iMac G5 1.8G - Fresh caps, but not quite there...

[Toasty]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Did you do what mrberad suggested in post #16?

Please don't give up and please don't toss them out.

Toast

[dbridgma]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Musing over what could be causing the iMac 1.8GHz LCD failure...I'm pondering whether the temporary presence of extra caps (see post on 06-01-2009 above), and perhaps, solder that entered the thru-holes might have produced/contributed to the condition that's stopping the logic board from posting, or booting.

I inadvertently added two extra caps to these Rev A boards (the Rev B actually populates the extra holes with additional caps) & am now wondering if residual solder in the holes might be affecting the board's ability to function normally.

Been back over everything I can think of, however, there's still an acute lack of video signal on either board...& I'm out of clever ideas. Guess as a last resort I'll clean the holes where the additional caps were once present...scan the leads for cracked/cold joins again, but it feels like I'm shooting aimlessly in the dark.

Daniel

[dbridgma]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Correction to the ESR readings on the caps that were replaced on 06-01-o9...Anatek Blue readings were uniformly .08 or .09, not .8 or .9 as I erroneously recorded in that earlier post.

Those readings check against Anatek's website, as good cap ESR values on 1800uF v6.3 caps.

I'm checking these boards for continuity, however, I'm doubtful that I know what I'm looking for. Somewhere continuity has been broken because now I can only get a single trickle (green) diagnostic light. The second (PSU) light will not stay on permanently. It used to, but now it's not staying on. So it looks like I'm causing more harm than good to this logic board.

Daniel

[Toasty]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Sounds like bad solder joints somewhere or a failed/ing cable. Both units are having the same problem?

The ESR readings sound wrong. In circuit ESR should be reading .00 to .01 MAX. Those banks of caps are connected in parallel. ESR will divide each time another cap is connected. Same as resistors in parallel. Capacitance in parallel is additive.

Example:
100uF caps in parallel -
1 cap ESR = 3.2 ohm / capacitance = 100uF
2 cap ESR = 1.6 ohm / capacitance = 200uF
3 cap ESR = 0.8 / capacitance = 300uF
4 cap ESR = 0.4 / capacitance = 400uF

Make sure the meter is zeroed and check the calibration with the supplied resistors. Make sure you are pressing firmly on the cap connections and also the leads together when zeroing.

Toast

[dbridgma]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Okay, I'll test again if you suspect that these ESR readings are wacked. The Blue ESR meter is consistently measuring .08 or .07, which I interpreted to be below the .1 max for caps above 500uF (as summarized on the Anatek chart). But you're saying the caps in circuit should be .00 or .01? Their chart shows .01 as the base of the chart, all readings are above it...

Will bad solder joints push the in circuit ESR numbers upward?
Daniel

[KeriJane]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Hi Daniel

Yes, bad solder joints will cause the ESR reading to be high.

It is important that the Blue ESR meter is calibrated correctly and that you zero it at the start of each session by shorting the leads and pressing the button momentarily. Mine usually reads .08-.09 before zero-ing.

My favorite way to zero:
1-Find a spot on the board near where you are working with a good amount of solder on it.
2- Turn the meter ON, then press both probe tips into the solder fairly hard. At this point mine reads .08-.09. Notice how the slightest movement causes a change in the resistance reading? Bas soldering will have the same effect.
3- while maintaing pressure on the probes, press the button on the meter to zero it. The "zero" is only maintained until the meter turns itself off in a few minutes.
4- It doesn't hurt to have a new, quality cap of known, published ESR handy for reference.

I agree about the multiple caps in parallel ... with practically any caps the ESR should be very low, like under .01.

Solder in through-holes usually isn't a problem as most are designed for a component lead + solder to pass through.

I've got one of these G5 iMacs too! After recapping the PSU and replacing the screen I use it as a Digital Picture Frame for my mom.

Have Fun,
Keri

Sequence of events:
1- Caps in PSU blew.
2- Previous owner punched out screen
2- I bought the remains on eBay

[dbridgma]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Keri,

Thanks for the detailed summary of how to work with the AnaTek Blue ESR meter. It was not obvious how to zero it out before taking readings...or that doing so was necessary. Glad you chimed in.

So I used your method to reset/restart the meter before each testing...when I do this the caps either test at .01 (not the .09/.08 that the meter registers without reset) or .00. So, I'm assuming this means that they're within the range of functionality...although a cold joint may be lurking somewhere.

Appreciate the voice(s) of experience.

Daniel

[Toasty]

Re: iMac G5 1.8G - Fresh caps, but not quite there...



>>It was not obvious how to zero it out before taking readings...<<

Ummm.... page 9 of the assembly, troubleshooting, and instruction manual:

"(3). Hold the test probes tightly together – the test lead resistance is displayed.
(4). With the probes still together, press the button again to give a zeroed reading of “.00”. You can repeat this at any time."

...and you completely missed my instructions....

>>Make sure the meter is zeroed and check the calibration with the supplied resistors. Make sure you are pressing firmly on the cap connections and also the leads together when zeroing.<<

Yes, I AM enjoying this!

Toast

[dbridgma]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

Didn't miss your instructions, Toast...just didn't process them fully while swimming in a sea of conflicting multi-meter advise, much of it generic...& since the Blue ESR meter came assembled...page 9 of "assembly" didn't leap out.

This is all news to me, so each phase is fraught with missteps, half-recollected instructions. I'm slow...but trainable:-)

Daniel

[fety]

Re: iMac G5 1.8G - Fresh caps, but not quite there...

any luck? updates?