Symptom: Runaway LC Fan, Bong, Nothing Else

[Elfen]

This one is interesting as I just ran into a few minutes ago as of this post.

Turn on LC III, it "Bongs!", and nothing else. No Video, no nothing. Strangely, though I did not notice it at first, the fan is howling at a high speed.

This was an LC III I recapped last year with Tantalum Caps that works. So I flicked the switched a couple more times - the same thing but at times it sounds like a half "Bong" instead of the full "Bong." A couple of times it did not even do that. I thought, "Did my LC III died after a recap like some other Macs did on this forum?"

I tried a known bad LC PSU that "Flubs." And it "Flubs" like it always did. But I notice that the fan was slower in speed and not howling. I took it as the PSU being bad and dismissed it. Then I tried a Good PSU from an LC. The LC III "Bongs" normally, video came up and then I got a flashing disk icon (it has no hard drive). So the LC III is fine! BUT the fan was spinning slower and not howling.

By the process of elimination, the PSU is bad. It's obvious that it will need a recap. That's a given.

But the fan over-revving, is that a sign of over-voltage? Is one of my regulars blown out?

What you experts think?

EDIT: This is the Astec LC PSU, not the TDK LC PSU.

 

[DragonKid]

A fan that works fine on a good 12V supply over-revving on a questionable PSU sure sounds like an overvoltage issue to me.  I wouldn't use that PSU with an actual logic board until you figure out what's wrong with it, if the overvoltage is high enough to make a fan run substantially faster, it could be high enough to cook the logic board.  I don't know much about how LC power supplies are regulated, but this does sound like a voltage regulator issue to me, maybe an issue with a reference voltage.  If you feel like confirming, you could connect some kind of dummy load (old junk case fan, large power resistors, etc.) to the PSU and check what the output voltages are on each rail.

 

[bibilit]

Those LC PSUs are easy and cheap to fix.

 

[Elfen]

LC PSUs are easy to fix if they just need a recap (as in a PSU that makes your LC "Flub" when you turn it on). Recapping the LC PSU fixes nearly all of the issues with it. But an overvoltage on a PSU means something else is going on and the regulator(s) might be blown or shorted.

I have not yet recapped this PSU, and hopefully it will fix the issues I'm having when I do. But if the regulator(s) is DOA and the overvoltage continues to exist, then I just wasted a recapping job until I get the regulator(s) fixed/replaced.

EDIT: The TDK Version of the LC PSU is the easiest of the LC PSUs to recap and fix. This is an Astec PSU, and it has a Heat Sink that goes over 3 large caps (without touching them) at the output section from a power regulator. It needs to be removed to replace those three caps. This does not exist on the TDK PSU.

 

[uniserver]

the astec psu's are known for caps going bad in other places.  so yes it could be the dead caps and cap goo is shorting out something causing an over voltage.

So the best thing to do before you trouble shoot is to re-cap it or have it re-caped.  but i seriously doubt that it needs anything more then new caps and for the cap goo to be cleaned up with some qtips, and sprayed with %91 ipa.

 

[uniserver]

and yes the astec psu's are tricky to re-cap

 

[Elfen]

This is good to know, Uniserver. As soon as the funds are available, I'll go get some caps for the Astec LC PSU as they differ from the TK LC PSU.

Many thanks!