Late 06 iMac 5,1 20" No Boot, PSU noisy

[howardc64]

PSU = AcBel API4ST03. Pic attached.

iMac automatically attempts to boot without pressing power button when power cord plugged in. LED1 is on, LED2 flashes on briefly every few seconds along with HDD and ODD fan kicking just slightly. LED3 never comes on. ODD makes no noise, HDD is removed. This pattern repeats indefinitely. Known good PRAM battery installed, PRAM reset performed. I understand we have trickle power with LED1, LED2 shows logic board is attempting to start and failing every few seconds.

PSU makes a 2 noticeable sounds

- Buzzing sound whenever power cord is plugged in

- ~2Hz clicking/arcing sound if PSU is disconnected from logic board (either at the 5 pin PSU output connector or the 2x7 logic board connector) Sound file is here https://drive.google.com/file/d/0Bwn...ew?usp=sharing

- Disconnect the Inverter board makes no difference on the sounds or boot pattern

- Can't really isolate which component is making the buzzing or cycling noise.

PSU output measurement shows 12V on both grey wires. 0V on brown

PSU did have 1 bulging cap (1000uF 16V). I've substituted with some cheap Chinese cap (1000uF 25V, black color next to output wires) I have for now (I've read this forum and know I should get better caps) But for now PSU doesn't behave any differently before or after this cap replacement.

Couple of resistors have blemishes on top. See yellow arrows on the pics. Just to the right of the lower left arrow is the 2 solder points for one of the blue caps with some blemish between. Looking at the capacitor legs from the other side, I see no sign of leaks.

======

I guess here is my question

- Based on the LED patterns + 12V PSU output, and noisy PSU, is the problem further downstream in the logic board or could still be the PSU?

Thanks for any thoughts in advance. My knowledge is digital+computer science guy so analog+power supplies are beyond me

[Toasty]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

Welcome to the forums!

For starters, recap the unit with proper caps. Trying to diagnose it with failed caps will leave you with bruises on your head...

The ticking sound is the switcher trying to start, failing, and restarting.

These need a little bit of a load to run correctly.

Place a small 12V lamp or two across the 12V output. Automotive 194 marker lights would be best.

T

[Toasty]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

The resistors in your pics -

Upper - 33kΩ marked 333 -> Reads ~27kΩ one way and ~33kΩ the other

Lower - 27Ω marked 270 -> Reads 270Ω both ways

Don't miss the 2 tiny caps under the Mains cap. They are:

Outer: 68uF / 25V / 6.5mm X 12mm / 105°C - Ltec TK series
Inner: 22uF / 50V / 5mm X 12mm / 105°C - Ltec LZG series

Both should have an ESR around 1.2Ω to 1.6Ω.

The Mains cap should have an ESR around 0.2Ω.

150uF / 420V / 18mm X 40mm / 105°C - Ltec TY series

PDFs attached.

Ltec product link: http://www.ltec.com.tw/product_e.html

T

[Toasty]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

Contacted OP via PM as there has been no response to the replies.

Response from OP in PM [EDITED]:

<howardc64>...the ticking sound is expected when I unplug the PSU from logic board load and goes away when plugged in(?). I guess a light bulb load likely will do the same.

I'll check the caps + resistors values you mentioned.

Was hoping to score another freebie/cheap iMac so I can just swap PSU to confirm upstream or downstream.

<Toasty>You can score one from Ebay for less than $20 and free shipping. Just search for the model number API4ST03.

It should not tick when disconnected from system.

<howardc64>...you think the ticking sound without load indicate something is definitely wrong with the PSU correct?

Have a friend that have more knowledge on analog power supply circuits, I'll huddle with him with your feedback+suggestions

<Toasty>I have 2 here that make no noise without a load.

Problems with the output caps -might- present as this problem. Quite honestly, I'd spend my bucks on a used working one from Ebay. Test it with the system, and if okay, recap that.

You could easily spend $10-$15 on the caps + shipping.

[End PM conversation]

T

[jazzie366]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

PSU = AcBel API4ST03. Pic attached.

iMac automatically attempts to boot without pressing power button when power cord plugged in. LED1 is on, LED2 flashes on briefly every few seconds along with HDD and ODD fan kicking just slightly. LED3 never comes on. ODD makes no noise, HDD is removed. This pattern repeats indefinitely. Known good PRAM battery installed, PRAM reset performed. I understand we have trickle power with LED1, LED2 shows logic board is attempting to start and failing every few seconds.

