Help Please with recapped Acbell in Imac G5

[x33]

Re: Help Please with recapped Acbell in Imac G5

PROBLEM SOLVED!!!

Toasty, you pointed me to the right direction (3845 ic), THANK YOU!

As I said, there were no signs of overheating, burn marks or anything like that and everything checked out ok...apparently

So, I started pulling out the smd transistors around 3845 to measure them.

One little sneaky bastard 1p transistor (2n2222a smd type, NPN bi-polar) right next to 3845 was damaged internally!!! Junctions B-C and B-E checked out ok but when checked the E-C, I've got 1.9kohms one way and infinite the other....

Replaced it in a second (thank you my trusty hot-air soldering station) and the psu started working like a charm

I hope this will help some other poor souls went down the path of madness trying to figure out these psu issues.

Cheers everyone and I'll stay in touch!

[godonr]

Re: Help Please with recapped Acbell in Imac G5

Just saw your post on getting the 661-3350 p/s up. I'm curious-- are you talking about Q1 and/or Q2 next to the PWM? Did replacing it restore 24v to pin 22 on a jumpered supply? That would be the best news ever.

[x33]

Re: Help Please with recapped Acbell in Imac G5

Yes the power supply works beautiful actually and I am using my Imac on a daily basis with no issues so far.

I regret that with all the excitement I forgot to take a note on which transistor is,before putting it back together.

I do, however, remember its location visually, so if you can upload the picture of that area, I'll be able to point it to you.

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

https://www.badcaps.net/forum/showpo...1&postcount=27

[x33]

Re: Help Please with recapped Acbell in Imac G5

Q1 was the culprit and by the looks of it, it's the same on your PSU as well.

See the attached pic.

Let me know if this worked for you too.

[godonr]

Re: Help Please with recapped Acbell in Imac G5

Have confirmed x33's findings on three supplies already. If after a recap on the 20" power supply you still get 20vdc on pin 22 when jumpered ON, replacing smd Q1 (it will show signs of overheating) next to the PWM will restore the 24vdc needed for backlighting. Thanks to Toasty and everyone else who knocked their brains out troubleshooting this supply. Way to go, x33.

[x33]

Re: Help Please with recapped Acbell in Imac G5

Please note that the signs of an overheated Q1 may or may not be so obvious.

In my case Q1 was showing a very slightly fogged band across the center of the transistor, which was of course extremely easy to overlook until I used the watchmaker loupe.

There's also an extreme situation pictured here:
https://www.badcaps.net/forum/showth...6&page=3&pp=20

where Q1 actually exploded..

Anyway, I am glad I have brought my modest contribution in solving this bizarre problem, and if more repairs are confirmed, I'd suggest to turn this thread into a sticky, for the solution to be more accessible.

Cheers everyone!

[murf64]

Re: Help Please with recapped Acbell in Imac G5

OK I've learked in here long enough. I'd like to thank you guys. I've been recapping motherboards for six months. Last month I ran across my first bad power supply, it was a celetronix. I checked another 20" I had fixed up for my wife also had a celetronix. I took both apart and I started using the good unit to get an idea of what the voltages should be at certain spots on the board. And then the good one died, damn. I don't know what I did but I killed it.
After reading this forum, I got the impression I would be unable to repair either celetronix, so I bought a dead Acbel on eBay. I knew I could do a recap job. Unfortunately, when I got the new dead power supply the capacitors looked perfect. I realize they could be why it didn't work, but I really doubted it. At my wits end, I read this thread. This morning I pulled the Q1 transistor and it tested bad! I replaced it with a similar one off one of the dead celetronix. I put the psu in my wife's 20" iMac and it started right up. Amazing.
I still have one 20" iMac I need a psu for. I'm waiting on an eBay auction for an Acbel, but I'm hoping you guys come up with a fix for the celetronix units before then. I still have two of them.
Thanks for the great work.

[jetadm123]

Re: Help Please with recapped Acbell in Imac G5

Interesting reading. Wonder if it's a bad batch of Q1's or using a transistor that's underrated for the the application? Or, maybe a lousy design?

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

Q1 supplies Vcc to the 3845 PWM chip. It's Collector is connected directly to the positive of the 100uF @ 50v cap near the TOP. Emitter is hooked to Pin 7 (Vcc) of the 3845.

I don't think its a bad batch of 1P's, as they are used all over these PSUs. Internally, the 3845 has 34v zener across Vcc and Gnd. If the supply voltage spikes above that, I'm wondering if that causes Q1 to overheat and die. There have been a few cases where Q1 and the 3845 have popped/cratered. Could that be from not just a spike but a constant overvoltage or continuous spiking?

Toast

[godonr]

Re: Help Please with recapped Acbell in Imac G5

may be a clue here... to date I've replaced 49 1Ps but never a C37 (100u/50v). this is in the AP13PC96-290 AcBel version. usually it's C40, C52, C49, C47, C45, C56 and C55 that have failed also. many repaired supplies have been on constantly for nearly a year now.

[godonr]

Re: Help Please with recapped Acbell in Imac G5

Should add... all showed signs of overheating and EC was shorted.

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

Yes, but those caps filter the power to the PWM and surrounding, and control the TOP. It's worth it to me for <$2 worth of caps to never see it again for that reason.

[godonr]

Re: Help Please with recapped Acbell in Imac G5

Duly noted