerned; once the caps are removed from the MOB, it is somewhat of a static test even if you are using a capacitance or ESR meter. The reason is that the bad caps can severely break down under heat and loaded conditions, and might check somewhat OK in a static mode, out of the circuit, with no voltage or ripple current applied. By the way, because a power supply can be putting out more ripple current under load, this can directly affect capacitors throughout the computer. For example, a defective power supply can weaken the caps on the logic card, and bad capacitors on the logic card can weaken the power supply. It is the law of the weak link in the chain that breaks first. That is why, it is important to verify both the PSU and the MOB have good capacitors.

    I suggest you take a look at some of the other articles I have written in regards to the iMacs for more detailed information. Follow my Apple link for the details.

    As far as the measurements on these voltages with the PSU out of the box are concerned, they are not under loaded conditions. A flaky power supply can act up under load and produce a lot more ripple, and reduce the voltage below the operation threshold. I have been looking for a power supply tester for an iMac; one that will measure the voltages under loaded conditions, but as yet have not found one. There are numerous PC PSU testers to be found, but none for the Macs that I know of. If anyone knows of one, let me know.

    Yes, I do ship to the UK and numerous international locations. Visit my ecommerce website at J West Sales for the Apple inside you, for more information on international shipping.

    March 27, 2010 at 4:22 PM
  • Athanasios says:

    Hi Jim, I replaced the caps I got from you. The Comp worked for about an hour then shut off and would not go on. I then came here and checked the pins and I only have voltage on the 24 line and the 5 volt line on pin 5. What should I check next? Diodes, rectifiers etc. I have an SMD re work station so this type of work is not too hard for me. I did use the heat gun to get some of the caps out as it was easier to do that way. Wonder if that hurt anything?


    June 29, 2010 at 10:37 AM
  • Jim Warholic says:

    Hello Athansios,

    It is possible that something else is blown out in the power supply. I know that there are a number of surface mount devices on the bottom of the power supply board, and it probably is a bad idea to use a heat gun to get the caps out. Note that I think you can do a better job with direct heat from the tip of a soldering iron, than you can do with the heat gun. The solder melts relatively easy with a soldering iron on the PSU. It’s a different story on the MOB. So, yes, it is possible something else is hurt by the use of it. At this point in time, it might be time for a new PSU.

    Write to be directly if you want to know where I recommend buying a new power supply.



    June 29, 2010 at 2:57 PM
  • Geoff Rogers says:

    First of all, thank you for all the info on the bad caps. My PSU went out Sept. ’09 and I was soon up and running with a new PSU that I installed. Lately my Imac G5 has been turning off, erratically at first, now within a minute of it booting up. There is a click or pop and it is dead. I pulled the new PSU and checked the pin outs and they all had the correct voltage. I left it powered up for a couple hours, the case was only warm, and the voltages were still ok. I reinstalled the PSU and the problem is still there. Is there anything else I can check, or is the next step changing the caps on the motherboard and seeing if that works?

    July 6, 2010 at 5:28 PM
  • Jim Warholic says:

    Hello Geoff,

    Many times the PSU is only the tip of the heat shut down problem. Sometimes the motherboard caps are bad, even though there might not be any visual signs of failure. Here is a recent article I published titled Fat Caps and Ripple Current. The article is an email conversation I had with an expert electronics designer about underrated capacitors and the excessive ripple current that results from poor electronics engineering designs in a wide array of consumer and commercial electronics equipment.

    Unfortunately, you might be in a situation where the parts failure cascading effect has resulted in the relatively new power supply, now being pushed to the limit because of weak capacitors on the MOB, and thus you may have a “new” bad power supply too. The popping sound is probably coming from the PSU.

