iMac P/S posts then shuts down

[godonr]

Re: iMac P/S posts then shuts down

Marky -- just checked this thread after long absence. on this supply, jumper 15 (on/off) and 16 (gnd) and 18 (gnd) and 22 (22/24v). I can tell you from experience if you don't see 24v the supply may not be fixed by a recap. The problem is somewhere else. Toasty is the best and has helped me alot. I'm still looking for where the failure is but didn't have the time (or his level of electronic knowledge) to solve this yet. Haven't given up but I can get the supply from Apple for $70 so I opted for that. If anyone has come up with any new info, I'd like to see it.

[RRack]

Re: iMac P/S posts then shuts down

Hello. I'm in the process of repairing the PSU on my son's G5 20" imac as well. This is after Apple replaced both the main logic board and then this PSU a little over a year ago. Anyway, it failed again and just wouldn't power up yesterday. I pulled it apart and did find three really swollen/leaking caps. I've removed them and will go grab some later today. The problem I'm having is that there is a burnt SMC on the back side of the board in the middle. I can't figure out it's specs nor can even figure out it's exact placement. I've attached links to some pics. Can anyone identify it? It's just to the right of R4. Other than that, I can't really find anything that is fried. I figured that I'd give it a shot before I shell out over $100 for a new PSU. Any tips or information would be greatly appreciated.









Thanks much,
Robert

[Toasty]

Re: iMac P/S posts then shuts down

Belongs in a new thread. Will get lost in here and does not relate to the OP's symptoms.

[RRack]

Re: iMac P/S posts then shuts down

Huh? I thought this thread was about repairing the bad caps on an Apple iMac PSU. I simply have a question about a certain component on that exact same PSU. Apple doesn't release any schematics so I'm stuck trying to guess what that SMC transistor is. I've also run into the same bad caps and have replaced them on the top side. I just thought someone might take a quick peek at the underside of their board and let me know the ID of this so I can dig up specs and track one down.

[RRack]

Re: iMac P/S posts then shuts down

Here's the solder side of the power supply. Upper left is M12 over M10. Middle of board is M2.

godonr has a great photo of exactly what I need that he posted back in May. It appears that the position might be Q1 from his photo. Mine is a little burnt along with the leads. I still can't make out the ID markings even with intense zooming. godonr, might you be able to read the surface mount transistor right next to R4?

[godonr]

Re: iMac P/S posts then shuts down

RRACK - just noticed your post. It is Q1 and carries a 1P Z marking. I have had about a 50 of these 1st generation supplies fail but never saw one take out Q1 like that. Hope this info helps!

[godonr]

Re: iMac P/S posts then shuts down

This was posted on x33's thread a while back but I realized others following this thread may not have seen the resolution. Hope you didn't trash those 661-3350s.
Have confirmed x33's findings on many supplies already. If after a recap of the 20" power supply you still get 20vdc on pin 22 when p/s is jumpered ON, replacing smd Q1 (it will show signs of overheating) next to the PWM will give you the 24vdc. EC short is the problem. Q1, Q2, Q5 and Q7 are all 1ps and I have encountered more than one bad at a time. Also, they have to be removed to verify (or at least have a few legs lifted). This is the solution to supplies which don't respond to just recapping.
Thanks to Toasty and everyone else who knocked their brains out troubleshooting this supply. Way to go, x33.

[stj]

Re: iMac P/S posts then shuts down

RRack,
the 8pin chip - probably uc3845b is fucked.

[RRack]

Re: iMac P/S posts then shuts down

thanks for the reply stj... I pretty much had forgotten I had posted this. I've since purchased a rebuilt PSU a while back. Still seems to be working strong.

[godonr]

Re: iMac P/S posts then shuts down

RRack -- your board is obviously an extreme example of what can happen. I support over 200 17 and 20" G5s and since x33's discovery have brought 27 machines back to life. stj's probably right on in your case, but it looks like the hot spot started with Q1. Knock on wood... so far I haven't had to replace any PWMs.

[godonr]

Re: iMac P/S posts then shuts down

Does anyone have a source for the 1P NPN (Q1) SMD found on the 20" power supply?

[Toasty]

Re: iMac P/S posts then shuts down

Yes. You need to do the research to learn to find these.

Look for a transistor marking code reference. There are several out there...
(This is actually one of the easier ones.)

Case style should be SOT-23-3.

Good Hunting!

Toast

PS:
While you're digging, look for the 2F transistor too, as that is the PNP complimentary one to the 1P.

[godonr]

Re: iMac P/S posts then shuts down

So it's not marked on the transistor? What does 1P stand for?

[Toasty]

Re: iMac P/S posts then shuts down

1P is the marking code that once you find the reference guide(s), you'll see. It substitutes for the 6+ digit code we're used to for transistors and such. You can't mark these with a number such as 2N1234A, there's no room. So the manufacturers came up with a 2 and 3 digit code system that covers *most* of the devices in use. You also need to take into account the package type and connections.

Example would be a 52:
In a 3 pin case (like the 1P or 2F), it's likely a Schottky diode from the single pin side to one of the 2 pins on the other side. BUT, in a 2 pin case, it could be Zener in either 45v or 3.3v. So you would need to know the case -type- to figure which it was.

Toast

[godonr]

Re: iMac P/S posts then shuts down

Toast... appreciate the patience. I've used the codes before but didn't know how to decide which 1P I was dealing with. Knowing the case certainly helps. Appears to be FMMT2222A/FMMT2907A for this although DigiKey has it as obsolete but carries a MMBT2222A. Only diff I see is 300 vs 330mW power max on devices so should be OK. Notice determining the exact case can be tricky, too. What's the diff between SOT-23 and SOT-23-3?

[Toasty]

Re: iMac P/S posts then shuts down

Not any difference I can see from sheets I've seen.

Get the 350mW parts instead of the 330mW. I have not looked at the specs for the 800mA and 60v, but that's an option too.

[godonr]

Re: iMac P/S posts then shuts down

That would be the Fairchild version (300mW) Mouser carries? Will try them. Thanks again

[Toasty]

Re: iMac P/S posts then shuts down

No, no, no. DK has them both in about 1/2 million quantities.

Search: BT2222 then BT2907
pick 350mW and In Stock

.

[godonr]

Re: iMac P/S posts then shuts down

Sorry... typo... the Fairchild is 350mW also. Seems to be readily available at both places.

[trex66]

Re: iMac P/S posts then shuts down

Hummm, I have a similar PSU and a similar problem. I have a 17" ALS that will just shut off whenever it wants to. I can't duplicate it, but the voltages are fine whenever I check, then it will just shut down, sometimes after posting, sometimes after 30 min, or two days, or a week. I don't know what could be causing this to happen, any advise would be appreciated