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Old 22nd March 2010, 09:10 PM   #11
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Yeah, the Mythbusters had some fun testing the hot water heater myth!

I think everyone should take a little more precaution in powering up vintage electronics with big caps, since we never know which ones will blow and which won't. A variac is a good tool to help, but without monitoring the current through the filter caps, you won't really know when they'll try to blow.

Kyle
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Old 22nd March 2010, 09:23 PM   #12
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I remember mentioning once that those old caps are like a shotgun shell going off in your face,and people thought I was being over-dramatic. It's no bull,Be Careful!
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Old 22nd March 2010, 09:30 PM   #13
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In a former lifetime I used to build computers for resale (the 286 era). Some friends and I pooled our money and bought out a vendor that sold components in the Computer Shopper magazine. There was a virtual mountain of returned goods in the back of the warehouse. I got at least a hundred "AT" power supplies. On the first pass I just tested them all using a car headlight for a load and sorted out the good ones. Later I opened each one and put a clip lead across the fuse (yeah even then I new better) and retested finding more good ones. Unfortunately I found one with shorted diodes and the result looked a lot like this, including the stuff stuck to the ceiling. It was one of those "ohnosecond" moments when you realize the instant the plug hits the outlet and the lights dim that you screwed up. Before the brain decides to yank the plug, and the body reacts and finds out that its welded into the outlet (the ohnosecond) the cap went BOOM!

After that I modified my technique to incorporate a 100 watt bulb in series with my clip lead "fuse".

Quote:
Some of those old caps have no vents
As I recently found out even a vented cap can explode when the Big Dumb Blonde One sets a 1000 watt power supply on 600 volts feeding a circuit that has a 450 volt cap. There was a 10K resistor in series with the cap which exploded too. Blew the 47uF 450V cap completely in half, and there was only a black stain on the board where the resistor used to be! The amp was rockin before the cap came a knockin!
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Old 22nd March 2010, 09:53 PM   #14
Arnulf is offline Arnulf  Europe
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Originally Posted by stalker View Post
I'm sure he's going to put the potentially go-bang parts under the cover next time around.

I had potential difference in a very long serial cable with no opto isolation blow up an UART once, it made a hole directly in the middle of the UART chip where the silicon die used to be. The cover was on fortunately. ICs (at least not the power stuff) are waaaaaay less prone to explosions than electrolytics but I learned my lesson.
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Old 22nd March 2010, 09:54 PM   #15
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If the die that punches the grooves for the vents isn't perfect, they will go boom. I have yet to blow up a cap in a tube amp. I have blown up a few in computers, especially cheap power supplies and VRMs. They use the cheapest caps they can find and they never seem to vent as they should after being cooked for years. A few times the PSU case was packed with aluminum foil and paper confetti after the bang.

Flamed plenty of tants, though. P U.
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Old 23rd March 2010, 05:51 AM   #16
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Working with solid state and high voltage always use safety specs and ear defenders. When an Isotop or bigger leaves it's footprint, it's a trip to the surgury to take out the scrapnel.

richy
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Old 23rd March 2010, 10:36 AM   #17
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Hello,

I hope these cans wont try to do the same..

Click the image to open in full size.

470F 450V Sic Safco 85% capacitor. They have to endure 280V DC.

George
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Old 23rd March 2010, 03:19 PM   #18
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Originally Posted by rknize View Post
If the die that punches the grooves for the vents isn't perfect, they will go boom.
I blew up a cap (C6) twice in my Tubelab-SE mono. Obviously something wasn't sized properly for this modification. Fortunately, the vent worked both times and ended up making a relatively small mess inside of chassis. Still not a good experience. The fume is nasty.
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Old 23rd March 2010, 03:50 PM   #19
rknize is offline rknize  United States
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Quote:
Originally Posted by Evenharmonics View Post
I blew up a cap (C6) twice in my Tubelab-SE mono. Obviously something wasn't sized properly for this modification. Fortunately, the vent worked both times and ended up making a relatively small mess inside of chassis. Still not a good experience. The fume is nasty.
That's good. I would expect the higher-quality caps that we generally buy from Panasonic and others to be built to better standards than the ultra-cheap ones found in most computers. I can't recall blowing any of these up, though.

I fried a whole bunch of tants one day as a teenager. I was inexperienced and didn't notice that the polarity marking on the body was for the POSITIVE terminal and not the negative like electrolytics (whose bad idea was that?). My dad got a chuckle out of that. Man those things stink, especially when a half dozen go at once.
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Old 23rd March 2010, 04:00 PM   #20
kevinkr is offline kevinkr  United States
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Originally Posted by antiquekid3 View Post
We had one go off at my local radio club...it went off like a rocket! All the insides spooled off like toilet paper, and luckily no one was injured. Those electrolytics can be scary!

Especially for old ones, reforming caps (and carefully watching the current) is a good idea. Don't let it get past a few milliamps!

Kyle
I had a very similar experience with one of those small orange axial electrolytics made by Cornell-Dubilier about 30yrs ago. I was also quite lucky.

The capacitor was brand new, installed correctly and operating well within its ratings. I smelled something burning, and it was too late. The capacitor decapitated a transistor in a TO-5 can which was in the driver stage of a mosfet power amplifier, needless to say its failure took out the output mosfets as well. What a mess...
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