PSU makes a 2 noticeable sounds

- Buzzing sound whenever power cord is plugged in

- ~2Hz clicking/arcing sound if PSU is disconnected from logic board (either at the 5 pin PSU output connector or the 2x7 logic board connector) Sound file is here https://drive.google.com/file/d/0Bwn...ew?usp=sharing

- Disconnect the Inverter board makes no difference on the sounds or boot pattern

- Can't really isolate which component is making the buzzing or cycling noise.

PSU output measurement shows 12V on both grey wires. 0V on brown

PSU did have 1 bulging cap (1000uF 16V). I've substituted with some cheap Chinese cap (1000uF 25V, black color next to output wires) I have for now (I've read this forum and know I should get better caps) But for now PSU doesn't behave any differently before or after this cap replacement.

Couple of resistors have blemishes on top. See yellow arrows on the pics. Just to the right of the lower left arrow is the 2 solder points for one of the blue caps with some blemish between. Looking at the capacitor legs from the other side, I see no sign of leaks.

======

I guess here is my question

- Based on the LED patterns + 12V PSU output, and noisy PSU, is the problem further downstream in the logic board or could still be the PSU?

Thanks for any thoughts in advance. My knowledge is digital+computer science guy so analog+power supplies are beyond me

That ChengX cap looks fake, I don't see a series anywhere, if that is real it should say CD110 or KM on it (likely). Either way that's not a low ESR cap, so don't use that.

Also, clicking could be it trying to start, but the cap responsible for that may have gone bad. Look for a small cap near the transformer that's gonna be very small, something in the region of 22uf 50v, etc.

[Toasty]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

Umm...., try reading posts 2 & 3 before posting.....

Cheers!
T

PS: Please don't quote -entire- posts. Unnecessary and just wastes time and space.

[PeteS in CA]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

It could be failing to start due to the start-up cap (common with 384x PWM ICs). It could be a shut-down-and-retry current limit trip due to something shorted in the secondary. If the power supply has an OVP crowbar, either a shorted SCR or the PWM going full pulse width could cause this with shut-down-and-retry current limit.

[Per Hansson]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

You can test the small startup cap in a cheap way (As Toasty said a replacement on eBay is only $20).
Find a hair drier with color of your choosing, then heat the area that Toasty described in post #3
(main filter capacitor, the startup caps are located under it).
Heat it up to at least 50°C or so but don't go crazy above 100°C...

Then try starting it, if it starts then replace the small startup caps.
And of course all the output caps because if one has leaked all are going to go soon anyway...
Rubycon YXH would be an excellent replacement for those. (But not from eBay, most there are counterfeit!).

[Toasty]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

Latest response from OP in PM:

<howardc64>...Been getting these iMacs for $0-$5 so planning on waiting for another cheap one to show up and swap the PSU

Well, that's the second time recently that all this advice has essentially been wasted. We'll never know what is wrong with this PSU.

T

[howardc64]

Re: Late 06 iMac 5,1 20" No Boot, PSU noisy

Thanks for all the suggestion guys. Have a friend much more knowledgeable on analogs (I'm digital+software) and we'll try various suggestions here and see what we get.

One problem with this PSU is everything is so tightly packed. Wary that component removal may result in more destruction. Don't know what type of solder they use but had turn up my Hakko FX-888D iron up to max temp (450C?) to barely get the 1 cap off so far.

BTW, there are problems further downstream of the PSU on this iMac. Previous owner took out the HDD for security but took the bracket, screws and temp sensor board+cable with it. Also ripped out the weak SATA connector on the logic board. The LCD have some prominent dimming along the edges so issues with CCFL backlight tubes. LCD was tested on a good iMac but nearly toasted the good iMac's PSU given PSU's exposed connections against the metal rear LCD frame, LCDs on these iMacs are really challenging to install as mounting screws are deeply inset and hard to put on. Anyhow, I thought I quit while ahead before ruining the perfectly good iMac. Anyway, more things to fix beyond PSU and probably not worth it unless another broken iMac provide all the missing+worn parts.