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PSU caps very slow charging (G3 iMac)

RickNel

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Joined
Apr 24, 2009
Messages
641
Location
Canberra, Australia
I have a machine that won't boot from overnight full power-off, but gradually goes further in the boot sequence for every hour that it is plugged into AC mains. Boots fully after about 5 hours on standby. Once booted, runs perfectly.

Standby draws a steady 5-6watts.

On another G3 iMac I've previously done a complete replacement of electrolytic caps on the Power-Analog-Video board (PAV) - around 30 through-hole caps up to 560uf.

I would prefer to identify which specific cap(s) could be causing this behaviour. Could the 5watt trickle current really be charging up some of the larger filter caps, given enough time? I thought the trickle current only feeds the start-up logic and switching control, which to my mind does not explain this slow conditioning effect. I've never seen behaviour like this in a PC-style PSU.

Rick
 
It could be a faulty component in the standby circuitry, one that has become temperature sensitive.
From cold, small current running through it slowly warms it up.
Do you have any electrical freeze spray?
 
I don't have freeze spray. My workshop (under house) gets close to zero degrees overnight at this time of year. I might try the temperature test in reverse - ie. warm the thing up with a blow heater and see if that makes it boot sooner. Still begs the question of which component to suspect.

Rick
 
I don't have freeze spray .. I might try the temperature test in reverse - ie. warm the thing up with a blow heater and see if that makes it boot sooner. Still begs the question of which component to suspect.
I have come across temperature sensitive capacitors, and semiconductors (mainly ICs).

If your 'temperature test in reverse' produces a result, then the use of electrical freeze spray (usually supplied with a narrow-tube nozzle) should allow you to pinpoint the component.
 
The startup circuit on my IIci locks up during the winter months and won't turn on, and works fine in the warmer months in my playroom which isn't adequately air conditioned. I usually have to set a space heater next to it in the winter for a few minutes before it will work. ;)
 
Sounds like Geoffm3 and I both have Macs that have acquired human characteristics. I start more slowly myself on winter mornings. Is this proof that The Singularity is approaching?:confused:
 
Sounds like Geoffm3 and I both have Macs that have acquired human characteristics. I start more slowly myself on winter mornings. Is this proof that The Singularity is approaching?:confused:

LOL! That's pretty accruate. I have that problem myself too.
 
I did a series of experiments booting the machine from a cold overnight environment (~6degreesC) and from "room temperature" in occupied part of the house. Also tried pre-warming a cold machine with a fan heater.

Neither the warm-room nor the pre-heat experiments delivered an instant bootup. Both cases resulted in boot being achieved faster than in stone-cold environment - that is, in less than an hour.

In all temperature environments, leaving the machine plugged in to AC for several hours produced instant boot.

I'm pretty convinced a full cure for this would be a complete re-cap of the PAV board. Now wondering whether its worth committing to that (mostly time + ~$$), or just leave it as is. It's not as if I have any need to boot the thing instantly on a regular basis. It's just a nagging feeling that I don't like leaving known issues unattended.

Rick
 
I don't have any specific product recommendations to make, but Googling "in circuit ESR capacitor" might help.
 
This simple DIY ESR in-circuit tester looks interesting. I saw some techniques using oscilloscope and signal generator - but since I don't have a signal generator, a self-contained in-circuit testing unit could be a good alternative. At least with hope of identifying which particular caps need replacing instead of committing to a complete re-cap of about 30 items.
 
I've been poking around for ESR meters lately, and while I haven't purchased it yet, the MESR-100 looks like it'll do a good enough job, is portable, designed for in-circuit testing (low test voltage) and already inside a nice case with probes etc. Cost is around $50ish available from a hundred different suppliers (yey China). The quality / recommended models are usually 150 plus, but I don't think need quality to determine if a cap is "stuffed" or not. If it's border line - replace it.

There is also the component testing clones which use a microcontroller - but the test voltage level is usually too high for in circuit ESR stuff (e.g. it may turn on various chips etc) from what I gathered. Those are super cheap (like $20).

Also worth having a whirl with the capacitance mode on your multimeter if you have it. I just replaced a dead cap - 10% of it's original value (was 10uF, now 1uF), kind of stood out :)

For the scope method, I think you can get away with using the 1V square wave build in to the scope which is normally used for calibration, I find it all too fiddly so want to grab a meter (but don't want to pay 200).
I'm not a pro at this, just been looking in to it myself.

I'd also recommend searching google / google groups, because I'm sure I've read about this issue before somewhere.
 
I don't even bother with ESR meters. If a device ever shows the slightest signs of capacitor related problems it's cheaper just to replace whatever caps I suspect.
 
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