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eMac Power Supply Issues

CC_333

CC_333

68040
Hi,

I have an early eMac (700 MHz) that seems to have a weak power supply. It won't start up like normal. The hard drive spins up, and the fan works, but the screen stays blank. It only starts up if I leave it in that state for about half an hour, turn it off, and turn it on again.

Obviously this isn't normal.

Once it's on, it works fine (except that the primary IDE channel is dead, but I've worked around that). However, if I turn it off, even for a split second, I have to let it "warm up" again before it'll restart.

Any ideas as to why this is so?

c

 
Do you see any bulging capacitors on the logic board or the power supply (attached to the logic board)?

I don't know about the original 700 MHz and 800 MHz models but the later ones had issues with this. Just today I swapped five blown caps to revive a "dead" 1.0GHz eMac.

 
Hi,

There aren't any bulging capacitors as far as I can tell.

I know about the later models of eMac having been afflicted with the "rotting cap syndrome" or whatever it's called, but this one (700/800MHz) isn't affected as far as I know.

In the past, I've entertained the thought of buying a replacement analog/power board, but there never seems to be any for this model of eMac (at least not for a reasonable price; One site sells replacements for something like $150, and this machine's hardly worth $50 anymore!)

c

 
The ones I replaced were on the down converter board - the one that sits vertically off to the side and perpendicular to the logic board. I couldn't inspect the capacitors until I had removed the digital assembly.

 
The ones I replaced were on the down converter board - the one that sits vertically off to the side and perpendicular to the logic board. I couldn't inspect the capacitors until I had removed the digital assembly.
Click to expand...
I swapped that with a known good one with no change in behavior.
I suspect the problem's on the analog board itself. I just don't know what the problem is!

I'm not opposed to making component level repairs, but I'm not opposed to replacing the whole board, either.

I'm still gathering up advice here so I can figure things out. I'm pretty much stumped.

c

 
use the blowdryer trick. If heating certain areas against others, allow the machine to fire up normally, You... have bad capacitors...

 
use the blowdryer trick.
Click to expand...
Thank you for the tip!
I will give it a try when I get some free time. Hopefully it's not as hard as I think!

If I determine that I do have bad capacitors (and the symptoms do seem to suggest that as a possible cause), how do I go about replacing them? To clarify, we're talking about the big power capacitors, not the little ones on the logic board, right?

c

p.s. Jeez!! Capacitors are failing all over the place these days! Power supplies... logic boards... Where will it end?!

 
Right... We are talking standard PCB mount radial electrolytics.

Capacitors play a critical role in electronics so they will NEVER go away. They pass AC and block DC.

Capacitors act as 0 ohms resistance to an AC waveform. while DC is infinite. This is an IDEAL capacitor BTW. Any real capacitor will not be 0 ohm, itll be whatever its ESR rating is. For example, if a capacitor reads 2 Ohms ESR, it means it will read 2 ohms reactance of an AC signal.

This property allows them to make excellent filters. Because again, if a capacitor is put in parallel with an AC signal source, or rectified AC, it appears as 0 ohms. (or its ESR). So basically it shorts the AC to itself, but it leaves the DC property alone. hence why you get a smooth signal at the other end.

This is why ESR plays a huge roll, the worse the ESR, the higher the resistance of the cap is on an AC load. so if the ESR is 30 ohms, it wont do as good of a job for filtering than if it had 2 ohms.

Of course, they have other properties as well especially needed for other circuits, like RC time constants, etc. But i decided to express the most important property. Aside from the actual capacitance itself.

 
Thank you for the brief "lesson" on capacitors, techknight!

Please forgive me if I sound dumb, but how does it relate to my eMac problem?

I assume that you were basically saying that a higher ESR (such as when capacitors age) could perhaps cause the problem by presenting a higher resistance to the incoming AC, correct?

If I'm wrong, it sure sounds like a good explanation!!

c

 
Hi,

I will definitely be working on this when I get the time (probably within the next couple of weeks).

I'll let you know what's up with it then.

c

 
Progress Report:

I accidentally did the heat-up-the-capacitors trick by leaving it locked up in an outbuilding at 100 degrees for a few hours today. Obviously this is not the best way to go about it, but it worked, so to speak.

It was hot enough that it worked flawlessly and immediately when I pressed the power button, so that would indicate failing capacitors, correct?

Anyway, it sounds like the diagnosis is confirmed pretty much, at least I think so.

c

 
I'm suspecting similar capacitor problems with my 700Mhz eMac are causing the screen problems discussed in this thread.

Tonight I got out the hairdryer to test, but I'm not sure how to go about it. Should I just blow-dry stuff while it's running? To do that, I'd have to take off the white plastic shell (and the power button with it) and the big emi cage.

Or am I supposed to take all that stuff off, randomly blow-dry suspected parts, then assemble everything back together and test it?

Note: Obviously running it while torn apart means staying far away from the CRT. As in DON'T touch anything.

 
blow-dry suspect parts while everything is apart, but hooked up enough to run. Careful of the high voltage lead.

I had a buddy once who is dead now, For other reasons, that tried to blow dry cure silicone on a 30kv lead while the TV was on. Lets just say, that was a shocking experience.

But the emacs are known to use shit caps, so keep this in mind.

 
Progress Report:
I accidentally did the heat-up-the-capacitors trick by leaving it locked up in an outbuilding at 100 degrees for a few hours today. Obviously this is not the best way to go about it, but it worked, so to speak.

It was hot enough that it worked flawlessly and immediately when I pressed the power button, so that would indicate failing capacitors, correct?

Anyway, it sounds like the diagnosis is confirmed pretty much, at least I think so.

c
Click to expand...
Hehe, at least I can be useful for something at times. Yup time to recap.

 
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