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Thread: Blown Cap (or how to make your heart beat faster)

  1. #1
    Join Date
    Jul 2008
    Chandler, AZ, USA

    Default Blown Cap (or how to make your heart beat faster)

    For those who have never seen, or heard a capacitor blow (and this was my first one ever):

    Sitting in front of my Televideo terminal this morning, I was happily typing away and drinking my coffee, when all of a sudden the video starting looking weird. In a normal reaction, I looked closer and more intently at the screen. At about the same time as I was in complete concentration viewing my recently typed but now fading characters, there was a loud bang, like a balloon popping or droping a book flat on a solid surface.
    It scared the crap out of me! My heart rate must have doubled (and that's after two cups of coffee too).
    Then smoke started pouring out of the vents at the top of the terminal.
    In a split second, I pulled the plug to cut the power.

    After about 15 minutes waiting for my heart to slow down, I opened it up and here's what I found.

    Bits of a cap inside the terminal:

    Cap parts.jpg

    And what's left of the blown (and infamous for Televideo 9xx owners) C306 capacitor:

    Blown cap.jpg

    Next time the video on one of my terminals starts looking weird, I'm going to get farther away, not closer !
    Last edited by Lorne; February 21st, 2010 at 09:25 AM. Reason: balloon has two L's not one

  2. #2
    Join Date
    Jan 2007
    Pacific Northwest, USA
    Blog Entries


    Old HV caps could blow rather frighteningly. I have particular memories of a Sprague "Vitamin Q" capacitor blowing. These were steel-jacketed axial-lead jobs with the ends sealed with glass and used on a lot of military gear.

    When it went, the noise left my ears ringing and jagged piecess of the steel jacket embedded in the ceiling above the chassis I was working on. 10 seconds earlier, I had been probing around that part of the circuitry...

  3. #3


    Yeah, I usually replace a lot of older capacitors (especially if they are paper capacitors though those appear more in Radios and TV's). Hopefully you get the terminal working again.

  4. #4
    Join Date
    Jul 2003
    Västerås, Sweden


    So far the only caps that I saw poff were power filter ones, and they all emitted a fizzling noise seconds before the smoke came out. Thus the last time it happened, I could predict that within four seconds the capacitor will break. I am not sure if this is good knowledge or not...
    Anders Carlsson

  5. #5
    Join Date
    Jan 2010
    Central VA


    Yes, "Vitamin Q" and other steel-jacket oil filled caps were pretty scary when they went! We've got a Tektronix storage scope at work that blows one now and then (it's an old tube model). Tantalum caps are pretty bad too.

    The worst scare I got, though, was losing a cap in the accelerating voltage filter of my Eico 460 was one of those wax-and-paper types, with nearly 1 kV. It failed with a chunk being blown out of the capacitor body. Unfortunately, someone had replaced the fuse with a much higher value than was originally in there, so the transformer was able to maintain an arc in the blown-out hole and started the wax smoking. It didn't catch fire, but it ruined the power transformer.

  6. #6


    Who said vintage computers were not exciting

    Quote Originally Posted by glitch View Post
    ...Tantalum caps are pretty bad too.
    Yes, I've had a few caps blow. Most just fizzelled but the tantalum cap exploded like a firecracker. It was in an Apple II disk drive. Sparks and material flew high in the air.

    Just before it went up I had been leaning over the device!

    I now make it a policy to inspect newly acquired computers and parts internally first to flush out any dead rodents, insects and dust. I then put the cover back on before applying the voltage. Then at least if anything explodes on the board it's not going to hit me!

    My vintage collection:
    My vintage activities blog:
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)

  7. #7
    Join Date
    Jan 2010
    Central VA


    I'd had a dipped tantalum explode off a 386 motherboard, with the top of the cap striking my forehead. It's a good thing I wear glasses!

    We've got a large stock of NOS tantalums at work, that someone had picked up in a large auction lot at some time. The polarity markings exist as a dot above one lead, so to determine the polarity of a specific batch before use, my co-worker and I usually hook them up to a bench power supply set to their full rated voltage. In one direction, they draw no current (the correct polarity); in the other, they invariably explode.

  8. #8


    I learned this 20 years ago in Vo--tech on how to blow up caps! Had so much fun booby-traping other test benches till someone wired MY bench with a big cap! It smoked up the class room and set off the fire alarms.... Good for us the instructor did have a sense of humor of it but after that we never did that again(school officals were pissed)

    Years later while working on a home power amp, I had a one of the those caps the size of a soda can blow up! That was not fun and loud enough to have ringing ears for a few hours.

  9. #9


    I'll probably hear a lot of them when I start powering up some older machines on a regular basis again. The one that got me was working on recreating disks on an Apple IIe. I had the case top off swapping that jumper stack so I could connect a computer up to the II and get it to type. It wasn't working so I connected it to a serial sniffer prior to the computer. I had an LED flashlight I was using and had put that down on the second shelf on the rack and while I'm working "POW!" and the sound of plastic hitting the wall and a piece hitting my forehead. I was looking around freaking out on what blew I grab my flashlight, hit the rubber power button but it feels weird. Evidently (I guess?) I left the flashlight on, put it face down and it heated up enough that the button exploded out of the rubber along with the spring that I found afterwards.

    I was happy that it wasn't a computer or component but disappointed in what I thought was a good quality metal led flashlight.
    Looking to acquire: IBM 5100, Altair 8800

  10. #10


    Hehe. Been there with exploding capacitors. Many years ago I used to play an online MUSH game, using an Esprit terminal. One particularly interesting memory was being up real late, like 1am, playing the game - I was weapons officer on a starship involved in a firefight with another ship. The other ship was firing... similar situation - the text started shaking a bit, then *BANG* - the screen died with a flash. Scared the hell out of me. I guess the Romulans had better weapons than I thought.



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