I suspect there is a poor connection internally with either a cold solder joint on one of the components, or something of an internal connection issue with one of the components. These type of problems can be difficult to track down. You may have to to try warming up selective areas, or even cooling down selective areas once you have it working to try and determine the localized area of the problem.

You might want to also visually look for a cold solder joint on the bottom of the PSU circuit board.

Here are a couple of things to consider. One, if this is the second power supply to go out in a short time, then I suspect the capacitors on the MOB are stressing out the capacitors inside the PSU. And, the other thing to consider is the capacitors don’t always look bad. In fact, the capacitors actually can dry out in the electrolytic juice, and no longer filter the voltages properly.

It would be nice if all the capacitors that went bad would show bulging signs, but this simply is not the case. I have experienced many of the MOB capacitors that when heating up of the legs to melt the solder, the legs tend to come out of the capacitors because the internals of the capacitors are so bad, that the legs (anode and cathode plates) no longer are making a solid connection internally to the guts of the cap. Refer to how electrolytic capacitors are constructed. Additionally, the caps can actually leak out from the leg area. This is why many times the caps will not bulge on the top surface, but will blow out the electrolyte at the bottom and will not be readily apparent.

Refer to Apple iMac G5 Logic Board repairs for more information on this area.

(Unfortunately, I think the ripple current may have burned out my graphics card permanently, but it only presents itself as periodic graphical disruption at high temperatures, so the computer is still usable.)

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!!!)

Si

Do you know where I can buy those WT7515 IC chips?

Hope you are well.

Paul

I replaced both PSU caps and tested them in a known good MOB and the status was the same

PSU 1 – shows trickle voltage LED1 and nothing else.
PSU 2 – shows trickle voltage LED2 and then LED2 comes on but goes out and I hear a squeak from the PSU case.

Any ideas?

Something in my heads says PSU2 has another bad cap (squeaking), this leaves the 2 x large caps and the other small ones – Alex B’s pic shows one of the PSU’s Apple p/n: 614-0326 the other os 614-0366

I should add, that I have tested both PSU voltages and I only get voltages on pins 22 & 9
Although pin 22 is quite low 20.53 compared with the 17? ALS PSU I have which is 23.3

Also PSU no longer does the flicker on LED 2

Are you sure it is coming from the PSU? I have actually heard a hard drive ticking when it was moving the read heads searching the disk, but could not read it (in this case, the disk had crashed). Did you try plugging in the PSU, just by itself, and measure the voltages from the pin connector? Refer to the article link in my Apple information page for measuring the PSU voltages.

If it is the PSU ticking, I suspect that either the small transformer is shorted internally or one of the other components is charging and discharging with an internal short. At this point in time, after double checking to make sure all the solder joints are good, it sounds like you might need a new PSU.

Before the repair I noticed that when plugged in the power supply “ticked”, and made a very rapid and quiet clicking noise… What could this be? It remains even after all the capacitors were replaced. When plugged in diagnostic led #1 is on, but that’s it; when you hit the power button there is no change. Any suggestions? Power supply bad? Is there anything I can fix on it besides the capacitors?

Thanks Steve, what a fantastic site with so much info!

Not sure on the graphics one.

On the 17 inch ALS one, I suspect there might have been a cascading failure affect. Sometimes when you pick and choose only certain caps to replace, the other ones that have not been replaced are next to go. It is kind of like the weakest link. This holds true from the MOB to the PSU and from the PSU to the MOB. By only replacing a few caps at a time, the others either in the PSU or in the MOB, end up in failure mode. Then, you end up wondering whether the few caps you installed are good anymore.

I would also double check the PSU voltages (refer to my article on how to measure the voltages on the Apple PSUs).

I would also check all the connectors. Try reseating all of them to make sure all the pins are making contact on the board.

One other thought comes to mind; are you sure you have good solder joints? Refer to my article on how to do lead free soldering on these thick Apple logic cards. If the capacitors are only tacked in place, sometimes they are not making a solid connection through the circuit board hole wall, and thus not getting a good connection to the top surface of the circuit board.

Thanks – sounds highly possible. I have the caps to do the logic board – just need to repair the PSU’s – not sure which I prefer but both need to be done now.

On a side note

I replaced the caps on my original logic board & the PSU mentioned above but then got square blocks on the screen that went away with the mouse being move over them – any ideas? Cooked graphics?

I also have a 17″ ALS that did not display internally I replaced the caps next to the CPU heatsink and it worked. I then decided to replace the others near the graphics connect and the back to the left and it stopped working internally but works fine on an external connection. I have been over every cap and even have another board that does the same. Any ideas?

Cheers Steve

I suspect the PSU now has a problem.

It is possible that some of the caps on the MOB, even though you do not see any signs of bulging, are bad. I have seen MOB caps that do not show any signs of problems, yet after replacing the caps, then everything worked fine. What typically happens with these older MOB caps, is the electrolytic juice drys out, and simply does not allow the capacitor to function, to properly filter out the AC ripple. Then, when a new or rebuilt PSU is plugged into the computer, the PSU has to work harder in order to produce the same DC voltages. This is when the cascading failure begins to happen.

I have a 20″ ALS with graphic issues (blocks all over the screen internal and external) so fairly certain it’s the graphics chip as had done the caps. So I just purchased another that had PSU issues.

When I powered the new unit on – LED1 is on, when you power up nothing happens – so assume needs some new capacitors?

I took the PSU from my old unit (I had previously replaced all caps) and plugged this in it booted fine. Did a few software updates left it over night and it woke from sleep etc fine. So upgraded the ram etc. went out and left it on doing an more updates. Came back and updates requested a reboot – let it do this and when I came back the unit was off? I could not remember shutting it down. So powered to find out but came back 1hr later and it was shutdown. Now concerned.

Long story short does not power up now – LED1 is on, LED 2 flashes quickly and then nothing ?
Again would a new cap set sort this – I am asking as I did these 4 months back and where previously fine so puzzled.

I have looked over the MB and cannot see any bulging caps at all. Is it possible that something on the MB or unit blew my good PSU?

I have also plugged in a 17″ ALS PSU (does not fit inside case but for test) and the unit powered up to the login prompt fine. Have not left it attached for fear of blowing this as well.

Thanks Steve