Acbel (from Intel iMac) power supply dead -- any hope?

[TechShui]

Model Acbel API4ST03
Apple iMac power supply (2007 model, Core2Duo, 512MB RAM).

I made the grave mistake of working on this iMac while it was off, but still PLUGGED in.

These PS don't have any metal shielding, they're mounted with the solder-side up, the LCD panel screws in on top of it. (there's a black rectangle of insulating paper adhered to the back of the LCD).

As I was reassembling the thing, the top-left corner of the screen's metal frame touched something on the Power PCB. I heard a very faint buzzing, so I lifted the Lcd out again.

The weird part is, it took a few seconds to happen, but then a spark emited from the area I circled!

I'm guessing ESD is to blame. Is there any hope for repair?

The system won't turn on at all now. I tested the smd resistors in that part of circuit but they all seem OK, not shorted. There's an SMPS IC above the area that sparked. I'm looking for a sch for it right now... will post the results soon.

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

There are no schematics. Proprietary information.

A clear close up of the area (macro) would help.

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Hi Toasty, I attached a closeup. Let me know if you also want an HQ version of the entire board?

I was referring to the SMPS IC schematic, which is not proprietary. Here it is: http://html.alldatasheet.com/html-pd.../TEA1654T.html

Bad news, I think my DMM is broken. Which basically nullifies all the readings I took off the smd resistors.

Only the Ohm portion of my meter is acting bad.
I was retesting the tiny 223s (22k ohm smd) resistors, which earlier had 17.50 with my dial on 20k. And now they have 0.55 and slowly rising.

Just to be sure, I checked a new 2.2k ohm resistor that I had left over from another project, and I got a reading of 0.16 and slowly rising.

[retiredcaps]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Bad news, I think my DMM is broken.

What brand and model?

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

I was looking to see if any of the resistors had fried or if that transistor was popped. I really can't see if they are.

There is a seller on eBay with a bunch of these for $40, free shipping:

http://www.ebay.com/itm/251030600287

Look at the IC pinout and see if you have 5v on pins 8 & 11. When the unit switches on, you would have that on pin 13 also.

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

What brand and model?

Innova 3300 Digital Multimeter

I was looking to see if any of the resistors had fried or if that transistor was popped. I really can't see if they are.

There is a seller on eBay with a bunch of these for $40, free shipping:

http://www.ebay.com/itm/251030600287

Look at the IC pinout and see if you have 5v on pins 8 & 11. When the unit switches on, you would have that on pin 13 also.

By "transistor" do you mean that tiny VR mosfet in between the smd resistors?

Pin 8 (vcc) - 0v
Pin 11 (vcc 5v) - 0v

Then switched to HV setting and checked these pins too.
Pin 7 (drain) - 161v
Pin 6 (HVS) - 0v
Pin 5 (HVS) - 0v

So can we conclude from this that the SMPS controller IC is dead and needs to be replaced?

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

No. 5vstb is dead.

See attached photo. The 2 resistors "look" suspicious, but it just could be the grain in the photo.

Since the spark came from that area, check the indicated transistor too.

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

I agree that they look suspicicious.

But it's very difficult to discern any markings, each of those smd resistors about 1mm in length. I'm using a magnifying lens, and I think it says 24x (last digit is scratched). The other one MIGHT be 2x2 (another scratch).

So assuming they're the same... 242 = 2.4k Ohm 1mm SMD resistor.

The transistor looks like 2FY or 2FU is printed on it. Believe it or not, what you see in the photo is better than what I see with my eyes and magnifier. Great zoom on that camera

I did a ton of searching This could be the component, maybe?
http://www.datasheetdir.com/DF3A8-2FU+download

Or maybe not, that one shows 8.2 as being the standard marking.

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

The "scratches" on the resistors may indicate these are burned out. Typically the surface should appear to be smooth and legible under magnification. A 10x jeweler's loupe is recommended for good examination.

The transistor is a 2F, quite common for AcBel and others. Device is a BC550B or 2N2907 PNP transistor.

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Thanks Toasty,

And what's the name of that transistor's package? Digikey only has 2 SMD versions to choose from.

3-CLCC package:
http://search.digikey.com/us/en/prod...AUB-ND/1636469

And 4-CLCC
http://search.digikey.com/us/en/prod...AUA-ND/1636468

No photo on either, but I think the number refers to the pins, so 3-CLCC is correct?

