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Modding a Vivitar flash unit

Discussion in 'Electronic Design' started by Peabody, Apr 5, 2011.

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  1. Peabody

    Peabody Guest

    Well this isn't exactly "design" but I figured someone here would

    I have a Vivitar Zoom Thyristor 3500 flash with a DM/C module at
    the bottom. Circa 1981 per the copyrights.

    The problem is that in Manual mode you always get full power, and
    what I need for off-camera "strobist" work is the ability to set
    the power to Full, 1/2, 1/4, 1/8,...,1/64 power.

    From the way the flash operates, it seems clear that it always
    charges to full power. Then it varies the power output by varying
    the amount of that charge that's dumped when the flash fires.

    But how does it do that? Does it vary the current? Or does it
    always start the same way, but cut off the dump earlier for lower
    power. Seems like it would have to do the latter to avoid heat
    problems. So if that's the case, then there has to be a timing
    circuit of some kind.

    Has anyone done this kind of mod to a flash unit? If so, any
    pointers would be useful. Thanks very much.
  2. Grant

    Grant Guest

    While I've not done the flash mod, I know of the technology they're using,
    put an IGT in the ground lead of the strobe tube and can turn off the flash
    at any point for the lower energy output. This is also used for pre-flash
    to avoid red-eye and perhaps focus.

    So you need to add one of the gate transistors in series with the tube to
    turn it off, turn on is still triggered by that small scr switched pilot
    pulse to the strobe's reflector (in a P&S cam I recently dismantled).

  3. Grant

    Grant Guest

    That's current technology, the series switch is tiny too, I got two examples
    from small cams I opened up now. I mistakenly called it an IBT or something
    in my reply, couldn't remember the name for the letters.

  4. Peabody

    Peabody Guest

    If I might interrupt my own thread to address a more fundamental

    I determined that the voltage on the flash's trigger pin was ok for
    my Canon XT, so this morning I mounted the flash on the camera.
    And it doesn't fire. I bought the camera used, and have never
    mounted anything in the hot shoe before, so it may have been dead
    from the beginning.

    So, in troubleshooting, I've found that:

    The built-in flash pops up and flashes properly.

    When the flash is mounted in the hot shoe, the built-in flash will
    not pop up. That means the little switch in the hot shoe is
    working, and the camera is responding properly.

    When I short the flash center pin to its shell, the flash fires.
    The test button on the flash also works.

    I inserted a popsicle stick on one side of the hot shoe to depress
    the "external flash is present", then tested resistance between the
    center pin and ground. I get infinite resistance measuring one
    way, and a diode drop the other way - I assume this is a protection
    diode of some sort for when the current shuts off.

    I then measured resistance when the shutter is triggered, and see
    nothing on my analog meter. Not a twitch. But, I don't know how
    fast it would be, so I don't know whether it should twitch or not.

    Well, unless there's a camera setting I haven't found to enable
    external flash, it seems my hot shoe isn't operating.

    Anybody have any words of wisdom on this?

  5. "let's do something useful for a change" (place a comma wherever you

    here is link to an interesting pdf document about Doc:

    Thanks, Jure Z.
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