Help Please with recapped Acbell in Imac G5

[x33]

Hello everyone and A Happy New Year!

Boy I am happy I've found this forum, but although using the "search" button offered me hours of delightful..lool...reading, I still can't find a solution for my recapped PSU that doesn't work.

Here's the short story:

Imac G5 20" no iSight, no light sensor.

PSU is Acbel 661-3350

Recapped the bad Nichicons (bulged with some brown residue, but not exploded) in the secondary.

I got 5Vsb and 20.1V unjumpered, and all correct voltages when jumpered ON.

Installed it back in the imac, all I got is the LED1 lit solid green.

Pulled it out, same thing as above.Works ok. Didn't try it under external load since I was getting ready for the holidays.

Came back from the holiday trip, re-installed the PSU in the imac, without doing anything else to it. Pushed the internal button on mobo and LO and Behold ! Scared the cr@p out of me because the fans started spinning and a loud beep told me theat the imac is alive(LED 1 and 2 lit green)!!! Did I mention it was 5:30 in the morning???

Anyway, I realized I left the RAM out, so I had to shutdown to insert the ram.

Did it and is back to its previous state: LED1 solid green ...Nothing else.

Removed the PSU to check the voltages again: 5Vsb and 20.1V present, but when jumpered ON the other voltages DO NOT show up at all anymore.

Opened the PSU and after a while I found diode D10 opened. It seems that the only other person with this type of fault was Jim Warholic as per his website. Anyway, the diode is marked T2D and then 48 with two lines above. Someone responded to Jim saying that any fast recovery rectifier would do the trick as long as it's 100v/1A.

I have tried one fast recovery rectifier rated 400v/1.5A, but that KILLS ALL the voltages. No more 5Vsb, no more 20.1V. Nothing!!! Just 45mV here and there..

I pulled it out and the 5V and 20.1V came back! What the heck is going on?

Is the TOP244Y switcher bad? There's no way I can determine this, but is definitely not showing any shorts. Checked the outputs for 3.3, 5, and 12 and no shorts...

Can someone please give me a hint, what am I missing here???

Thanks and sorry for the long post

[x33]

Re: Help Please with recapped Acbell in Imac G5

Anybody??

Please, I am getting desperate here.

Toasty, I've seen that you have the exact same power supply. Can you please point me to the right direction with this issue (and D10 rectifier)?

[ratdude747]

Re: Help Please with recapped Acbell in Imac G5

hang on, toasty is the expert on these... he should read this...

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

If 5vsb & 20v are there, TOP is okay and so is diode. Put it back. You can't check (forward-bias) that diode unless you have a 200v current limited supply. It's an ultra fast diode capable of 100kHz+.

Go back to where you had no RAM. Push button again and see if it fires up. Then put RAM back in and push button again.

>>Recapped the bad Nichicons <<
Just the bad (visible) ones?


Did you have the mobo out? Another thread concluded with a pinched wire causing the problem.

[x33]

Re: Help Please with recapped Acbell in Imac G5

Oh thank you guys for chiming in!


Toasty, I am troubleshooting the PSU now. I've recapped all the Nichicons I've found, bulged or not.

And then I've found this diode which shows nothing when I put the dmm on it (for diode reading).

So Toasty, you're saying this is normal? I'll put it back then, but I still can't find out why the PSU refuses to turn on anymore (out of the imac, just jumpering ON).

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

You pulled the diode and measured? Or was it in circuit?

The 5vsb & 20v are now gone when you just plug it in and NOT jumpered on?

[x33]

Re: Help Please with recapped Acbell in Imac G5

You've got me really confused with your answer about the diode, since I've never heard of a diode that doesn't indicate anything on the dmm.

Of course I pulled it out to measure it. It shows open both ways. Nothing, dead..

I am getting 20v and 5Vsb with this diode installed (psu jumpered or not) that's all.

I tried to replace it with another fast recovery diode from a good PSU and that kills all the voltages. I have checked the polarity on the diode and it was correct.

So, what kind of diode is this D2T (D10 on the board)???

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

Ultra fast. < 75ns recovery

What brand/series caps did you use for recapping?

[x33]

Re: Help Please with recapped Acbell in Imac G5

so something like UF4003 would do it?

I have used Nichicons HZ series from mouser.com

[x33]

Re: Help Please with recapped Acbell in Imac G5

Toasty, just for the fun of it, I have left the psu WITHOUT the diode and it does exactly as it does with the original diode: 20v and 5Vsb and nothing else (jumpered on or not).

This re-confirms the original D10 diode is dead.

I will try getting an ultrafast replacement for it to see what happens.

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

I think that a cause of recapping failures in these units, is using caps of too low ESR, like the HZ's. It seems odd, but I've seen people with working units with blown caps, not work after recapping with excellent replacements. I don't know if it's a particular trait of the AcBel's or not.

I've 2 of these somewhere in the pile. Let me grab them and see if either work. I can give you some voltages to check later. I can't guarantee that it will be tonight or tomorrow.