    I believe that many folks have experienced the Apple iMac G5 merry-go-round failure effect over the years with having their iMacs repaired by Apple. I have heard numerous stories (actually many have written directly to me) over the years, about how Apple Genius Bar folks have first replaced the motherboard, then months later the power supply was replaced, then again months later the motherboard again. This story has been repeated over and over, and one has to wonder, what’s wrong with the design? Is it bad caps, or is it a bad design, or is it Apple squeezing the maximum amount of dollars from each unit they sell by having the designer save a penny here and a penny there, just long enough to get it past the warranty period? The article Fat Caps and Ripple Current addresses some of these issues and is worth reading.

    As far as where you currently are at with your iMac G5, you might want to read some of the other articles about the iMac computers.

    Below are a listing of the articles for Apple iMac G5 repairs and capacitor replacements:

    *Apple iMac G5 Logic Card (Mother Board) Repair and Capacitor Locations

    *Apple iMac G5 Power Supply Repairs

    *iSight Logic Board Capacitor Locations

    *Disassembly Procedure for iMac G5 First and Second Generation iMacs

    *Measuring the DC Voltages on the iMac G5 Power Supply Pinouts: (You are here)

    *Comparisons of the Different PSU Types Used in iMac G5 Computers

    *Hard Drive Replacements

    *Hard Disk Drive USB Adapter For Mac and PC

    *Add a Second Monitor to Your iMac G5 Computer – Extend the Desktop

    *Soldering Tips for Lead-Free Soldering

    *Installing Capacitors on MOB and PSU Tips and Helps

    I hope that helps everyone out a bit.



    July 6, 2010 at 8:03 PM
  • Geoff Rogers says:

    Thanks for the reply. I had already started reading the other articles on repairing the motherboard. It is good to have a concise listing of all the articles so I don’t miss one. “Fat Caps and Ripple Current” is definitely worth reading to understand what is going on. I am glad you pointed out the cascading effect because I was just looking it at as the PSU or the MOB and not the effect of one on the other, and that is key. I’ll read it all twice before I get started.
    My rant is about “Just in time manufacturing” since I constantly see signs of “Junk in time” and the cascade effect of bad components being shipped and turning millions of finished products into junk.

    July 7, 2010 at 9:14 AM
  • Glen says:

    Hi Jim,

    Are you sure that the pin 15-16 power supply activation test is safe and proper to run on all 661-3350 power supplies? The last unit I tried to activate (supposedly new) began making a sizzling sound when I hooked it up. The power supply I tried before that, had just had a capacitor replacement and did the exact same thing.

    July 15, 2010 at 2:53 AM
  • Jim Warholic says:

    I have had many folks confirm this before I posted it.

    Yes, I am sure that pin 15 and 16 are proper to turn on the power supply. I tested it in the past, but just to make sure, I decided to repair one on the bench that I have as a spare dead iMac G5 for trouble shooting purposes just now. When I first opened the iMac, I found 7 blown caps on the MOB and then opened up the PSU and found 6 additional bulging caps in there too. I tried jumpering across pins 15 and 16 and nothing happened. Then, I replaced the total caps in the PSU and then tried the jumper test again. Success for the PSU repair! The power supply turned on, using the jumper technique, and I measured all the voltages using a multimeter, set on DC measurements.

    Once again, the numbers on the pins are located on the wired side of the connector. So, you need to verify that you are on the correct pins before you hook up your jumper. If you are not careful, it is easy to get the pin numbers mixed up. Also, if you trace pin 15, the gray wire, you will see that it goes to the PSU circuit board and the circuit board is labeled on/off right at the point where the wire enters the circuit board. This is a good way of making sure you are on the correct pin, though you would need to open up the case of the power supply in order to verify this.

    July 15, 2010 at 9:25 AM
  • Adam H says:

    Hi Jim,

    First, thanks for this amazing info you’ve got here for PSU troubleshooting. I’m beginning to test a 661-3350 PSU and am going to start by checking for the correct pin/voltage relationships. I’m looking at the pinout you’ve got labeled for this supply and everywhere that you’ve got the label “gray” or “gray/purple” I’ve got a brown. Everything seems to jive as far as the pattern of colors it’s only the gray that seems to be replaced with brown. Maybe they ran out of gray wire that day in the factory?? Any thoughts?

