iMac 24'' Early 2009 A1225 Delta ADP-250AF Bad Caps

[mitchw]

Hi,
I got this iMac for free, and it was said to me that it has dead logic board.
I discovered that the +12VSB came for 3-4 seconds and then disappeared. Hitting the power button for about 20 times, I got it to start up.

Even with the logic board disconnected, the 12VSB was pulsing after few seconds. PSU looked nice with no caps blown. I decided to put my ESR meter on few of the dark green Taicon caps.

I was shocked that one 100uF 35V read 45 ohms, next to it was a 16V 470uF that read almost 4 ohms. These were on the primary side.

After checking the ESR of all the caps, I found two 35V 220uF completely O/C, reading more than a 100 ohms ESR out of circuit, and about 40uF each. Those were decouplers for 24V supply.

There was also on 25V 820uF that read on the high side, so I replaced it.

Now the PSU works like new, I will probably do a complete recap in near future, but I must first replace the GPU in as it is also defective. Also, those light green Taicon caps seem to be much better quality than the dark green ones.

Just my 2 cents, this can help someone

[bobzbob]

Re: iMac 24'' Early 2009 A1225 Delta ADP-250AF Bad Caps

Thank you for the helpful information. I will give it a try.

[mitchw]

Re: iMac 24'' Early 2009 A1225 Delta ADP-250AF Bad Caps

Just to add an update, got another PSU like that one to repair few days ago, but was still kinda working, but computer sometimes shut off by itself.

Well, same problem, caps went bad. Casualties:
25V 820uF (bulging top)
2x 35V 220uF (high ESR, but measured kinda OK in circuit)
16V 470uF (slightly high ESR)
35V 100uF (low capacitance and very high ESR)
35V 33uF (good capacitance, but like 10 ohms ESR)

All those replaced, and now working 100%.

[PeteS in CA]

Re: iMac 24'' Early 2009 A1225 Delta ADP-250AF Bad Caps

It's probably not a popular message, but getting 10-11 years of useful life from electrolytic capacitors in a stressful environment is pretty decent. It does not help that Apple (and Dell, and HP, and ...) beat the @#$% out of their power supply vendors over price, forcing the use of second-tier quality vendors such as Taicon. Delta probably does better than average in getting good life out of second-tier electrolytics.

[mitchw]

Re: iMac 24'' Early 2009 A1225 Delta ADP-250AF Bad Caps

Both of those computers were all out of use since 2015-17. They were just used as backup machines, but probably saw just 5-10 hours use since 2017. Who knows how long those caps actually lasted in terms of being in spec ESR wise. Probably 4-5 years.

And yes, because if its bad thermal design of those computers, all the hot air from CPU and GPU gets pushed through the PSU and then out at the top. I saw temperatures as high as 75 deg C on the PSU sensor when testing the machine, before the fan kicked in high and cooled it down to about 60 deg C

[PeteS in CA]

Re: iMac 24'' Early 2009 A1225 Delta ADP-250AF Bad Caps

75C air being the "cooling" for the power supply might be unhealthy for the semiconductors as well as the electrolytic capacitors.