iMac P/S posts then shuts down

[godonr]

Re: iMac P/S posts then shuts down

Toast -- the rail from 24v goes through SB340 diode > 330uf/35v > L1 > D26 (marked UGIODCT 0420) with a symbol under it that looks more like a MOV. Is that what we're looking for?

[Toasty]

Re: iMac P/S posts then shuts down

No. That's a dual Schottky (probably common cathode) switchmode rectifier. Symbol sort of looks like 2 Zeners cathode to cathode? That's an extremely heavy piece (20A) for a 24v line that's rated what? 1/3 amp?

C59 is the 330uF cap? You should have the 20v across that cap at standby, and 24v when "on".

You have L1 out of circuit, correct? C59's positive is connected to L1. Does it exit the coil at the lead directly across from it (center pin of D26)?

You should have continuity between that center pin on D26 and the 24v lead to the plug. Ohmmeter positive tip to center D26, neg to plug #22.

If that's all correct, then this uses a much different method (buck regulator) of generating the 24v line than the 17's. Yet, so many of the components for the 24v on the 17's are in the same places.

Toast

[godonr]

Re: iMac P/S posts then shuts down

Toast -- You have it exactly right. That answers why I couldn't find a 24v regulator. What exactly is a buck regulator? I can ID all of the components if it helps. The heatsink @ HS3 has that dual schottky and another MBR20100CT (0440), a PS30L45CT, a KA78M15, and 2 MBR2045CTs. There's several TL431s, two 20N60C3s (one is JW434, tone is NK434 Darlingtons?), an A1013 (Q14). Does this info help? And, yes C59 is a 330uf. I appreciate all the info so far. Is there a next step?

[Toasty]

Re: iMac P/S posts then shuts down

Can you get a pic of the backside of the board? If you can identify the chips back there (PaintShop-Photoshop?) it will make some explanations a bit easier.

>>You should have the 20v across that cap at standby, and 24v when "on". <<

Do you?

>>Does it exit the coil at the lead directly across from it (center pin of D26)?<<

??

I may be mistaken on the buck regulator (properly: buck converter) setup there. Since I can not get a proper identification on the UGIODCT device. It certainly appears to be connected as a dual-Schottky would for a buck converter.

I did a bit of research and am going to say that this is actually a push-pull converter. Starting with the 2 transistors Q12 & 13 in front of T1 which alternate driving T1's primary. Then T1's outputs go through D26 and the 2 similar devices right next to it. On to L1 and the cap and out through the diode D21 to the +24v lead. I'd also venture a guess that there is a PWM chip on the solder side down near Q12 & 13 that is getting feedback from one leg of L1.

T1 is not just providing 24v, it also appears to be producing the 12v & 5v also. The inputs to L1 seem to go directly across for output. Going from down of the stripped board to up. On the 5 output legs, I would expect to see, going left to right, 24v, 12v, GND, GND, 5v.

See what you can confirm from my theories please. My puzzlement is: If the 12v & 5v come up when PSU is jumpered "on", then why doesn't the 24v? What's keeping it at 20v?

Toast

[godonr]

Re: iMac P/S posts then shuts down

Give me a bit to get it all together and I'll get that pix. Also, I may have confused things as to which supply I've been working on. The one we're talking about gives nothing but 20 & 5vsb when jumpered. That's the more common problem encountered before and after recapping. I do have one supply which comes all the way up (all voltages present, plugged into the mobo) then shuts down and I know I mentioned it first. Sorry if that confused things but my focus has been the non-responsive units.
I can work whichever is best for you!
If it helps, as you suspect, there is a pwm 3845B Z442 (m2) right under T1. Also, where the two feed wires off T1 go to the board just below C53, there are two solder side pwms LM339M (M10) and WT7515 (M12).
Let me double check the C59 -- I know on the non-working it stays at 20v. I'll need to pop the top on the working one to verify the 20 to 24v swing. Also, as far as I can tell the coil lead is directly to the center pin of D26. On an intact board, continuity from pin 22 to center pin of D26 can be read.
Let me get more answers for you. If responses are a little slow it's because I'm on the job and having to fit this in all the other things we do around here. Thanks for your help and patience!

[Toasty]

Re: iMac P/S posts then shuts down

>>The one we're talking about gives nothing but 20 & 5vsb when jumpered.<<

Okay. So the 5v, 12v, and 3.3v are missing then? We're working in the wrong area then.

What of the other supplies? I thought they all had the same symptoms. Are you saying each one is/has a different problem?

You actually have a great solution in front of you. If all of these are identical, then take your working unit and start comparing voltages at matching points across the board(s). Set them up and jumpered on. Then start checking from the component side at the transistors, transformers, diodes, anything that has a clear leg that you can meter.

Take a clear pic, blow it up and print it. Then mark off each point as you go with the voltages. You can make one for each unit, and keep it with it so you know where to go looking for the problem.

Looks like I picked a bad week to stop drinking...

Toast

[godonr]

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.

[Toasty]

Re: iMac P/S posts then shuts down

BTW:
3845B is a PWM
LM339M is a quad comparator
WT7515 is a protection IC that monitors 3.3v, 5v, 12v for over/under voltage

[godonr]

Re: iMac P/S posts then shuts down

I apologize for the confusion. To date, I have repaired 16 of these just by recapping. I have about 20 more problem supplies. As I started on them, they would come up without the 24, 3.3, 5 and 12v present. So that's the batch I'm trying to determine "why" on. This problem appears common to this supply after a multiple cap failure. Whatever area of the supply tells it to switch on the voltages is where the problem is, right? Believe me, I know what you mean about the stop drinking.

[Toasty]

Re: iMac P/S posts then shuts down

No. The area that tells it to switch on is finding a fault somewhere and telling it NOT to switch on. Most likely. Possibly. Perhaps.

