Imac G5 20"' black screen only :(

[koektrommel]

Hi,

I have problem with an 20" Imac G5 i bought recently. It's a model from i quess 2005.

The problem is i think somewhere on the logicboard, since the PSU is 100%. I checked that by simple putting in a good working PSU.

Symptons, in folowing of happening:

- first, it bootet up normally. Chime, screen, all oke and lasting for hours.
- When you shut it down and start up again it sometimes worked well, sometimes it won't start up at all or the screen stayed black (but.. Chime!) The fan obviously blowing fast.
- Now, i can do whatever i want, but it won't boot at all. Only the fan start blowing fast, no chime at all. It simply won't boot and the screen stays black

That''s all... the Imac won't boot at all and it's not the inverter. Otherwise i'd hear some noise, the chime etc.

I've read about reflowing the GPU, what do you think? Is this the problem?
Does a failing GPU leads to a Imac that won't boot complete?

I checked all the capacitors and they look great. No bulging caps at all and the ESR value is also ok. Most were +- 0.10-0.12 or so when measured onboard with a ESR Micro. When we had a screen, we did see a perfect screen. No lines, no failures like often described.

Nevertheless, somethign isn't good at the logicboard, but i don't have a clue what it can be. It's not the PSU, its not the HDD. I swapped them, no difference


Someone an idea?

[koektrommel]

Re: Imac G5 20"' black screen only

I forgot to ask the question whether it could have something to do with RAM-memory? Or a defective RAM-slot?

[Toasty]

Re: Imac G5 20"' black screen only

Motherboard (Apple calls it midplane) has bad caps. You can't test most of the motherboard caps while on the board as they are connected in parallel. The individual, outlying caps you can, but the main bank next to the CPU and the ones down by the PSU are all tied together.

What does this statment mean?:
"Most were +- 0.10-0.12 or so when measured onboard with a ESR Micro."
Plus or minus -what- unit of measure?? If that's the reading in ohms, then the caps are (real) bad. If it's the variance between them, it's meaningless.

Are you getting 1, 2, or 3 lights on motherboard? How many and when?

Not RAM, nor GPU problem.

Pictures of your motherboard caps please.

Toast

[koektrommel]

Re: Imac G5 20"' black screen only

Hi Toasty,

I measured the ESR from each capacitor. I thought the ESR was plusminus 0.10 ohm or so. The capacity was also ok as far as i could measure. None of them was bulging. I've repaired many settopboxes and know from that experience that the chance of a bad cap is big but to my opinion these are all ok, although they could have a lower esr offcourse. Otherwise i would have measured bigger ESR-values .

I got two lights, which indicates a problem with the logicboard communication. I simply won't start up anymore now. Harddisk spins up, that's it and the fans are making a lot of noise. No chime, nothing further than the fans and a black screen Software isn't loading; it's not that the screen is defective or so otherwise i would hear a click when i switch the volume with the keyboard.

I found the similar symptons for a Macbook and there the reason seems to be found in the RAM.

I'll try to make a picture but that will take some time. Don't know where my camera is.

[Toasty]

Re: Imac G5 20"' black screen only

I repeat-
"You can't test most of the motherboard caps while on the board as they are connected in parallel. The individual, outlying caps you can, but the main bank next to the CPU and the ones down by the PSU are all tied together."

So unless you unsoldered those major banks of caps and tested them individually, you can not say the caps are not bad.

These caps are -common- failures in these units and will give the problems you are experiencing. I have the same unit with the same problem and I know it's the motherboard caps.

Your description of declining functionality points right at the caps. RAM or GPU failure would be that immediately, not eventually.

There are many iMac G5 threads in here. Read those that are motherboard related (many are PSU topics). Also see Topcat's (site owner) full Poly-mod of the G5. He sells the kit for both a "regular" electrolytic recap and the poly-mod recap.
See: https://www.badcaps.net/pages.php?vid=21

Macbook is not iMac. Different beastie.

2 Lights is normal. 3rd light when powered up fully is correct. Also mentioned in a couple of iMac threads.

In lieu of pictures, report cap make and series of those next to CPU and those right above PSU. Also, what is date of unit manufacture?

Just because PSU is working -now- does not mean it will be in another month. To do these correctly, you need to investigate PSU and confirm no problems. Those caps you can test as individuals, as they are not tied together.

Your reading of 0.10Ω is way out of line. Especially for a bank of low ESR capacitors. Max acceptable ESR of any bank would be <0.001Ω = 1 milliohm.

Verify your readings and meter scale with a precision low ohms resistor. 0.47Ω or less.

Toast

[koektrommel]

Re: Imac G5 20"' black screen only

You were right. I forgot that these were low esr caps. For general purpose capacitor the esr is quite right according to my opinion. But most of the time i see really swollen or bad caps with complete gone capacity and extreme high ESR-values.

I tried the hairdryer on it and with hot capacitors it started how it should be ... this thing is lying here for more than eight months. Tried to change the batterie, measured the capacitors but i couldnt find out which one. I think i made a mistake somewhere.

Didnt know what i did first, but i removed the battery and perhaps i heated all the caps and then it worked. Later on i heated the beds of capacitors and it's in the midplane somewhere.

[koektrommel]

Re: Imac G5 20"' black screen only

I''m not convinced yet

I thought i found the capacitors that gave the problem. Heating that place would give the chime and startup back. So i thougt, let's remove those and check them to see if they're really the ones that are giving the problems:

The answer: nope I measured all the 120Uf/16V Chemicon KY and these were also OK! Capacitance of 116 and 121, ESR-value of 0,19 which is also OK. ( I had some 100uf here, but they're new and having an ESR which is 0,25 so i didnt replace them.

Nearby, at the otherside of the board, there's a chip: BROADCOM bcm5971KPB. Is there a connection between the startup (chime) ?

And, another question. What does the GPU look like?

[Toasty]

Re: Imac G5 20"' black screen only

Why are you testing 120uF caps? They are not the problem. The huge banks of caps are.



Toast

[koektrommel]

Re: Imac G5 20"' black screen only

Because when i put the hairdryer there and getting them really hot, the chime came back and it started directly. I heated the different capacitors several times separately and only when i heated those 120uF, the chime and startup came.

I've checked the great banks, removed all the right 2 rows of capacitors from the bank: all good: >1800uF and esr-values of 0.02 ohm each. None of them was bad! I made a mistake by telling earlyer that i measures values of 0.12ohm or so.

Replaced the 1000uF chemi-con KZJ caps, these where 0,10ohm and replaced them with new KZE's with ESR 0.02.

Right now, i don't have the 1800uF caps, but i'll replace them later on. But when heating, the 120uF caps, it starts so that was the area i was looking for. After testing and resoldering those, concluding they were also OK i decided to give the chip a reflow, thinking that might be the problem cause it's directly next to it beneath the board (after all, that area held the problem so i directly reflowed that chip since the 120uf's were ok. After that, it started with a questionmark (solved) and now it's starting again but only after heating but less heating necessary.

I'll check the 1800uF left to the 1000uF again, since they also get some of the heat.


Perhaps it's heating up the surrounded capacitors of the left bank (4x 1800uf and 3x 1000uF)