    Thanks in advance.

    July 26, 2010 at 4:37 PM
  • Jim Warholic says:

    The main thing is to verify the pins for the jumper.

    As far as the colors are concerned, I guess it is possible that the gray wires are now brown on some of the PSUs. Once again, the voltages should still check the same between the appropriate pins.

    July 26, 2010 at 5:46 PM
  • Dave K says:

    Hi Jim, Excuse my stupidity, but what does the PG mean on pin number 8? I’m getting around 5 volts on pin 8.


    August 24, 2010 at 2:23 PM
  • Jim Warholic says:

    Hi Dave,

    In this case, I believe the PG acronym represents the Power Good signal. I found the following reference in the Wikipedia when it comes to talking about power supplies in general for PCs.

    In addition to the voltages and currents that a computer needs to operate, power supplies also provide a signal called the Power-Good signal, sometimes written as Power_OK or Power_Good or you can distinguish it by its gray color. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present at startup, the CPU is held in reset state. If a Power-Good signal goes down during operation the CPU will shutdown. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself.

    The ATX specification defines the Power-Good signal as a +5 volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard, where it is received by the processor timer chip that controls the reset line to the processor.

    So, it sounds like your PSU is saying that the voltages are good, however, I suspect you are measuring this under no load conditions.

    August 24, 2010 at 4:54 PM
  • Cid says:

    Hello Jim.

    Cheers for all your hard work on the website.

    I have replace the caps in a couple of G5′s and managed to get working units for my children.

    I have picked up another G5 and I’m pretty sure that the logic board is working fine as it was a good unit that a customer salvaged the working PSU from.

    I have another couple of PSU’s on hand which I’m trying to get running with limited luck.

    One of the PSUs is a DD type PSU. It was only putting out 24VDC on pin 24. No other VDC on any of the other pins. It did have a couple of popped caps, which I replaced, however the unit still won’t turn on when I jumper pins 15 and 16. I think maybe this PSU is a dead…

    I have another PSU which is behaving a little weird. It will power on fine but after approx 60-90 seconds it will power off completely. So that none of the diagnostic lights on the logic board are lit. If I take of the mains and leave it for 60 seconds, then plug it back in, the unit will fire up again for approx 60-90 seconds.

    This PSU unit doesn’t look like any on your website.
    The sticker reads Delta Electronics INC
    Model DPS-180QB A
    Apple PN 614-0329

    I’m half thinking this might be a replacement unit as opposed to an original Apple part.

    None of the caps look bad, so I’m not sure if there is anything i can do to fix it. It’s a little frustrating as the unit does power on. It seems as if the load on the unit causes it to trip out.

    Any ideas or comments would be welcome. Otherwise I will wait for another salvage G5 to come my way.


    September 16, 2010 at 12:57 PM
  • Jim Warholic says:

    Hello Cid,

    Thank you for the comments about the website.

    I was thinking about your missing voltages problem child PSU, and think that there is probably something else blown out completely, closer to the front end of the PSU. There are all types of other components that could get taken out when capacitors become so bad that it draws excessive current and causes a loading effect internally. This is why it is always a good idea to replace bad caps as soon as they become visually apparent instead of waiting for the entire PSU to die completely.

    The other PSU probably has one of the surface mount transistors or control chips blown out. I’d be curious if the PSU will power up by itself, and stay on under no load. It would be interesting to note what the voltages are in this condition.

    You also might be able to check the surface mount transistors on the bottom of the PCB and see if any of the emitter base or base collector junctions are shorted or open. If you find any that do not compare the same as the others, then you might be able to replace it from one power supply to the other. Just look for the same p/n on the surface mount transistor, though you might need a magnifying glass to read it.



    September 16, 2010 at 10:25 PM
  • Cid says:

    Hello Jim.