For the SMD resistors, is there any way to tell what Wattage I need? 1/4 Watt maybe? So far I keep getting no results found Digikey. I had selected Vishay Film, then I tried selecting ALL manufacturers with CTRL + click.

Still no results. At least not the ones I need
http://search.digikey.com/us/en/cat/...69?k=resistors

Maybe Digikey only carries Panasonic ERJ SMD resistors? I will try Mouser.

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

SOT-23 is the package. MMBT2907AFSCT

http://search.digikey.com/us/en/prod...FSCT-ND/458970

You have to measure the resistor (in mm) and find the appropriate metric that will fit.
Best guess would be in the 1/8w size.

Why are you looking at the transistor before you have tested it?

This still does not answer the 5vstb missing problem.

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

The new closeup pic is pretty much self explanatory.

I retested everything, then I retested again while someone else pressed the power button. There was absolutely no change. All the readings stayed the same with or without the power button held down.

I don't know why I got 0.00v at pin 11 when I tested it 2 days ago -now I see 0.31v. The likely explanation is that the probes were not making good contact with the pin. (those pins are so tiny after all

All readings use pin 10 of the smps IC for GND.

Btw, is that pin 10 (GND) supposed to be live? While I was measuring one of my fingers slipped and touch the black probe while it was touching that pin. I got a nice jolt, could feel it all the way to my elbow

One more note, I don't think the transistor is bad after seeing these test results. Needless to say the 2 resistors definitely bad, right?

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Everything to the right (underside pic) of the 2 optoisolators is Mains HOT.

Ground would be from the [-] of the mains cap for the circuit. That could easily be live, especially if using an extension cord or a bad outlet (mis-wired). Isolation transformers are strongly suggested.

As I do not have one of these to go by and offer you more precise information, I am advising that you consider purchasing the one from eBay.

Cheers!
Toast

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Yes, but just for the record, there IS hope for this PS?

That was the original question after all.

My biggest hurdle will be deciphering the ruined markings on those resistors. If I'm lucky that's all that was damaged.

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Yes, nearly anything can be repaired... for a price.

I doubt that it's just the resistors, as the failure (spark) occurred -after- you removed the short. That means some other component failed and the resistors failed because of that (unknown) device's failure.

[Agent24]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

It will likely be difficult, without schematics and with burnt SMDs

Best chance is probably to get another PSU, at the very least you'll have a working PSU, but you could also do a comparison of component values, voltages, signals etc which might help fix the first one.

[TechShui]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Yeah, I'm still searching for good pictures of an Acbel API4ST03. Hard to believe with all the iMac PS-related posts on this forum, not even one picture is in focus.

The best I've found so far is on iFixit
http://guide-images.ifixit.net/igi/D...xxeugkAEa.huge

From that I can (barely) see the value 243 on the burnt resistor that's closest to the SMPS IC.

It also appears from that photo, that the other burnt resistor has a different value... I think?

[Agent24]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

You could try asking around if someone with one of those PSUs can take a better photo...

I wonder if that IC is shorted considering the stuffed resistor appears to be connected to it.

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Well, the eBay ones are gone, so that ship has sailed.

You still have no 5vstb.... hint hint

Between yours and the iFixit pics, I'd call 242 on the resistors, 2.4kΩ.

[Toasty]

Re: Acbel (from Intel iMac) power supply dead -- any hope?

Got one! Fixed it!

The resistors are indeed marked 242 and as previously noted are 2.4kΩ.
The size code is 1508 (metric) or 0603 (inch).
e.g., 1.5mm x 0.8mm or .06" x .03"

On pin 8 of the IC TEA1654T you should have, at standby (no load), around 10v. With the unit loaded and running, voltage rises to around 14v.
Use the negative(-) of the bridge rectifier as your ground for the meter.

If that is missing, you need to trace the circuit back and find where it has gone missing.

Across the mains cap @ standby = 166v, run = 385v

These units provide a constant 12vdc to the system. They sense the system being turned on and come up to full power once a load above 0.2 amperes is presented. A 194 automotive marker lamp works well. Below that they provide 12v, but the current is low and the 12v may appear to "wobble".

The mysterious brown lead** is a shutdown lead that outputs to the system in the event of a PSU malfunction (Over Voltage). The signal is provided via the 5 pin SIP, M51957B IC on the top side of the board next to the output wires.

** The brown lead was thought to provide the turn on signal via the onboard battery to these units. I have now confirmed, this is not the case.