iMac G5 PSU - no power

[Toasty]

Re: iMac G5 PSU - no power

Diodes can read like that if they're across a coil. But that seems abnormally low -both- ways.

You'll have to remove the diode to get a proper reading. Because this is a double-sided board, you can't just unsolder the legs on the underside and get a reading. The vias or thru-holes will still make contact.

Pull it out and check it.

105v means that they're good and current is being drawn.

AC & DC voltages seem correct. The voltage at the cap seems low and that diode being shorted could be the reason.

<<crosses fingers>>

Toast

[seanc]

Re: iMac G5 PSU - no power

I hope so - be interesting to see what happens next.

If I take that diode out and results are the same, I'm thinking the procedure for replacing it is to measure another one that looks the same and try and find a replacement? I'd have to take a good one out of circuit for an accurate reading too?

I might pop downstairs shortly and remove the diode, if not it's a job for tomorrow - Saturday!

[Toasty]

Re: iMac G5 PSU - no power

On that diode, I'll bet the upper leg (cathode) is connected, via the topside trace, to the (+) leg of the mains cap. The lower leg (anode) is hooked directly to the Drain of the MOSFET.

Now betting heavily that diode is bad.

It is a high speed diode, so no regular diode will work here. Also, probably rated 600v and 8A.

If it is and you go to fix it, get a new mains cap too. It's odd for that diode to go bad for no reason. Something -might- be wrong with the mains cap, even though it is a Rubycon.

Cheers!

I like this smiley:

[seanc]

Re: iMac G5 PSU - no power

Picture 1 in post #22 - bottom right, D7 is a small zenner - I should have made it clear which type of diode was shorted.

Searching RS components for your suggestion reveals big diodes!

Hmm :/

[Toasty]

Re: iMac G5 PSU - no power

<<takes all his money off the table>>

You never checked the 2 legged diode next to the MOSFET mounted on the heatsink? I thought that is what you referred to in the post where you said the diode read shorted...?

Go back and check that diode.

On the heatsink it's - bridge rectifier - diode in question - MOSFET


D7 is NOT a Zener. ZD9 is a Zener

Toast

[seanc]

Re: iMac G5 PSU - no power

Diode in question test fine.

[Toasty]

Re: iMac G5 PSU - no power

I checked on the PSU's here. Again, I don't have anything that matches, but the circuits are fairly similar I'm sure.

None of the diodes on my board(s) read that low. .3-.6Ω is typical with a DMM. I use an analog meter to sort out the in circuit diodes. Hate the DMM's because they show identical both ways on many that I test. The analog does not.

A reading of .003Ω is either shorted or across a coil. Trace the circuit 1st and see if it is. Follow the paths from the diode to the next device etc. You may find a more likely suspect, or that it is across a coil/trafo etc.

You're going to have to pull the diode off to test it. Remember these are soldered -and- firmly glued to the board. Use solder wick to get it as cleaned up as best you can and see if you can get it loose. If necessary, a replacement would still need to be a high-speed diode of possibly 1A capability.

Can you identify for me the 3 8 legged IC's on the solder side IC1,3,5 and the 2 on top near the big cap? I'm also looking for the switcher/chopper here. These have been using a TOPSwitch device that looks like the other mounted transistors, but has 5 or 6 legs. Does this have one?

Toast

[seanc]

Re: iMac G5 PSU - no power

I've had a nice break from this now and gotten a few other things out of the way, so have come back to it.

I went on a mad raving loony session and gutted a few power bricks and ATX PSUs tonight.

I found the same looking diode on one, so took it off. It tested 0.620. I took the one off the iMac PSU and it also tested normally with the same value. I swapped it for the hell of it, but I don't think it's the issue.

IC1: (ST Logo) L6562D 9437
IC3 & IC5: LM393 48J Z33L

ICs near big cap:
1. TOPSwitch K433 TNY267PN 30320E
2. 4H2 1001DL

[seanc]

Re: iMac G5 PSU - no power

Took the heatsinks and all attached devices out of the PSU and tested - all seem to be OK. Tested a diode that I found hidden beneath it all, showed shorted in circuit, but was fine out of circuit.

Replaced input cap - no change.

Tested L6562D - seems OK
Tested both LM393 - they both give readings totally different from each other. Now I just need to work out which of the many different parts are the correct ones to replace them with.

I have a choice of:

LM393M/NOPB
LM393DRG4
LM393DG4
LM393DG
LM393D
LM393ADG4
LM393AD

[seanc]

Re: iMac G5 PSU - no power

Admitted defeat.
Bought another used power supply off of eBay, it arrived, shock horror, it's the same model!

Popped it in, got 5vsb trickle light showing on midplane, hit power button and the iMac worked fine (only tested 20 mins). My first ever re-cap on the iMac G5 logic board was a success!

I think I'll have to open up the power supply in the name of badcaps and compare it to my broken one.

[Toasty]

Re: iMac G5 PSU - no power

I'm glad you acquired a working one. Recap it and enjoy!

I read back over this and going way back to one of my early posts, the TOPSwitch is a likely suspect. It generates the 5vsb and without that working, nothing else will. Get the datasheet for it and compare the voltages from old one to new one. It or its accompanying circuit has a failure. We were tracking the power in side of this and it seems so much time passed between that I lost track of where we were.

TNY263-268 link:

In essence, power flow is:

Mains > rectifier & filtering > TOPSwitch > transformer > 5vsb & 24v circuit > filtering > feedback through opto to TOPSwitch > standby power > unit startup circuit.

The LM393M/MX is what you're after, if you need one. That's the 8 pin SOIC package.

Toast

[seanc]

Re: iMac G5 PSU - no power

Ah I forgot to post, replaced the TOPSwitch with the same version - still nothing.

I'll poke around the TOPSwitch and see if it's putting out anything, then I guess we arrive at a transformer.

I have damaged a very small surface mount capacitor, it's lost the tinning on the end. I will try and measure this against another one and scavenge one from a dead board.

I'll get hold of an LM393M when I can too, then I just need to work out how to remove it. I'm about to attempt swapping a RTL8201BL package (network), so if I manage that, the LM393 should be easier.

[Toasty]

Re: iMac G5 PSU - no power

Look at TOPSwitch datasheet. (Actually TinySwitch) Several basic circuits in there that should allow you to troubleshoot this psu.

Transformers in these are not typical failures. Haven't had one yet.

[Toasty]

Re: iMac G5 PSU - no power

Did this ever get done seanc? Just in the back of my mind and pokes me once in a while...

[seanc]

Re: iMac G5 PSU - no power

I've moved house recently, I know which box it's in
It isn't fixed yet. Saving it for a day when I'm feeling brave.

The iMac in question definitely works, because I got an identical replacement PSU for it.

[Toasty]

Re: iMac G5 PSU - no power

Ok. Just that we are 5 months on from then and I thought it might have hit the bin since the replacement was working. Good to know the headache is in the future...

[Toasty]

Re: iMac G5 PSU - no power

Update - I've acquired one of these in operating condition and it should be in my hands within 10 days.

Hopefully, you still have yours...

[seanc]

Re: iMac G5 PSU - no power

Yes Toasty, it has survived another house move! Still have the iMac for testing too... need to get rid of it, since I have no need for it - it survived a total motherboard recap about 6 months ago.

[Toasty]

Re: iMac G5 PSU - no power

Got the PSU. Some slight variations in top layout. I don't know how different the foil side will be.

What is the label on yours? Pic?

This is DPS-180SB-2A REV:01

[Toasty]

Re: iMac G5 PSU - no power

Sean? Helllloooooo!



Also, what's the number on the edge of your board?