Re: Caps a poppin!

[Tristan Mumford <xtristan.xmumford@xgmail.xcom>]

Charlie Springer wrote:

> On Sat, 6 Jan 2007 02:38:23 -0800, Tristan Mumford wrote
> (in article <459f7c41$0$38156$c30e37c6@lon-reader.news.telstra.net>):
> 
>> There were 2x 0.1uF burnt matchheads for a start.
>> I also found evidence of a few other caps that have been replaced. The
>> original caps (not the replacements) had also fried judging from remnant
>> scorchmarks.
>> However no other components show any signs of stress.
>> I replaced the burnt caps (with unfortunately different values. better
>> than lumps of carbon)
> 
> Wow!  A 0.1 mic is usually ceramic and rated at a lot more than 5 volts
> and without a failure mode due to age. I would suspect a power supply
> problem, especially with more than one failed cap. Would it wipe out caps
> but not the chips? It depends on how it happened. The anti-static
> protection in the chips might handle a spike that would cause a burn
> through in a cap, after which the cap would do a low resistance melt down
> at 5 volts.

It is kind of scary. Because it looks like they went off with a violent
explosion. I've seen other things go up that way like resistors and ICs,
but never a capacitor.

> 
> Now, if they were tantalum electrolytic, I would suspect either backwards
> replacements or a voltage spike failure (and don't breath the vapor and
> wash hands after handling).
Washed hands, but as for breathing the vapour... I have a feeling they have
been like that for maybe a decade.


> 
> Either way, I would check the power supply. I get a lot of //e's with bad
> power supplies. Something in those Apple switchers does not age well.
> Often they will display symptoms much like you describe.

My thoughts too. I attacked it with a multimeter. Unfortunately after I
discovered a grave mistake I made which may have killed one of the RAM
chips. Anyway. Yes. I checked the voltages. +5 and +12 were only about 0.3v
too high. Not too worrying.
However the -5 and -12 were exhibiting some really strange behaviour.
Sometimes -12 was ~-6v or not there at all, like a partial shutdown of the
supply. -5 would intermittently vanish too.
Correct me if I'm wrong, but if -5v dies wouldn't that kill the video sync?
It didn't seem to be affecting it.


> Sometimes pulling 
> all the boards out will lower the current enough for the supply to run
> correctly and boot OK. I have a box of new (new old stock, as they say)
> supplies from Sun Remarketing (and a box of new keyboards for Platinum
> //e) which 95% of the time is all that is needed to fix up a dead Apple.
Ooh. //e keyboards. Nice.

I'm tempted to do another external AT(/X) powersupply mod like I did for
my //e. Although in this ones case the type of supply I used for the //e
probably would actually fit inside the case.

I also have an unidentified powersupply. It's either Compaq style or Apple.
It has a connector with blade-style contacts. I might see if it works and
possibly adapt it. Assuming it's not a IIgs supply, which I have never
actually seen before.
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