    Cheers for the swift reply and advice.
    Very decent of you to help people in this fashion.

    The power supply which doesn’t work at all will go into my junk electronics box from where i salvage bits if required.

    The other PSU I will test the voltages when not under load and see if it still cuts out after 60-90 seconds and then post back with the results.

    Cheers once again…..


    September 17, 2010 at 3:59 AM
  • al says:

    Hi Jim,
    Well I have a 20″ iMac G5 2.1ghz 3rd which would turn on but not display anything but just the white corner light. I removed the logic board and found 2 popped caps. I replaced both of them, put everything back together and now it won’t turn on at all. The four leds are not even lit. I plugged the iMac without the logic board installed and measured the pw supply but everything checks out. I can even hear the hard drive. did I miss something here?

    September 20, 2010 at 2:56 PM
  • Jim Warholic says:

    Well, first off, I can pretty much guarantee that you have other bad caps besides the ones that you replaced on the MOB that were bulging. Second off, you might have missed something on the cabling when putting it back together. Third off, you didn’t mention anything about checking the capacitors in the PSU itself.

    Now, it is possible that just replacing the visually bad caps on the MOB has now progressed to the next weak link. I always recommend replacing the entire two types and banks of capacitors on the MOB. You simply can not be sure the other ones are good and will forever be second guessing yourself as to are they or aren’t they good caps.

    By the way, if you are hearing the hard drive, then something is turning on. Maybe you need to reset the PRAM and the SMU. It could be the PSU is faulty under loaded conditions too.

    September 20, 2010 at 3:37 PM
  • Diskyro says:


    Does anyone have a schematic or a pin out diagram of a power supply PA-3241-02A1 from a IMAC A1225; I want to test and repair that power supply and for that I need a schematic or at least a pin out to identify the power on pin.Thanks

    October 27, 2010 at 7:08 AM
  • jenni porter says:

    Jim, You are by far one of the kindest and knowledgeable guys out there. Can you answer a couple of questions for troubleshooting my bad iMac G5 power supply?

    I got the thing out and jumpered the pins. I started testing with my multimeter between pins 11 (gnd) and 22 (24v). I got the 21.3 volt reading. How do I test the other pins? With one end at pin 22 or at ground?

    When I test between pin 22 and any other pin I get the same 21.3 v reading. When I test between 11 and any other pin I get zero. Could you explain a little more on how to test for voltage on each of the pins?

    Thanks and keep up the good work.


    November 16, 2010 at 11:30 AM
  • Jim Warholic says:

    Hi Jenni,

    Basically, once the power supply is turned on, by jumpering the appropriate pins [15 (gray) and 16 (blk/gnd)] as outlined above, then you would proceed to measure the voltages from ground pins to the voltage pins using the multimeter set on DC. You could also check to see if there is any AC ripple on each of the voltage pins referenced to ground. To do this measurement simply set your meter from DC to AC readings. If there is excessive ripple, it will show up as an AC voltage reading on the DC pinout.

    Hope that helps explain it a bit more.

    November 16, 2010 at 2:04 PM
  • jinni porter says:

    Thanks for the response. You’re confirming that I was doing it right, I’m just not getting any readings except between the 24v pin and ground.

    All the grounds are common so it doesn’t matter which one is probed with the meter right? The only voltage read is from ground to pin 11 at 21 volts. I thought that I was doing something wrong because measuring between a ground and any other pin has no voltage.

    Do you have any ideas on where to start looking to fix with these readings?

    Thanks again,


    November 16, 2010 at 4:08 PM
  • Jim Warholic says:

    I believe all the grounds are the same. You can verify that with the PSU unplugged and see if the pins are common with one another. I also certainly recommend opening up the PSU and inspect the capacitors for any signs of bulging as can be seen in my PSU article. If the caps do not show any signs of being bad, and still the voltages are missing on some of the pins, and you do have voltages on other pins, which means the internal fuse on the circuit board is good, then it is most likely another physical component that is bad. Note that there have never been any schematics published online for the PSUs, which makes it a bit difficult to troubleshoot down to a component level.

