Newbie - Imac 24 - Liteon PSU - no power at all

[StraitNet]

Hi all...I have very little experience when it comes to troubleshooting boards but have always wanted to learn.
Yesterday I picked up an Imac 24 for $20 that wouldn't power on and figured it was a good first project.

Imac 24 A1125 - PSU is Liteon PA-3241-02A Rev: A

So far I have checked that it has power to the incoming line that plugs into the power supply board.
It has NO l.e.d. lights so it's not getting a trickle charge.
I can't see any caps that have bulged tops.

I will attach a photo of the multi meter I have..again not even sure how to use that all the time...sigh..lol (I know how to set it to check 120 volts and change it to check continuity)

If someone can give me a hand and tell me what I should check, and how to check it that would be great. I have done a lot of searching but many of the terms are foreign to me.

Thank you for your time.
Christian

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

I may be way off here but just pulled out a magnifying glass and looked around the board.

Next to the AC input there is a small black box that reads:
T 5A 250V

I flipped the board over and checked between each post on that box for continuity and got nothing.

Is this by chance a fuse that could be blown?

[budm]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

That is the Time delayed (SLO-BLO) 5AMP 250V fuse, so it means the board has shorted circuit some where that you will have to figure out.

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

That is the Time delayed (SLO-BLO) 5AMP 250V fuse, so it means the board has shorted circuit some where that you will have to figure out.

Did I test this fuse properly? and is it bad if there's no beep when I touch both contacts with my tester?

Also would a power surge cause this to blow or just a short somewhere?

Thank you.

[budm]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

If it does not show <1 Ohms (depends on your meter probe wires resistance) then it is bad.
Yes, it can blwo due to the surge or shorted circuit, you will have test the bridge rectifier, MOSFETS, etc.

[selldoor]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

Any idea why your meter is set on ACvoltage 200 and has a reading of 282 - what were the leads on. If there is a lull in responses I recommend you set to and clean the fan.

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

If it does not show <1 Ohms (depends on your meter probe wires resistance) then it is bad.
Yes, it can blwo due to the surge or shorted circuit, you will have test the bridge rectifier, MOSFETS, etc.

I only had my meter on the setting that would beep if there is continuity..didn't check ohms...mainly because I didn't know to do that...sigh
and the other stuff you mentioned is still a foreign language to me as of now...lol That's what I want to try and learn how to do or test.

Thank you.

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

Any idea why your meter is set on ACvoltage 200 and has a reading of 282 - what were the leads on. If there is a lull in responses I recommend you set to and clean the fan.

That was just a photo I had already showing voltage on another power supply.

I will be cleaning the fan once I get my compressor back home

Spent today digging out the various boards I had been saving for a day that I could learn this stuff...and also pulled open a few other power supplies to see if I could find this same fuse..no luck of course.

[selldoor]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

Ok maybe the reading is 28.2 not 282.
You are going to have to remember to change settings and lead sockets
for different tests. Black lead remains in the COM socket.
Testing Voltage AC
Red lead in V socket turn dial to ~V If expected result is more than
200 VAC turn dial to 600. If in doubt use highest figure - you can then
turn the scale down and test again for more accuracy.
Testing Voltage DC
Red lead in V socket turn dial to (Blacklineover 3 dots V) 200,20 ,2
depending on expected result - as with AV if in doubt start high.

Resistance test (OHMS)
Put the red lead in the Ohms (omega sign/ upsidedown horseshoe) socket
and turn the dial to 200 2k 20k depending on expected result.


There are lots of utube videos on how to test components

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

Selldor...I just didn't know what the expected voltage was on that given power supply so I was just trying to get a reading anyway I could. There was no markings or info on the internet for that supply. Someone ended up helping me out and telling me the voltage looked good though.

I have watched many youtube videos this year but still most of it is over my head since I haven't got the basics down yet and most people in the videos use much higher quality testers with different dials or functions it seems.

This Imac board will be a good way to start if it's more than just this fuse...I really want to learn how to find a problem when something doesn't power on.

Do I start at the input and check resistance on each piece on the board one at a time?
Do I check the actual printed circuit for shorts?
Do those components on the boards (cap,fuses,mosfets etc,) all have the expected readings written on them or do I have to research each before hand?

That sort of thing...process of elimination I guess.
Thank you.

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

Update:

I finally got the pack of fuses in the mail today, re-soldered a new one to the power supply board and as soon as I turned it on..POP goes the new fuse

I am searching and searching on how to trace a board out..check for shorts, bad MOVS, CAPS, etc. (still foreign language to me).

If anyone wants to give this newbie a head start it would be greatly appreciated.
Thank you.

[stj]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

check the fets - black things with 3 legs

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

check the fets - black things with 3 legs

Do I just check for continuity? Sorry not sure what to check..or how

Update:
I also just did another test..
I replaced the fuse again and had everything unplugged..Except the power in clip (two wires blue and brown) before trying to give it power.
As soon as I even touched the outlet the same fuse popped again...didn't even have to plug it in.

So it must be a short on the Power supply I am "thinking".??

Not sure where to go from here though.

I am very very slow at this desoldering and soldering process...but don't mind practicing if that's what it takes.

[stj]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

just check the thingsi mentioned for shorts between the pins

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

It looks like all the black things with three legs are bolted to aluminum cooling blocks or something....is that right?

Just making sure I google for the right thing to figure out how to test for shorts.

Thanks

[stj]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

usually they are.

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

So I have found info on how to test those Mosfets...but I am still trying to figure out what they are connected to and why?

Do I need to take the aluminum blocks all a part and remove each Mosfet before testing?

Is there any sort of grease etc. on the mosfet contacts? similar to CPUS (cant think of what that is called right now)

Thank you for all help so far...I am trying

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

Ah..
Thermal compound and heat sinks..those are the terms I was looking for.

Now that I found an article on this stuff..it sure seems more complicated than I imagined.

[StraitNet]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

I just created an image showing different parts of this power supply board.

When I set my multi-meter to ohms (beep test) and check the legs on item B (both of them) I get no reading...no matter how I test them, straight across, diaganol etc.

Should I get a beep here? and what is this called?

Also if you can let me know the names of any other items that would be great.

Thank you and Happy Thanksgiving

[TELVM]

Re: Newbie - Imac 24 - Liteon PSU - no power at all

I'll try to help:


A - If the fuse pops there is something shorted, it will keep popping until you fix the short.

B - Look like optoisolators, I think it's OK if they don't 'beep' for continuity any way.


C3 - Bridge Rectifier.

C2 - Active Power Factor Correction (APFC) Pulse Width Modulation (PWM) MOSFET (transistor).

C1 - APFC boost diode.


E1 & E2 - Primary switching transistors ('switchers').


D1 & D2 - Secondary rectifiers (probably diodes, some modern PSUs also use transistors for rectification).

F - Looks like a Y-capacitor.



Recommended readings:

http://hardwareinsights.com/wp/how-a...-supply-works/

http://www.techpowerup.com/articles/...king/psu/160/1

http://www.hardwaresecrets.com/artic...Supplies/327/3

http://www.tomshardware.com/reviews/...ce,3061-2.html

http://www.repairfaq.org/sam/smpsfaq.htm