I bought a 17″ non iSight, non ALS G5 iMac, and it seems quite dead.

I took the back off, plugged it in and noticed that the first diagnostic LED is on but flickering. If I unplug it, the flickering stops, and after a moment, the LED turns off.

My guess is that there’s at least one bad cap here, and some AC is getting through.

Thoughts???

If upon inspection, there are absolutely no physical signs of bad capacitors, then the only way to absolutely verify is to try replacing them and see what happens. Yes, there are low ESR meters that will sometimes work to establish a good or bad capacitor, however they do not always indicate properly due to other components in the circuits.

Kristin

As I had one of these in my repair shop for ages, and after months of hunting and testing for the problem, I managed to narrow my issue right down to the Northbridge Chip on the front of the Logic Board, under the silver heatsink with the Infrared Sensor on it.

The iMac was sometimes booting, then shutting off or freezing completely. PRAM zaps would often help, but only for another 2 mins or so. Sometimes I could remote login via SSL when the screen had turned black, and this led me to think that it was a GPU problem, but testing other GPU’s didn’t help or change anything. Testing replacement PSU’s did not change anything.

In the end, I found that I could reflow (Hot air gun heat) the Intel Northbridge Chip, and the iMac would instantly work again fine for hours/days, then slowly exhibit the same problems.

In the end I realised that I needed to do a full Re-Ball of the BGA Northbridge Chip, and attempted it myself, but accidentally didn’t heat up the board enough to start with, and ripped off traces under the chip as I removed it, =p
*whoops*.

So I wasn’t able to finish repairing it and test it to confirm the problem, but if you still have your logic board, you could try taking it to a place that does BGA re-balling (not just Re-flowing which is only heating, in Re-Balling they actually remove the chip and replace each of the solder balls with new ones, preferably leaded-solder balls to protect against future cold solder joints and cracking).

Hope this info helps you. ^_^

Good Luck!
– Sylph

I believe that iMac is an earlier Intel based iMac. Some of those had problems with the graphics chip and/or the ball grid array (BGA) connections. Depending on the type of symptoms, i.e. lines down through the screen, it is probably a bad MOB logic board or video display unit if you do not see any bulging capacitors. These are very expensive to fix. Here is a link to a video that is showing an iSight G5 model that may be similar to yours. Though, I am not sure if that video is the G5 model or simply a iMac. Here is a link to the history of the iMac.

Hope that helps you out.

Regards,

Jim

Sorry, no information in that area. Maybe someone else that reads the comments knows the pinout voltages for the inverter or can point us in the right direction.

Regards,

Jim

Would you happen to know the pinout voltage for the backlight inverter? For a imac G5, 1.6ghz. (6 pinout)

First off, you should always replace all the caps of both groups on the MOB. Even though, you only replaced the ones that were physically bad, I suspect you have others that are also bad.

I would also check the PSU capacitors too. Refer to my other articles at my Apple menu link for more information links.

Additionally, only the first three LEDs should come on. The fourth LED is over temperature. This can be caused by bad caps, fans that are not working, or a PSU that has too much ripple due to bad caps or other components.

Regards,

Jim

Any ideas what is the problem now?
thanks!
Steve

That is what someone supplied to me for labeling. It may be where the wires go on the circuit board as a reference point. However, don’t worry about those specific references, just the wire numbers when measuring the voltages.

Jim

Also, the capacitors on the main logic board could be causing the PSU to go into a failure mode. I have seen this happen on more than one occasion, even if the capacitors are not bulging. Note, capacitors do dry out and stop filtering the voltages, especially when heated up.