    November 16, 2010 at 5:40 PM
  • Ted Hammer says:

    Hi Jim,

    First of all, compliments on the amazing work and info on iMac G5 caps and PS issues, diagnostic and fixes.

    I have read a fair amount of the relative info on your site and am working on a dead iMac.

    It had the motherboard replaced a few years back, then started getting flaky a few months ago.

    Shuts off by itself. Intermittent start up, but would always eventually, until….
    Two days ago power in my house went off for a short while now it wont start up. Outlet has power :^).

    The main board looks good (caps). I yanked the PS and opened it. Numerous bulging caps with a couple with small indication of brown guts leaking.

    Here’s the question:
    Apple site says that if first indicator light (LED) on board light up, the PS is OK, if the second does not the board is bad. Is it in your experience possible that the PS indicator light can be illuminated but the power supply is bad causing the system to not start up?

    I’m considering delving into replacing the PS caps if it seems like the path to resolution.

    Thanks very much in advance for your response,

    Ted (SJ California)

    April 12, 2011 at 1:16 PM
  • Jim Warholic says:

    Hello Ted,

    The Apple trouble shooting guide trips up a lot of people in this area of the power supply and logic board diagnosis.

    The first LED is only an indicator of trickle voltage getting through from the PSU to the MOB. The Apple trouble shooting guide is off quite a bit on their flow chart in regards to this issue with LED number 1 as it relates to the power supply unit.

    If LED number 1 is lit, AND the PSU capacitors are bulging, which means the power supply is bad, then the power supply is a good candidate for capacitor replacements to fix the problem. However, if LED #1 does not light when plugged in, then something else in addition to the capacitors is probably blown on the PSU. Some folks have trouble shot down to either a blown inline fuse (soldered in to the circuit board of the PSU), or there are some surface mount transistors on the bottom of the board which have been known to go out. I do not have the spec on the transistors, but I know some folks have taken two PSUs and made one good one by replacing the surface mount transistor from one to the other.

    Hope that clarifies the number one LED being on or off on the MOB when the PSU is plugged in.



    April 12, 2011 at 2:11 PM
  • Ben says:

    Hi love the site thank you, but…my problem is as follows-if you can help?
    Second hand g5 imac 2ghz 20″ told it shut down many times then gave up.
    I had it chime twice, but then nothing. Opened it up. Always have light1. Sometimes 2 would flicker but not and more. Got it to chime once since opened.

    Psu…1 obvious dead cap. Replaced about 5 of them so far, very hard to get out….all caps in the MOB look perfect. Result

    Light1 always on….never 2 anymore (btw lcd pulses sometimes.)

    Testing voltages on PSU. Sometimes when jumped get results but very off from the list. ad it seems the grounds yield very different results too.

    So there it light scorching inside thePSU im sure this is the problem. Surely if it chimed ever, (fans spun, HDD spun then nothing) then the MOB is good? Please set me straight as everything in the PSU now looks good.

    July 8, 2011 at 4:11 PM
  • Jim Warholic says:

    @ Ben

    It is always a good idea to replace all the caps that are included in the PSU capacitor kits. Just because you do not see any others bulging, does not mean the other caps are good.

    It is possible you have something more than blown caps as being the problem on the PSU. The fact the voltages are bad is a sign of a faulty PSU. If you are not careful when soldering on the bottom of the PSU circuit board, it is possible that now, one of the other components is damaged. I would inspect your soldering work carefully to make sure there are no solder bridges or cold solder joints.

    August 6, 2011 at 9:51 AM
  • nj says:

    Hi Jim, great site and many thanks of reputing time into it and responding to everybody as it helps build up a bigger picture.

    I have a question for you, I can see on the underside a small SMD diode has blown. Is there anything that can be done about this or is it time for a new PSU?

