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Monitor IIc exploding caps! But still works?!

VERAULT

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Jan 30, 2012
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Connecticut, USA
So I was given a monitor IIc 9" a2m4090 CRT monitor that was dead. This is the better samsung variant (vs the hitachi) sams info here: https://ia803004.us.archive.org/22/...s_Apple_IIc/Sams_Computer_Facts_Apple_IIc.pdf

Anyway, it was dead. replaced a bad fuse, and all the eletrolytics, and both power leds.. which is weird. And the monitor works again. But the weird this is capacitor c514 which is 100uf keeps exploding! As in heat builds up so quickly it shoots off the metal jacket like a bullet. And the monitor keeps working after it explodes.. Amazingly.

So I'm a bit lost here. Why is it exploding, and why is the monitor still working? I DID NOT replace the non polarized caps but did inspect them for leakage.

Could use some help; exploding caps make me think back to the time I was almost blinded and lost vision in one eye for a couple weeks (because of exploding caps)
 
I would check the diodes D14/D15 and D17/D18
What voltage rating is C514 ? Correct polarity ?
Is the supply voltage of the Horizontal output within range ? (11.8 Volts)
 
Yes triple checked. Its originally (well the schematic doesnt show voltage) a 100uf 50v. I have replaced it with a 100uf 50v and a 100uf 100v. Both exploded both were in the correct polarity orientation.
I am wondering if my 100uf caps might be garbage. I put the original cap back in and so far its been in there 15 minutes with no explosion.

Another note, the original 1.6amp 250v fuse was blown when I got it. I replaced it with a 2a fuse as I thought I had ran out of 1.6a fuses.
I found a 1.6a fuse and replaced it the same time I put the old 100uf 50v fuse in as well. could that possibly be related or just coincidence?

here is a photo of the un-jacketed caps for kicks:20200831_143653_resized.jpg

In the past when I put a cap in backwards (hey we all make mistakes) they usually just boil out the electrolytic or pop out the rubber cap a little. Complete launch of the casing never happens unless something is really wrong.
 
Seems your caps are garbage then since the old one is working....

You could try and get some real brand caps like Panasonic, Nichicon, Epcos from a respectable supplier.
 
I am inclined to agree. I am not sure about the variable of the fuse which I replaced with the correct specs when I put the old one in. Maybe someone can clarify. But yeah I am leaning to my supply of 100uf caps being inferior if the fuse doesnt matter.

Still running by the way going on 45 minutes.
 
I imagine they are miss labeled much lower voltage caps. Certainly, experimenting with caps in my apprenticeship (along with the bin bag full of hydrogen + a long stick with a match on the end :) ), a good 10V small bodied cap would explode impressively at 240V but most bigger ones would just fizzle.
 
I imagine they are miss labeled much lower voltage caps. Certainly, experimenting with caps in my apprenticeship (along with the bin bag full of hydrogen + a long stick with a match on the end :) ), a good 10V small bodied cap would explode impressively at 240V but most bigger ones would just fizzle.

That sounds like fun!

You know the same idea occurred to me about them being labeled wrong. Anyway never happened before. I recap stuff pretty regularly and that is the first time that happened.

The old cap is still working in the unit. I wish I knew if the 2amp fuse (vs the 1.6amp which is what the unit calls for) could possibly have something to do with it.
 
The value of the fuse will have no influence on the effect you had with the 'bad caps'.
A fuse is normally used to protect the device (and its surroundings) from burning in case of an internal short circuit, it almost never serves as a protection for the components (which already failed causing the short circuit).


The 2 Amp fuse will still blow in case of a short, it might take a few milliseconds longer than a 1.6 Amp fuse.
 
+1 to the above. Using a bigger fuse just means the scorch mark around a short might get a little bit bigger before the power cuts off.

If the old capacitor isn't exploding then bad replacements sounds like the most reasonable explanation.
 
Ha, and even I added to that thread in 2016 :)
But still, the old cap is oriented in the same way as the new one and does not pop ?
 
Yes same orientation. But come on you cant ignore the coincidence. Can a brand new cap alter its polarity?

There are manufacturers who mark the positive side with a stripe... (I am looking at you Epcos!) :eek:
epcos_positive_stripe.jpg
 
I thought this subject sounded familiar. C514 failure is noted in a schematic found online. Usually when I make a note in a schematic,
it means I've seen the problem more than once. Makes me wonder if some caps were put in reverse at the factory or by subsequent
tech repairs. Notice the polarity of C513 and the polarity of the supply? That is definitely drawn backwards C514.PNG
 
Indeed the diagram is wrong, and what is the text C514 doing inside the LOT ?
The whole circuit is just awkward with the horizontal output transistor being used as 'high side' driver of the LOT and the negative voltages on base and such..
In most monitors the transistor pulls the primary winding of the LOT to ground and the other side of the primary is connected directly to the power supply.
Much easier to drive the horizontal output transistor too that way.

As for C514:
If the effective voltage is -16 Volt, a quality capacitor specified for 50V might handle that for quite some time.
 
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