iMac a1225 Power Supply Board LITEON PA-3241-02A1 Repair

[TheDaneH3]

Hi! This will be my first post on these forums, but after browsing for a long time I trust you guys the most to help me out with switchmode power supply issues.

For a preface, I just came into possetion of two 2009 iMacs, a 20" a1224 and the 24" a1225 that I am currently working on. (The a1224 has a problem with either the inverter or the screen, but there's an inverter on the way - so we shall see!)

The 24" iMac I am working on has a problem with the power supply. For those that don't know, iMac's have four diagnostic LED's that are used to diagnose hardware problems. The first LED - when the unit is plugged in - denotes whether or not the logic board is receiving trickle power from the PSU. This light is not on, and the iMac does not power on, so there is definitely a problem with the power supply here.

In the attatched images, it appears as though there are two bulging capacitors (in the close-up) that I plan on replacing once the replacement 470uF 16V 105C Nichicon caps come in the mail. The main fuse is not blown.

Unfortunately, this is about the extent of my knowledge of switch-mode power supply repair, and this is my first time working on a SMPS. Essentially, I am making this thread to see if there are any other things I should be looking out for on this board that may be faulty. I would appreicate any and all SMPS related wisdom and repair tips. Thanks in advance!

[TheDaneH3]

Re: iMac a1225 Power Supply Board LITEON PA-3241-02A1 Repair

I replaced the capacitors in question, and there is still no output or trickle voltage. Any help figuring out the next step in troubleshooting? Thanks.

I did some basic tracing of AC input voltage up until the rectifier (A GBU808 Bridge Rectifier). The output there is 160V DC as measured by my scope. Is that a standard output that should be expected?

[R_J]

Re: iMac a1225 Power Supply Board LITEON PA-3241-02A1 Repair

160vdc is the normal voltage out of the bridge with 120vac input. There are a number threads on this power supply that might help ie: https://www.badcaps.net/forum/showthread.php?t=38233
Search for "LITEON PA-3241-02A1"

[TheDaneH3]

Re: iMac a1225 Power Supply Board LITEON PA-3241-02A1 Repair

There are a number threads on this power supply that might help

I read through the other threads for this power supply, but I seem to have a different problem than them. For instance, there is no trickle power in my case. This leads me to think that there is another different problem that has not been addressed by that thread.

That being said, while poking around while the board was live I was distracted by something and blew up one of the power transistors. I might just cut my losses and buy a new board. Or better yet, just get rid of the iMac since it's really not that valuable.

[MikeCebula]

Apple 24" iMac A1225 PSU

I have a donated early 2009 iMac8,1 that has quit.

Working on any Apple product is new to me. I received the unit 3 years ago along with a MacBook and a MacBook Pro and loaded Linux on them. All 3 (and others) run 24 hrs/day doing cancer markers research. (In other words, very valuable computing.)

In narrowing down the problem I discovered the 4 LEDs that indicate power progression. LED 1 comes on as I plug the iMac in. As soon as I hit the power button, LEDs 2 then 3 momentarily flash and all three go dark. Unplug then plug back in gives me LED 1 lit.

I pulled the PSU and a quick look told me that many of the electrolytic caps were bulging. I guess 14 years isn't a bad lifetime for them. My son helped me out and replaced 15 on the board. I was pleased that the iMac then powered up and was able to put it right back to work for the next 5 days....then the problem reared its ugly head again. This time, all 4 of the 2200uf caps were bulged. These 4 were of a different brand (Chongx) than the rest that we replaced and I had not specifically ordered them to be low impedance. I have a package of 6 (Panasonic) that are low impedance coming shortly.

Replacing the entire field of caps on other, old electronic equipment has been a rewarding process and brought them back to life. Monitors, TVs, stereos, etc.

I found schematics for the two Apple laptops mentioned above, but not the iMac. A schematic and boardview would greatly help me identify the circuitry involved.

My question right now is if this failure of just these 4 caps is indicative of anything other than old age or could something else cause this? (Possibly the 2200uf ones I installed were many years on the shelf, just a bad brand, or not suitable for the application?)

Can anyone give me advice on my question?

Thank you,
Mike

[MikeCebula]

Re: iMac a1225 Power Supply Board LITEON PA-3241-02A1 Repair

OK. I received the 4 Panasonic 2200mfd caps and replaced them last Wednesday, 11/08/2023. Today is 11/16 and so far, the new caps are performing as expected. (The previous 4 caps lasted 5 days.) The iMac continues to run 24/7 on both cores for the previously mentioned research. Some of the other caps that were replaced are of the brand that failed these 4. Hopefully, they are less stressed and will last. Without a schematic, it is difficult to predict much of anything.

Mike