    Thanks very much in advance for your response


    October 18, 2011 at 8:28 AM
  • Jim Warholic says:

    I suspect something else is blown besides the surface mount diode. I’ll bet other components are bad too even if the capacitors themselves look bad on the PSU.

    October 18, 2011 at 10:50 AM
  • Simon Hewitt says:

    Notes on Electrolytic Capacitors

    All electrolytic capacitors have a finite life:

    The voltage across the plates of the capacitor, in normal use, causes electrolysis in the electrolyte solution. i.e. breaking down the solution into its constituent atoms/molecules. This process ‘consumes’ the solution because the solution is converted into various gases. The rate of consumption is highly dependent on the voltage applied (higher voltage=more electrolysis!).

    As the capacitor ages, this process begins to accelerate and eventually sufficient gas is produced to cause a pressure build-up within the capacitor, eventually causing the capacitor to ‘vent’ (pop or explode!). The score-markings on the top of the capacitor are there to ensure the capacitor ‘vents’ in a controlled manner.

    A partial venting will also cause electrolyte solution to leak out, which further accelerates the capacitor’s demise.

    Testing capacitors:

    While measuring the capacitance of a capacitor seems like the right thing to measure, actual measurements on bad capacitors will either show a good reading, matching the value stated on the side, or, most likely a higher reading. Strangely the capacitance actually INCREASES as the capacitor is dying!

    In DC filter applications higher capacitance is normally a good thing because it provides more filtering of the high-frequency ripple from the SMPSU. However in the case of comparing an electrolytic capacitor’s stated value to the measured value, this indicates a dying capacitor – This provides a poor-man’s capacitor test: Look for higher than normal readings (in addition to basic visual tests)!

    The real parameter to check is the Equivalent Series Resistance (ESR). This resistance is in series with the capacitor and so cannot be measured with an ohm-meter – A special ESR meter is required (I use a Peak ESR60 £90).

    The ESR of a capacitor is important because it IS the parameter that increases as the electrolyte is consumed. The ESR is also represents the power loss in the capacitor (Ploss = Resr x I^2), this power loss causes heating of the capacitor, which further accelerates the electrolyte consumption.

    It is very normal to see capacitors in a SMPSU that look normal (no signs of bulging, etc) but actually have a high ESR reading and are in fact faulty – Always test the ESR of all capacitors (individually out-of-circuit).

    Again a poor-man’s test for ESR is to simply feel the temperature of the capacitor. If it is hot, then there are significant losses caused by the ESR.

    Finally, increasing the capacitance values of replacement parts may seem like a like a good idea to reduce ripple currents, but be warned that going too high may mess-up the dynamics of the SMPSU causing it to become unstable!

    Hope that helps, sorry if it’s already been said (haven’t waded through the comments!!!)


    October 19, 2011 at 10:57 AM
  • Jim Warholic says:

    Thank you Simon.

    Excellent information!

    October 19, 2011 at 11:48 AM
  • Kim Damsgaard says:

    Hello Jim. First of all, thanks for a great site! My iMac G5 started 3 years ago on some pixel-mistakes. It quickly got worse and at last the display was flickering black and grey. I didn’t throw it out because I love the machine, and just the other day I decided to figure out what was wrong with it. I read at your page to check the capacitors, and 21 out of the 25 on the motherboard was bulging! And here is my question: When you push the on button it is kind of starting up. – There is a little noise from it, but the fan is not running as fast as I recall it to do the first 3 seconds, but I might be wrong about this. But, there is no light in the white light on the front, and I can´t shut it down again, even if I hold the button for several seconds, it still doesn’t turn off. – And now the question: Is the psu also broken, or can this failure be on behalf of the 21 bulging capacitors on the motherboard? (There are no bulging capacitors in the psu) Thanks again for all your great help. (Sorry for my spelling, I am from Denmark:-))
    Kim Damsgaard

    February 8, 2012 at 2:04 PM

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