The diode is fine, BTW. These always show as open on a DMM or VOM. As I said before, you need to be able to present ~200v to forward bias it. I am beginning to think they may be avalanche types used as a "snubber" or clamp for overvoltage spikes. If you look at the circuit, you'll see it's across the primary of the 5vsb transformer from the TOP output (drain - pin 6).

[stj]

Re: Help Please with recapped Acbell in Imac G5

I think that a cause of recapping failures in these units, is using caps of too low ESR, like the HZ's. It seems odd, but I've seen people with working units with blown caps, not work after recapping with excellent replacements.

that makes sense,
i'v seen similar things with too much capacitance on stuff that's been home-designed.

the inrush as the caps charge is tripping the over-current circuit.

[x33]

Re: Help Please with recapped Acbell in Imac G5

Ok Toasty.

I've put the original D10 back in the psu and I'll wait for some info from you.

I'll just try replacing the small caps in the primary (Teapo's) rated at 50v and one@25v to see if it makes any difference..

Isn't it weird though that the psu gives the 20v and 5vsb WITH OR WITHOUT D10 installed???

[x33]

Re: Help Please with recapped Acbell in Imac G5

Toasty, here's what I've managed to measure:

387 Vdc across drain and source on TOP switch !

5.78 V across the SCR1 thyristor.

Any other ideas what to check?

[x33]

Re: Help Please with recapped Acbell in Imac G5

Toasty ?

I hope you haven't totally forgotten about me

[x33]

Re: Help Please with recapped Acbell in Imac G5

err...Santa?

[x33]

Re: Help Please with recapped Acbell in Imac G5

Well, since I haven't got too many suggestions on this psu issue, and being too tired of wasting my time on it, I guess I'll do myself and everybody else a favor and turn this iMac cr@p into mush and throw it in garbage...

After all, that's where Apple products belong anyway!

Good weekend all!

[x33]

Re: Help Please with recapped Acbell in Imac G5

Well, after a good night sleep, I decided to give this piece of junk another chance... so after reading multiple posts here and across the net, I have managed to start it with a modded CoolerMaster ATX plus a laptop psu (for backlight 24V).

The modding was fun, but I'd like to clarify that this Imac's backlight doesn't work without the 24V.

I am still trying to fix the damn psu, since there's no way in hell I'll be paying 100+ bucks for such a lame psu..

So Toasty, I've sent you a PM with some new things I've discovered on the psu (trafo T1 has a lead in the primary that should be connected to negative from main rectifier bridge?? I can't measure anything on the lead..)

Anyone knowledgeable in these psu's is welcome to pitch in an idea.

Cheers!

[Toasty]

Re: Help Please with recapped Acbell in Imac G5

I have no news to report at this time. I have not been on the bench in over a week. I have been reading and following yours and others responses. Answering has not been forefront as I have no info to report. I do have the other units out, but I have not checked them.

I ask that PM's be limited to exactly that. Technical information and questions should always be posted on the forums. Info supplied or requested in a PM may very well trigger a response from others that are following the involved thread.

From your PM of several days ago:

"...but you're the only member that can still help me with this insane psu."

That is not true. There are many others here who are much more knowledgeable than I. They have most likely chosen to steer clear of Apple products and discussions as they are not the everyday repair. They see 1,000 regular PSU's to 1 Apple.

When it comes right down to issues with these PSU's, if a recap doesn't work, replacement is the best option. Mainstream shops have given up working on them once a recap doesn't work. They are a PITA to work on as you can see and tracing a signal can be a royal pain after it's made it 4th switch from top to bottom via through-holes under a transformer.

"...as a last resort I pulled out T1 trafo. It turns out that its primary has 3 leads. Two are going into the q12 and q13 fet's and the third one goes to the ground. Well measuring with the ohmeter, I get nothing from the 3rd one! I can see the wire going inside the trafo, yet I can't measure anything from it. Could it be burned inside???"

No. It appears to be a noise suppression winding. In this iteration, there is only 1 end showing that wraps the trafo post. In others, it's 2 ends of the winding. In all cases, it goes to common.

As a start, are the SMD transistors around the 3845 chip -all- okay?
Does it in any way look as though the board has been overheated around the 3845?

It's a very inexpensive part, < $1, and as much as I hate guessing and playing 'swapsies' with parts, it might be a best choice to replace it.

I would choose the TL2845, as it has a higher temperature rating (85°C) than the TL3845 (70°C). That is the only difference I see between them. Mouser has them in stock.

[x33]

Re: Help Please with recapped Acbell in Imac G5

Welcome back Toasty!

When I said "...but you're the only member that can still help me with this insane psu." I seriously meant : you're the only member I've read about having the exact same psu as I have. I obviously didn't mean in any way to be disrespectful to the other members and their knowledge.

Just wanted to clarify that.

I have just found out late last night about the noise supression winding on T1, and you just confirmed it. Thank you.

The bizarre part is that absolutely nothing on this psu shows signs of overheating... Checked everything with my trusty magnifying glasses.

I am afraid I'll have to play "swapsies" after all, since everything checks out ok...

I'll post back as soon as I get any new findings.

Thank you!!