Try the troubleshooting with the good vs. bad unit voltages first. If you have a scope, you can check the TOPSwitch and see if it's running. Or, just lay the probe down along each of the transformers and see if there's a waveform being generated.

Toast

[godonr]

Re: iMac P/S posts then shuts down

By topswitch, we're talking M6? TOP244Y with 6 legs? What would should I expect there?

[Toasty]

Re: iMac P/S posts then shuts down

Ya gotta start grabbin' datasheets man...



Happy reading!

Device will output about 60-130kHz signal, depending on setup. The newer supply that nwd had was running about 60-63kHz under load. I was getting 130kHz in between the transformers with a loop probe to my freq. counter.

Toast

[godonr]

Re: iMac P/S posts then shuts down

i agree... but do appreciate the spoon-feeding so far. Haven't worked much with newer power supplies so i was trying to get a feel for where to look. You gave me some good tips... appreciate it. Will post back if i find the source. Thanks again

[Toasty]

Re: iMac P/S posts then shuts down

hmmmm? Much too quiet...

How's the "homework" going?

[godonr]

Re: iMac P/S posts then shuts down

Haven't given up but I may have reached the wall that separates the newbies from the real techs. I'm having trouble locating the "source" for the various voltages- 3.3, 5, 12 and 24. Example- on the LM339 of a working supply, I see input 2 with +20 & -20 and +24 (Vcc)Supply yields output 2 of 0; input 3 has 0 & 2.5 and 0 (Vdd) Supply yields 3.3vdc; input 4 compares -1 & +2.8 to yield 3.3; input 1 does -2.6 & +3.1 to yield 3.3 at output 1. This is on a psu turned on with the 15/16 jumper. Now the one thing I've noticed on supplies which either run and shut down or won't go beyond LED1 lighted on the mobo (I think I saw where you guys discussed the 4 LEDs this iMac uses to indicate good power-- if not, LED1 just senses voltage from the psu, LED2 acknowledges startup and peripherals, LED3 is for display (to 20" needs the 24v here) and LED4 doesn't come on unless there's and overheat problem,) is the lack of 24 Vcc. And, while I can find it at TPs I lack the knowledge/skill/good eyesight to track it to its source. I think its at UGIODCT 0420 because after T1 puts out two 12v rails UGI shows 12v on its outside legs and 24v on the center. This carries up through L1, C59, D21 and out +24 to pin 22. Bingo that should be it, right? Not exactly, I've tested some supplies where 20v appears on the cathode side of D21 and 0 on the anode side. I've no idea where that's coming from. Anyway, I think the UGI rectifies the two 12s from T1 to yield +24. Also in that area under T1 are a bunch of smds including M2 which is a 3845B Z442 PWM which I know what it does but not how to tell what it's doing here. I'm also not sure how to read the TOP242 correctly and wondering exactly what its "multi-functions" are being used for. It seems like its protection circuits like WT7515? All this regulation is getting me confused!!

[marky]

Re: iMac P/S posts then shuts down

hi, i'm fixing my brother imac g5 with psu pn: 661-3350

does anyone have the pinout for the 22pin connector on the output of
the powersupply? i want to make sure the psupply is good first.

i have repaced c40, c63, c49, c47, c52, c45, c1 in the supply so far.

tnx, mark

[Toasty]

Re: iMac P/S posts then shuts down

Welcome Marky -

>>i'm fixing my brother imac g5 with psu pn: 661-3350<<
- So I'm clear, it looks like the pics in post # 8 in this topic, correct?

>>i have repaced c40, c63, c49, c47, c52, c45, c1 in the supply so far.<<
- Were the caps failed and you're replacing them or are you doing a preemptive recap?

- What brand & series have you replaced them with?

>>does anyone have the pinout for the 22pin connector <<
- See post # 12 in this topic for picture of connector. FYI - Numbers are on the wire side of the connector (tiny).

Voltages measured from ground are as follows:

3, 5, 7, 11, 14, 16, 17, 18 - Ground (-)

1, 2, 12 - 3.3v

4, 6, 20, 21 - 5.1v

8 - (Power Good) - 5v

9* - (Standby) - 5v

10, 13, 19 - 12v

15 - On/Off - jumper to ground (14 or 16) to run supply

22* - 24v (may read about 21v until switched on)

* Always present when unit is plugged in but not switched on.

If you encounter problems with the PSU or system, please open a new thread. It's easier to focus on one unit at a time and keep on track as to which problem we're dealing with.

Cheers!
Toast

[marky]

Re: iMac P/S posts then shuts down

thankyou toasty, that's exactlly what is was looking for.

6 caps were bad- c 40, c49, c47, c52, c1, c53 - i actually tested them
with my ESR meter. I replaced all of the other electrolytic caps too. the brands i used - decent brands from my local electronics supplier that i have used for years, i didn't pay attention to the manufacturers, they're already installed. i've used the his stock in the past with no problems, but i do recall 'rubycon' & 'elna'.

series - again i didn't look.

i installed the ps into the mac, i get led 1 on, but it doesn't power up via top switch nearest fan. i'm going to remove ps, check more voltages & then force ps 'on' via jumperring of pin 15.

mark

[Toasty]

Re: iMac P/S posts then shuts down

See Post # 10 in this thread for LED indications:

https://www.badcaps.net/forum/showthread.php?t=7124

You may have some motherboard caps to deal with also.

Toast

[marky]

Re: iMac P/S posts then shuts down

hi, i do get the 20vdc & 5vdc standby voltage, but the rest of the ps doesn't fire up when shorting pin 15 to 16. so i'll check the 'not quite their' thead u refered to toasty,

tnx, mark