Modding a Vivitar flash unit

[Peabody]

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

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.

 

[Grant]

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

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.

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).

Grant.

 

[Grant]

I've seen series IGBTs used to cut off the flashtube.

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.

Grant.

 

[Peabody]

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

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?

 

[Jure Newsgroups]

My wife worked for "Doc" while I was a student at MIT.


What I did in the mid-60's...

http://analog-innovations.com/SED/Pat-3496411.pdf

Patented in a flurry of applications when I left Motorola in 1968 and
went off to Philco-Ford with John Welty

IIRC this also dumps all the charge remaining :-(


...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Remember: Once you go over the hill, you pick up speed

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


here is link to an interesting pdf document about Doc:

http://mit.edu/6.933/www/Fall2000/edgerton/EdgertonWW2.pdf

Thanks, Jure Z.